-At the high-level, LLVM code is translated to a machine specific representation -formed out of MachineFunction, MachineBasicBlock, and MachineInstr instances -(defined in include/llvm/CodeGen). This representation is completely target -agnostic, representing instructions in their most abstract form: an opcode and a -series of operands. This representation is designed to support both SSA -representation for machine code, as well as a register allocated, non-SSA form. -
+At the high-level, LLVM code is translated to a machine specific + representation formed out of + MachineFunction, + MachineBasicBlock, + and MachineInstr instances (defined + in include/llvm/CodeGen). This representation is completely target + agnostic, representing instructions in their most abstract form: an opcode + and a series of operands. This representation is designed to support both an + SSA representation for machine code, as well as a register allocated, non-SSA + form.
Target machine instructions are represented as instances of the -MachineInstr class. This class is an extremely abstract way of -representing machine instructions. In particular, it only keeps track of -an opcode number and a set of operands.
- -The opcode number is a simple unsigned number that only has meaning to a -specific backend. All of the instructions for a target should be defined in -the *InstrInfo.td file for the target. The opcode enum values -are auto-generated from this description. The MachineInstr class does -not have any information about how to interpret the instruction (i.e., what the -semantics of the instruction are): for that you must refer to the -TargetInstrInfo class.
- -The operands of a machine instruction can be of several different types: -they can be a register reference, constant integer, basic block reference, etc. -In addition, a machine operand should be marked as a def or a use of the value -(though only registers are allowed to be defs).
+ MachineInstr class. This class is an extremely abstract way of + representing machine instructions. In particular, it only keeps track of an + opcode number and a set of operands. + +The opcode number is a simple unsigned integer that only has meaning to a + specific backend. All of the instructions for a target should be defined in + the *InstrInfo.td file for the target. The opcode enum values are + auto-generated from this description. The MachineInstr class does + not have any information about how to interpret the instruction (i.e., what + the semantics of the instruction are); for that you must refer to the + TargetInstrInfo class.
+ +The operands of a machine instruction can be of several different types: a + register reference, a constant integer, a basic block reference, etc. In + addition, a machine operand should be marked as a def or a use of the value + (though only registers are allowed to be defs).
By convention, the LLVM code generator orders instruction operands so that -all register definitions come before the register uses, even on architectures -that are normally printed in other orders. For example, the SPARC add -instruction: "add %i1, %i2, %i3" adds the "%i1", and "%i2" registers -and stores the result into the "%i3" register. In the LLVM code generator, -the operands should be stored as "%i3, %i1, %i2": with the destination -first.
- -Keeping destination (definition) operands at the beginning of the operand -list has several advantages. In particular, the debugging printer will print -the instruction like this:
- + all register definitions come before the register uses, even on architectures + that are normally printed in other orders. For example, the SPARC add + instruction: "add %i1, %i2, %i3" adds the "%i1", and "%i2" registers + and stores the result into the "%i3" register. In the LLVM code generator, + the operands should be stored as "%i3, %i1, %i2": with the + destination first. + +Keeping destination (definition) operands at the beginning of the operand + list has several advantages. In particular, the debugging printer will print + the instruction like this:
+ +- %r3 = add %i1, %i2 +%r3 = add %i1, %i2+
If the first operand is a def, and it is also easier to create instructions whose only def is the first -operand.
+Also if the first operand is a def, it is easier to create + instructions whose only def is the first operand.
Machine instructions are created by using the BuildMI functions, -located in the include/llvm/CodeGen/MachineInstrBuilder.h file. The -BuildMI functions make it easy to build arbitrary machine -instructions. Usage of the BuildMI functions look like this: -
+ located in the include/llvm/CodeGen/MachineInstrBuilder.h file. The + BuildMI functions make it easy to build arbitrary machine + instructions. Usage of the BuildMI functions look like this: +- // Create a 'DestReg = mov 42' (rendered in X86 assembly as 'mov DestReg, 42') - // instruction. The '1' specifies how many operands will be added. - MachineInstr *MI = BuildMI(X86::MOV32ri, 1, DestReg).addImm(42); +// Create a 'DestReg = mov 42' (rendered in X86 assembly as 'mov DestReg, 42') +// instruction. The '1' specifies how many operands will be added. +MachineInstr *MI = BuildMI(X86::MOV32ri, 1, DestReg).addImm(42); - // Create the same instr, but insert it at the end of a basic block. - MachineBasicBlock &MBB = ... - BuildMI(MBB, X86::MOV32ri, 1, DestReg).addImm(42); +// Create the same instr, but insert it at the end of a basic block. +MachineBasicBlock &MBB = ... +BuildMI(MBB, X86::MOV32ri, 1, DestReg).addImm(42); - // Create the same instr, but insert it before a specified iterator point. - MachineBasicBlock::iterator MBBI = ... - BuildMI(MBB, MBBI, X86::MOV32ri, 1, DestReg).addImm(42); +// Create the same instr, but insert it before a specified iterator point. +MachineBasicBlock::iterator MBBI = ... +BuildMI(MBB, MBBI, X86::MOV32ri, 1, DestReg).addImm(42); - // Create a 'cmp Reg, 0' instruction, no destination reg. - MI = BuildMI(X86::CMP32ri, 2).addReg(Reg).addImm(0); - // Create an 'sahf' instruction which takes no operands and stores nothing. - MI = BuildMI(X86::SAHF, 0); +// Create a 'cmp Reg, 0' instruction, no destination reg. +MI = BuildMI(X86::CMP32ri, 2).addReg(Reg).addImm(0); +// Create an 'sahf' instruction which takes no operands and stores nothing. +MI = BuildMI(X86::SAHF, 0); - // Create a self looping branch instruction. - BuildMI(MBB, X86::JNE, 1).addMBB(&MBB); +// Create a self looping branch instruction. +BuildMI(MBB, X86::JNE, 1).addMBB(&MBB);+
-The key thing to remember with the BuildMI functions is that you have -to specify the number of operands that the machine instruction will take. This -allows for efficient memory allocation. You also need to specify if operands -default to be uses of values, not definitions. If you need to add a definition -operand (other than the optional destination register), you must explicitly -mark it as such. -
+The key thing to remember with the BuildMI functions is that you + have to specify the number of operands that the machine instruction will + take. This allows for efficient memory allocation. You also need to specify + if operands default to be uses of values, not definitions. If you need to + add a definition operand (other than the optional destination register), you + must explicitly mark it as such:
+ ++MI.addReg(Reg, RegState::Define); ++
One important issue that the code generator needs to be aware of is the -presence of fixed registers. In particular, there are often places in the -instruction stream where the register allocator must arrange for a -particular value to be in a particular register. This can occur due to -limitations of the instruction set (e.g., the X86 can only do a 32-bit divide -with the EAX/EDX registers), or external factors like calling -conventions. In any case, the instruction selector should emit code that -copies a virtual register into or out of a physical register when needed.
+ presence of fixed registers. In particular, there are often places in the + instruction stream where the register allocator must arrange for a + particular value to be in a particular register. This can occur due to + limitations of the instruction set (e.g., the X86 can only do a 32-bit divide + with the EAX/EDX registers), or external factors like + calling conventions. In any case, the instruction selector should emit code + that copies a virtual register into or out of a physical register when + needed.For example, consider this simple LLVM example:
+
- int %test(int %X, int %Y) {
- %Z = div int %X, %Y
- ret int %Z
- }
+define i32 @test(i32 %X, i32 %Y) {
+ %Z = udiv i32 %X, %Y
+ ret i32 %Z
+}
+The X86 instruction selector produces this machine code for the div -and ret (use -"llc X.bc -march=x86 -print-machineinstrs" to get this):
+The X86 instruction selector produces this machine code for the div + and ret (use "llc X.bc -march=x86 -print-machineinstrs" to + get this):
+- ;; Start of div - %EAX = mov %reg1024 ;; Copy X (in reg1024) into EAX - %reg1027 = sar %reg1024, 31 - %EDX = mov %reg1027 ;; Sign extend X into EDX - idiv %reg1025 ;; Divide by Y (in reg1025) - %reg1026 = mov %EAX ;; Read the result (Z) out of EAX - - ;; Start of ret - %EAX = mov %reg1026 ;; 32-bit return value goes in EAX - ret +;; Start of div +%EAX = mov %reg1024 ;; Copy X (in reg1024) into EAX +%reg1027 = sar %reg1024, 31 +%EDX = mov %reg1027 ;; Sign extend X into EDX +idiv %reg1025 ;; Divide by Y (in reg1025) +%reg1026 = mov %EAX ;; Read the result (Z) out of EAX + +;; Start of ret +%EAX = mov %reg1026 ;; 32-bit return value goes in EAX +ret+
By the end of code generation, the register allocator has coalesced -the registers and deleted the resultant identity moves, producing the -following code:
+By the end of code generation, the register allocator has coalesced the + registers and deleted the resultant identity moves producing the following + code:
+- ;; X is in EAX, Y is in ECX - mov %EAX, %EDX - sar %EDX, 31 - idiv %ECX - ret +;; X is in EAX, Y is in ECX +mov %EAX, %EDX +sar %EDX, 31 +idiv %ECX +ret+
This approach is extremely general (if it can handle the X86 architecture, -it can handle anything!) and allows all of the target specific -knowledge about the instruction stream to be isolated in the instruction -selector. Note that physical registers should have a short lifetime for good -code generation, and all physical registers are assumed dead on entry and -exit of basic blocks (before register allocation). Thus if you need a value -to be live across basic block boundaries, it must live in a virtual -register.
+This approach is extremely general (if it can handle the X86 architecture, it + can handle anything!) and allows all of the target specific knowledge about + the instruction stream to be isolated in the instruction selector. Note that + physical registers should have a short lifetime for good code generation, and + all physical registers are assumed dead on entry to and exit from basic + blocks (before register allocation). Thus, if you need a value to be live + across basic block boundaries, it must live in a virtual + register.
MachineInstr's are initially selected in SSA-form, and -are maintained in SSA-form until register allocation happens. For the most -part, this is trivially simple since LLVM is already in SSA form: LLVM PHI nodes -become machine code PHI nodes, and virtual registers are only allowed to have a -single definition.
+MachineInstr's are initially selected in SSA-form, and are + maintained in SSA-form until register allocation happens. For the most part, + this is trivially simple since LLVM is already in SSA form; LLVM PHI nodes + become machine code PHI nodes, and virtual registers are only allowed to have + a single definition.
-After register allocation, machine code is no longer in SSA-form, as there -are no virtual registers left in the code.
+After register allocation, machine code is no longer in SSA-form because + there are no virtual registers left in the code.
This section documents the phases described in the high-level design of the code generator. It -explains how they work and some of the rationale behind their design.
+The MachineBasicBlock class contains a list of machine instructions + (MachineInstr instances). It roughly + corresponds to the LLVM code input to the instruction selector, but there can + be a one-to-many mapping (i.e. one LLVM basic block can map to multiple + machine basic blocks). The MachineBasicBlock class has a + "getBasicBlock" method, which returns the LLVM basic block that it + comes from.
-Instruction Selection is the process of translating LLVM code presented to the -code generator into target-specific machine instructions. There are several -well-known ways to do this in the literature. In LLVM there are two main forms: -the old-style 'simple' instruction selector (which effectively peephole selects -each LLVM instruction into a series of machine instructions), and the new -SelectionDAG based instruction selector. -
-The 'simple' instruction selectors are tedious to write, require a lot of -boiler plate code, and are difficult to get correct. Additionally, any -optimizations written for a simple instruction selector cannot be used by other -targets. For this reason, LLVM is moving to a new SelectionDAG based -instruction selector, which is described in this section. If you are starting a -new port, we recommend that you write the instruction selector using the -SelectionDAG infrastructure.
+The MachineFunction class contains a list of machine basic blocks + (MachineBasicBlock instances). It + corresponds one-to-one with the LLVM function input to the instruction + selector. In addition to a list of basic blocks, + the MachineFunction contains a a MachineConstantPool, + a MachineFrameInfo, a MachineFunctionInfo, and a + MachineRegisterInfo. See + include/llvm/CodeGen/MachineFunction.h for more information.
-In time, most of the target-specific code for instruction selection will be -auto-generated from the target description (*.td) files. For now, -however, the Select Phase must still be -written by hand.
-The SelectionDAG provides an abstraction for code representation in a way that -is amenable to instruction selection using automatic techniques -(e.g. dynamic-programming based optimal pattern matching selectors), It is also -well suited to other phases of code generation; in particular, instruction scheduling. Additionally, the SelectionDAG provides a host representation where a -large variety of very-low-level (but target-independent) -optimizations may be -performed: ones which require extensive information about the instructions -efficiently supported by the target. +The MC Layer is used to represent and process code at the raw machine code +level, devoid of "high level" information like "constant pools", "jump tables", +"global variables" or anything like that. At this level, LLVM handles things +like label names, machine instructions, and sections in the object file. The +code in this layer is used for a number of important purposes: the tail end of +the code generator uses it to write a .s or .o file, and it is also used by the +llvm-mc tool to implement standalone machine codeassemblers and disassemblers.
-The SelectionDAG is a Directed-Acyclic-Graph whose nodes are instances of the -SDNode class. The primary payload of the SDNode is its -operation code (Opcode) that indicates what operation the node performs. -The various operation node types are described at the top of the -include/llvm/CodeGen/SelectionDAGNodes.h file. Depending on the -operation, nodes may contain additional information (e.g. the condition code -for a SETCC node) contained in a derived class.
- -Although most operations define a single value, each node in the graph may -define multiple values. For example, a combined div/rem operation will define -both the dividend and the remainder. Many other situations require multiple -values as well. Each node also has some number of operands, which are edges -to the node defining the used value. Because nodes may define multiple values, -edges are represented by instances of the SDOperand class, which is -a <SDNode, unsigned> pair, indicating the node and result -value being used, respectively. Each value produced by an SDNode has an -associated MVT::ValueType, indicating what type the value is. +This section describes some of the important classes. There are also a number +of important subsystems that interact at this layer, they are described later +in this manual.
--SelectionDAGs contain two different kinds of values: those that represent data -flow and those that represent control flow dependencies. Data values are simple -edges with an integer or floating point value type. Control edges are -represented as "chain" edges which are of type MVT::Other. These edges provide -an ordering between nodes that have side effects (such as -loads/stores/calls/return/etc). All nodes that have side effects should take a -token chain as input and produce a new one as output. By convention, token -chain inputs are always operand #0, and chain results are always the last -value produced by an operation.
+-A SelectionDAG has designated "Entry" and "Root" nodes. The Entry node is -always a marker node with an Opcode of ISD::EntryToken. The Root node is the -final side-effecting node in the token chain. For example, in a single basic -block function, this would be the return node. +MCStreamer is best thought of as an assembler API. It is an abstract API which +is implemented in different ways (e.g. to output a .s file, output an +ELF .o file, etc) but whose API correspond directly to what you see in a .s +file. MCStreamer has one method per directive, such as EmitLabel, +EmitSymbolAttribute, SwitchSection, EmitValue (for .byte, .word), etc, which +directly correspond to assembly level directives. It also has an +EmitInstruction method, which is used to output an MCInst to the streamer.
-One important concept for SelectionDAGs is the notion of a "legal" vs. "illegal" -DAG. A legal DAG for a target is one that only uses supported operations and -supported types. On PowerPC, for example, a DAG with any values of i1, i8, i16, -or i64 type would be illegal. The legalize -phase is responsible for turning an illegal DAG into a legal DAG. +This API is most important for two clients: the llvm-mc stand-alone assembler is +effectively a parser that parses a line, then invokes a method on MCStreamer. In +the code generator, the Code Emission phase of the code +generator lowers higher level LLVM IR and Machine* constructs down to the MC +layer, emitting directives through MCStreamer.
+ ++On the implementation side of MCStreamer, there are two major implementations: +one for writing out a .s file (MCAsmStreamer), and one for writing out a .o +file (MCObjectStreamer). MCAsmStreamer is a straight-forward implementation +that prints out a directive for each method (e.g. EmitValue -> .byte), but +MCObjectStreamer implements a full assembler.
+-SelectionDAG-based instruction selection consists of the following steps: +The MCContext class is the owner of a variety of uniqued data structures at the +MC layer, including symbols, sections, etc. As such, this is the class that you +interact with to create symbols and sections. This class can not be subclassed.
--
-
- Build initial DAG - This stage performs - a simple translation from the input LLVM code to an illegal SelectionDAG. - -
- Optimize SelectionDAG - This stage - performs simple optimizations on the SelectionDAG to simplify it and - recognize meta instructions (like rotates and div/rem pairs) for - targets that support these meta operations. This makes the resultant code - more efficient and the 'select instructions from DAG' phase (below) simpler. - -
- Legalize SelectionDAG - This stage - converts the illegal SelectionDAG to a legal SelectionDAG, by eliminating - unsupported operations and data types. -
- Optimize SelectionDAG (#2) - This - second run of the SelectionDAG optimized the newly legalized DAG, to - eliminate inefficiencies introduced by legalization. -
- Select instructions from DAG - Finally, - the target instruction selector matches the DAG operations to target - instructions, emitting them and building the MachineFunction being - compiled. -
After all of these steps are complete, the SelectionDAG is destroyed and the -rest of the code generation passes are run.
--The initial SelectionDAG is naively peephole expanded from the LLVM input by -the SelectionDAGLowering class in the SelectionDAGISel.cpp file. The -intent of this pass is to expose as much low-level, target-specific details -to the SelectionDAG as possible. This pass is mostly hard-coded (e.g. an LLVM -add turns into an SDNode add while a geteelementptr is expanded into the obvious -arithmetic). This pass requires target-specific hooks to lower calls and -returns, varargs, etc. For these features, the TargetLowering interface is -used. +The MCSymbol class represents a symbol (aka label) in the assembly file. There +are two interesting kinds of symbols: assembler temporary symbols, and normal +symbols. Assembler temporary symbols are used and processed by the assembler +but are discarded when the object file is produced. The distinction is usually +represented by adding a prefix to the label, for example "L" labels are +assembler temporary labels in MachO.
+MCSymbols are created by MCContext and uniqued there. This means that +MCSymbols can be compared for pointer equivalence to find out if they are the +same symbol. Note that pointer inequality does not guarantee the labels will +end up at different addresses though. It's perfectly legal to output something +like this to the .s file:
+ +
+ foo: + bar: + .byte 4 ++ +
In this case, both the foo and bar symbols will have the same address.
+The Legalize phase is in charge of converting a DAG to only use the types and -operations that are natively supported by the target. This involves two major -tasks:
++The MCSection class represents an object-file specific section. It is subclassed +by object file specific implementations (e.g. MCSectionMachO, +MCSectionCOFF, MCSectionELF) and these are created and uniqued +by MCContext. The MCStreamer has a notion of the current section, which can be +changed with the SwitchToSection method (which corresponds to a ".section" +directive in a .s file). +
+ +-
-
Convert values of unsupported types to values of supported types.
-There are two main ways of doing this: promoting a small type to a larger - type (e.g. f32 -> f64, or i16 -> i32), and breaking up large - integer types - to smaller ones (e.g. implementing i64 with i32 operations where - possible). Type conversions can insert sign and zero extensions as - needed to make sure that the final code has the same behavior as the - input.
-
+
+Eliminate operations that are not supported by the target in a supported - type.
-Targets often have wierd constraints, such as not supporting every - operation on every supported datatype (e.g. X86 does not support byte - conditional moves). Legalize takes care of either open-coding another - sequence of operations to emulate the operation (this is known as - expansion), promoting to a larger type that supports the operation - (promotion), or using a target-specific hook to implement the - legalization.
-
-
-Instead of using a Legalize pass, we could require that every target-specific -selector supports and expands every -operator and type even if they are not supported and may require many -instructions to implement (in fact, this is the approach taken by the -"simple" selectors). However, using a Legalize pass allows all of the -cannonicalization patterns to be shared across targets which makes it very -easy to optimize the cannonicalized code because it is still in the form of -a DAG. +The MCInst class is a target-independent representation of an instruction. It +is a simple class (much more so than MachineInstr) +that holds a target-specific opcode and a vector of MCOperands. MCOperand, in +turn, is a simple discriminated union of three cases: 1) a simple immediate, +2) a target register ID, 3) a symbolic expression (e.g. "Lfoo-Lbar+42") as an +MCExpr. +
+ +MCInst is the common currency used to represent machine instructions at the +MC layer. It is the type used by the instruction encoder, the instruction +printer, and the type generated by the assembly parser and disassembler.
-The SelectionDAG optimization phase is run twice for code generation: once -immediately after the DAG is built and once after legalization. The first run -of the pass allows the initial code to be cleaned up (e.g. performing -optimizations that depend on knowing that the operators have restricted type -inputs). The second run of the pass cleans up the messy code generated by the -Legalize pass, allowing Legalize to be very simple since it can ignore many -special cases. -
+This section documents the phases described in the + high-level design of the code generator. + It explains how they work and some of the rationale behind their design.
--One important class of optimizations that this pass will do in the future is -optimizing inserted sign and zero extension instructions. Here are some good -papers on the subject:
+
-"Widening
-integer arithmetic"
-Kevin Redwine and Norman Ramsey
-International Conference on Compiler Construction (CC) 2004
-
- "Effective
- sign extension elimination"
- Motohiro Kawahito, Hideaki Komatsu, and Toshio Nakatani
- Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language Design
- and Implementation.
-
Instruction Selection is the process of translating LLVM code presented to + the code generator into target-specific machine instructions. There are + several well-known ways to do this in the literature. LLVM uses a + SelectionDAG based instruction selector.
+ +Portions of the DAG instruction selector are generated from the target + description (*.td) files. Our goal is for the entire instruction + selector to be generated from these .td files, though currently + there are still things that require custom C++ code.
The Select phase is the bulk of the target-specific code for instruction -selection. This phase takes a legal SelectionDAG as input, and does simple -pattern matching on the DAG to generate code. In time, the Select phase will -be automatically generated from the target's InstrInfo.td file, which is why we -want to make the Select phase as simple and mechanical as possible.
+The SelectionDAG provides an abstraction for code representation in a way + that is amenable to instruction selection using automatic techniques + (e.g. dynamic-programming based optimal pattern matching selectors). It is + also well-suited to other phases of code generation; in particular, + instruction scheduling (SelectionDAG's are very close to scheduling DAGs + post-selection). Additionally, the SelectionDAG provides a host + representation where a large variety of very-low-level (but + target-independent) optimizations may be + performed; ones which require extensive information about the instructions + efficiently supported by the target.
+ +The SelectionDAG is a Directed-Acyclic-Graph whose nodes are instances of the + SDNode class. The primary payload of the SDNode is its + operation code (Opcode) that indicates what operation the node performs and + the operands to the operation. The various operation node types are + described at the top of the include/llvm/CodeGen/SelectionDAGNodes.h + file.
+ +Although most operations define a single value, each node in the graph may + define multiple values. For example, a combined div/rem operation will + define both the dividend and the remainder. Many other situations require + multiple values as well. Each node also has some number of operands, which + are edges to the node defining the used value. Because nodes may define + multiple values, edges are represented by instances of the SDValue + class, which is a <SDNode, unsigned> pair, indicating the node + and result value being used, respectively. Each value produced by + an SDNode has an associated MVT (Machine Value Type) + indicating what the type of the value is.
+ +SelectionDAGs contain two different kinds of values: those that represent + data flow and those that represent control flow dependencies. Data values + are simple edges with an integer or floating point value type. Control edges + are represented as "chain" edges which are of type MVT::Other. + These edges provide an ordering between nodes that have side effects (such as + loads, stores, calls, returns, etc). All nodes that have side effects should + take a token chain as input and produce a new one as output. By convention, + token chain inputs are always operand #0, and chain results are always the + last value produced by an operation.
+ +A SelectionDAG has designated "Entry" and "Root" nodes. The Entry node is + always a marker node with an Opcode of ISD::EntryToken. The Root + node is the final side-effecting node in the token chain. For example, in a + single basic block function it would be the return node.
+ +One important concept for SelectionDAGs is the notion of a "legal" vs. + "illegal" DAG. A legal DAG for a target is one that only uses supported + operations and supported types. On a 32-bit PowerPC, for example, a DAG with + a value of type i1, i8, i16, or i64 would be illegal, as would a DAG that + uses a SREM or UREM operation. The + legalize types and + legalize operations phases are + responsible for turning an illegal DAG into a legal DAG.
SelectionDAG-based instruction selection consists of the following steps:
+-
-
- Optional whole-function selection. -
- Select is a graph translation phase. -
- Place the machine instructions resulting from Select according to register -pressure or a schedule. -
- DAG Scheduling. -
- Auto-generate the Select phase from the target description (*.td) files. - +
- Build initial DAG — This stage + performs a simple translation from the input LLVM code to an illegal + SelectionDAG. + +
- Optimize SelectionDAG — This + stage performs simple optimizations on the SelectionDAG to simplify it, + and recognize meta instructions (like rotates + and div/rem pairs) for targets that support these meta + operations. This makes the resultant code more efficient and + the select instructions from DAG phase + (below) simpler. + +
- Legalize SelectionDAG Types + — This stage transforms SelectionDAG nodes to eliminate any types + that are unsupported on the target. + +
- Optimize SelectionDAG — The + SelectionDAG optimizer is run to clean up redundancies exposed by type + legalization. + +
- Legalize SelectionDAG Ops — + This stage transforms SelectionDAG nodes to eliminate any operations + that are unsupported on the target. + +
- Optimize SelectionDAG — The + SelectionDAG optimizer is run to eliminate inefficiencies introduced by + operation legalization. + +
- Select instructions from DAG — + Finally, the target instruction selector matches the DAG operations to + target instructions. This process translates the target-independent input + DAG into another DAG of target instructions. + +
- SelectionDAG Scheduling and Formation + — The last phase assigns a linear order to the instructions in the + target-instruction DAG and emits them into the MachineFunction being + compiled. This step uses traditional prepass scheduling techniques.
After all of these steps are complete, the SelectionDAG is destroyed and the + rest of the code generation passes are run.
+ +One great way to visualize what is going on here is to take advantage of a + few LLC command line options. The following options pop up a window + displaying the SelectionDAG at specific times (if you only get errors printed + to the console while using this, you probably + need to configure your system + to add support for it).
+ +-
+
- -view-dag-combine1-dags displays the DAG after being built, + before the first optimization pass. + +
- -view-legalize-dags displays the DAG before Legalization. + +
- -view-dag-combine2-dags displays the DAG before the second + optimization pass. + +
- -view-isel-dags displays the DAG before the Select phase. + +
- -view-sched-dags displays the DAG before Scheduling. +
The -view-sunit-dags displays the Scheduler's dependency graph. + This graph is based on the final SelectionDAG, with nodes that must be + scheduled together bundled into a single scheduling-unit node, and with + immediate operands and other nodes that aren't relevant for scheduling + omitted.
+To Be Written
To Be Written
To Be Written
To Be Written
The initial SelectionDAG is naïvely peephole expanded from the LLVM + input by the SelectionDAGLowering class in the + lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp file. The intent of + this pass is to expose as much low-level, target-specific details to the + SelectionDAG as possible. This pass is mostly hard-coded (e.g. an + LLVM add turns into an SDNode add while a + getelementptr is expanded into the obvious arithmetic). This pass + requires target-specific hooks to lower calls, returns, varargs, etc. For + these features, the TargetLowering + interface is used.
+This section of the document explains features or design decisions that -are specific to the code generator for a particular target.
+The Legalize phase is in charge of converting a DAG to only use the types + that are natively supported by the target.
-There are two main ways of converting values of unsupported scalar types to + values of supported types: converting small types to larger types + ("promoting"), and breaking up large integer types into smaller ones + ("expanding"). For example, a target might require that all f32 values are + promoted to f64 and that all i1/i8/i16 values are promoted to i32. The same + target might require that all i64 values be expanded into pairs of i32 + values. These changes can insert sign and zero extensions as needed to make + sure that the final code has the same behavior as the input.
+There are two main ways of converting values of unsupported vector types to + value of supported types: splitting vector types, multiple times if + necessary, until a legal type is found, and extending vector types by adding + elements to the end to round them out to legal types ("widening"). If a + vector gets split all the way down to single-element parts with no supported + vector type being found, the elements are converted to scalars + ("scalarizing").
- -A target implementation tells the legalizer which types are supported (and + which register class to use for them) by calling the + addRegisterClass method in its TargetLowering constructor.
+ +-The X86 code generator lives in the lib/Target/X86 directory. This -code generator currently targets a generic P6-like processor. As such, it -produces a few P6-and-above instructions (like conditional moves), but it does -not make use of newer features like MMX or SSE. In the future, the X86 backend -will have sub-target support added for specific processor families and -implementations.
+The Legalize phase is in charge of converting a DAG to only use the + operations that are natively supported by the target.
+ +Targets often have weird constraints, such as not supporting every operation + on every supported datatype (e.g. X86 does not support byte conditional moves + and PowerPC does not support sign-extending loads from a 16-bit memory + location). Legalize takes care of this by open-coding another sequence of + operations to emulate the operation ("expansion"), by promoting one type to a + larger type that supports the operation ("promotion"), or by using a + target-specific hook to implement the legalization ("custom").
+ +A target implementation tells the legalizer which operations are not + supported (and which of the above three actions to take) by calling the + setOperationAction method in its TargetLowering + constructor.
+ +Prior to the existence of the Legalize passes, we required that every target + selector supported and handled every + operator and type even if they are not natively supported. The introduction + of the Legalize phases allows all of the canonicalization patterns to be + shared across targets, and makes it very easy to optimize the canonicalized + code because it is still in the form of a DAG.
-The following are the known target triples that are supported by the X86 -backend. This is not an exhaustive list, but it would be useful to add those -that people test. -
--
-
- i686-pc-linux-gnu - Linux -
- i386-unknown-freebsd5.3 - FreeBSD 5.3 -
- i686-pc-cygwin - Cygwin on Win32 -
- i686-pc-mingw32 - MingW on Win32 -
- i686-apple-darwin* - Apple Darwin -
The SelectionDAG optimization phase is run multiple times for code + generation, immediately after the DAG is built and once after each + legalization. The first run of the pass allows the initial code to be + cleaned up (e.g. performing optimizations that depend on knowing that the + operators have restricted type inputs). Subsequent runs of the pass clean up + the messy code generated by the Legalize passes, which allows Legalize to be + very simple (it can focus on making code legal instead of focusing on + generating good and legal code).
+ +One important class of optimizations performed is optimizing inserted sign + and zero extension instructions. We currently use ad-hoc techniques, but + could move to more rigorous techniques in the future. Here are some good + papers on the subject:
+ +"Widening
+ integer arithmetic"
+ Kevin Redwine and Norman Ramsey
+ International Conference on Compiler Construction (CC) 2004
"Effective
+ sign extension elimination"
+ Motohiro Kawahito, Hideaki Komatsu, and Toshio Nakatani
+ Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language Design
+ and Implementation.
The x86 has a very flexible way of accessing memory. It is capable of -forming memory addresses of the following expression directly in integer -instructions (which use ModR/M addressing):
+The Select phase is the bulk of the target-specific code for instruction + selection. This phase takes a legal SelectionDAG as input, pattern matches + the instructions supported by the target to this DAG, and produces a new DAG + of target code. For example, consider the following LLVM fragment:
+- Base+[1,2,4,8]*IndexReg+Disp32 +%t1 = fadd float %W, %X +%t2 = fmul float %t1, %Y +%t3 = fadd float %t2, %Z+
In order to represent this, LLVM tracks no less than 4 operands for each -memory operand of this form. This means that the "load" form of 'mov' has the -following MachineOperands in this order:
+This LLVM code corresponds to a SelectionDAG that looks basically like + this:
+-Index: 0 | 1 2 3 4 -Meaning: DestReg, | BaseReg, Scale, IndexReg, Displacement -OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg, SignExtImm +(fadd:f32 (fmul:f32 (fadd:f32 W, X), Y), Z)+
Stores, and all other instructions, treat the four memory operands in the -same way, in the same order.
+If a target supports floating point multiply-and-add (FMA) operations, one of + the adds can be merged with the multiply. On the PowerPC, for example, the + output of the instruction selector might look like this DAG:
++(FMADDS (FADDS W, X), Y, Z) +
The FMADDS instruction is a ternary instruction that multiplies its +first two operands and adds the third (as single-precision floating-point +numbers). The FADDS instruction is a simple binary single-precision +add instruction. To perform this pattern match, the PowerPC backend includes +the following instruction definitions:
-+def FMADDS : AForm_1<59, 29, + (ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRC, F4RC:$FRB), + "fmadds $FRT, $FRA, $FRC, $FRB", + [(set F4RC:$FRT, (fadd (fmul F4RC:$FRA, F4RC:$FRC), + F4RC:$FRB))]>; +def FADDS : AForm_2<59, 21, + (ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRB), + "fadds $FRT, $FRA, $FRB", + [(set F4RC:$FRT, (fadd F4RC:$FRA, F4RC:$FRB))]>; ++
-An instruction name consists of the base name, a default operand size, and a -a character per operand with an optional special size. For example:
+The portion of the instruction definition in bold indicates the pattern used + to match the instruction. The DAG operators + (like fmul/fadd) are defined in + the include/llvm/Target/TargetSelectionDAG.td file. " + F4RC" is the register class of the input and result values.
-
-ADD8rr -> add, 8-bit register, 8-bit register
-IMUL16rmi -> imul, 16-bit register, 16-bit memory, 16-bit immediate
-IMUL16rmi8 -> imul, 16-bit register, 16-bit memory, 8-bit immediate
-MOVSX32rm16 -> movsx, 32-bit register, 16-bit memory
-
The TableGen DAG instruction selector generator reads the instruction + patterns in the .td file and automatically builds parts of the + pattern matching code for your target. It has the following strengths:
+ +-
+
- At compiler-compiler time, it analyzes your instruction patterns and tells + you if your patterns make sense or not. + +
- It can handle arbitrary constraints on operands for the pattern match. In + particular, it is straight-forward to say things like "match any immediate + that is a 13-bit sign-extended value". For examples, see the + immSExt16 and related tblgen classes in the PowerPC + backend. + +
- It knows several important identities for the patterns defined. For + example, it knows that addition is commutative, so it allows the + FMADDS pattern above to match "(fadd X, (fmul Y, Z))" as + well as "(fadd (fmul X, Y), Z)", without the target author having + to specially handle this case. + +
- It has a full-featured type-inferencing system. In particular, you should + rarely have to explicitly tell the system what type parts of your patterns + are. In the FMADDS case above, we didn't have to tell + tblgen that all of the nodes in the pattern are of type 'f32'. + It was able to infer and propagate this knowledge from the fact that + F4RC has type 'f32'. + +
- Targets can define their own (and rely on built-in) "pattern fragments". + Pattern fragments are chunks of reusable patterns that get inlined into + your patterns during compiler-compiler time. For example, the integer + "(not x)" operation is actually defined as a pattern fragment + that expands as "(xor x, -1)", since the SelectionDAG does not + have a native 'not' operation. Targets can define their own + short-hand fragments as they see fit. See the definition of + 'not' and 'ineg' for examples. + +
- In addition to instructions, targets can specify arbitrary patterns that
+ map to one or more instructions using the 'Pat' class. For example, the
+ PowerPC has no way to load an arbitrary integer immediate into a register
+ in one instruction. To tell tblgen how to do this, it defines:
+
+
++++// Arbitrary immediate support. Implement in terms of LIS/ORI. +def : Pat<(i32 imm:$imm), + (ORI (LIS (HI16 imm:$imm)), (LO16 imm:$imm))>; +
+
+ If none of the single-instruction patterns for loading an immediate into a + register match, this will be used. This rule says "match an arbitrary i32 + immediate, turning it into an ORI ('or a 16-bit immediate') and + an LIS ('load 16-bit immediate, where the immediate is shifted to + the left 16 bits') instruction". To make this work, the + LO16/HI16 node transformations are used to manipulate + the input immediate (in this case, take the high or low 16-bits of the + immediate).
+
+ - While the system does automate a lot, it still allows you to write custom + C++ code to match special cases if there is something that is hard to + express. +
While it has many strengths, the system currently has some limitations, + primarily because it is a work in progress and is not yet finished:
+ +-
+
- Overall, there is no way to define or match SelectionDAG nodes that define + multiple values (e.g. SMUL_LOHI, LOAD, CALL, + etc). This is the biggest reason that you currently still have + to write custom C++ code for your instruction selector. + +
- There is no great way to support matching complex addressing modes yet. + In the future, we will extend pattern fragments to allow them to define + multiple values (e.g. the four operands of the X86 + addressing mode, which are currently matched with custom C++ code). + In addition, we'll extend fragments so that a fragment can match multiple + different patterns. + +
- We don't automatically infer flags like isStore/isLoad yet. + +
- We don't automatically generate the set of supported registers and + operations for the Legalizer + yet. + +
- We don't have a way of tying in custom legalized nodes yet. +
Despite these limitations, the instruction selector generator is still quite + useful for most of the binary and logical operations in typical instruction + sets. If you run into any problems or can't figure out how to do something, + please let Chris know!
+ +The scheduling phase takes the DAG of target instructions from the selection + phase and assigns an order. The scheduler can pick an order depending on + various constraints of the machines (i.e. order for minimal register pressure + or try to cover instruction latencies). Once an order is established, the + DAG is converted to a list + of MachineInstrs and the SelectionDAG is + destroyed.
+ +Note that this phase is logically separate from the instruction selection + phase, but is tied to it closely in the code because it operates on + SelectionDAGs.
+ +-
+
- Optional function-at-a-time selection. + +
- Auto-generate entire selector from .td file. +
To Be Written
Live Intervals are the ranges (intervals) where a variable is live. + They are used by some register allocator passes to + determine if two or more virtual registers which require the same physical + register are live at the same point in the program (i.e., they conflict). + When this situation occurs, one virtual register must be spilled.
+ +The first step in determining the live intervals of variables is to calculate + the set of registers that are immediately dead after the instruction (i.e., + the instruction calculates the value, but it is never used) and the set of + registers that are used by the instruction, but are never used after the + instruction (i.e., they are killed). Live variable information is computed + for each virtual register and register allocatable physical + register in the function. This is done in a very efficient manner because it + uses SSA to sparsely compute lifetime information for virtual registers + (which are in SSA form) and only has to track physical registers within a + block. Before register allocation, LLVM can assume that physical registers + are only live within a single basic block. This allows it to do a single, + local analysis to resolve physical register lifetimes within each basic + block. If a physical register is not register allocatable (e.g., a stack + pointer or condition codes), it is not tracked.
+ +Physical registers may be live in to or out of a function. Live in values are + typically arguments in registers. Live out values are typically return values + in registers. Live in values are marked as such, and are given a dummy + "defining" instruction during live intervals analysis. If the last basic + block of a function is a return, then it's marked as using all live + out values in the function.
+ +PHI nodes need to be handled specially, because the calculation of + the live variable information from a depth first traversal of the CFG of the + function won't guarantee that a virtual register used by the PHI + node is defined before it's used. When a PHI node is encountered, + only the definition is handled, because the uses will be handled in other + basic blocks.
+ +For each PHI node of the current basic block, we simulate an + assignment at the end of the current basic block and traverse the successor + basic blocks. If a successor basic block has a PHI node and one of + the PHI node's operands is coming from the current basic block, then + the variable is marked as alive within the current basic block and all + of its predecessor basic blocks, until the basic block with the defining + instruction is encountered.
+ +We now have the information available to perform the live intervals analysis + and build the live intervals themselves. We start off by numbering the basic + blocks and machine instructions. We then handle the "live-in" values. These + are in physical registers, so the physical register is assumed to be killed + by the end of the basic block. Live intervals for virtual registers are + computed for some ordering of the machine instructions [1, N]. A + live interval is an interval [i, j), where 1 <= i <= j + < N, for which a variable is live.
+ +More to come...
+ +The Register Allocation problem consists in mapping a program + Pv, that can use an unbounded number of virtual registers, + to a program Pp that contains a finite (possibly small) + number of physical registers. Each target architecture has a different number + of physical registers. If the number of physical registers is not enough to + accommodate all the virtual registers, some of them will have to be mapped + into memory. These virtuals are called spilled virtuals.
+ +In LLVM, physical registers are denoted by integer numbers that normally + range from 1 to 1023. To see how this numbering is defined for a particular + architecture, you can read the GenRegisterNames.inc file for that + architecture. For instance, by + inspecting lib/Target/X86/X86GenRegisterNames.inc we see that the + 32-bit register EAX is denoted by 15, and the MMX register + MM0 is mapped to 48.
+ +Some architectures contain registers that share the same physical location. A + notable example is the X86 platform. For instance, in the X86 architecture, + the registers EAX, AX and AL share the first eight + bits. These physical registers are marked as aliased in LLVM. Given a + particular architecture, you can check which registers are aliased by + inspecting its RegisterInfo.td file. Moreover, the method + TargetRegisterInfo::getAliasSet(p_reg) returns an array containing + all the physical registers aliased to the register p_reg.
+ +Physical registers, in LLVM, are grouped in Register Classes. + Elements in the same register class are functionally equivalent, and can be + interchangeably used. Each virtual register can only be mapped to physical + registers of a particular class. For instance, in the X86 architecture, some + virtuals can only be allocated to 8 bit registers. A register class is + described by TargetRegisterClass objects. To discover if a virtual + register is compatible with a given physical, this code can be used:
+ +
+bool RegMapping_Fer::compatible_class(MachineFunction &mf,
+ unsigned v_reg,
+ unsigned p_reg) {
+ assert(TargetRegisterInfo::isPhysicalRegister(p_reg) &&
+ "Target register must be physical");
+ const TargetRegisterClass *trc = mf.getRegInfo().getRegClass(v_reg);
+ return trc->contains(p_reg);
+}
+
+Sometimes, mostly for debugging purposes, it is useful to change the number + of physical registers available in the target architecture. This must be done + statically, inside the TargetRegsterInfo.td file. Just grep + for RegisterClass, the last parameter of which is a list of + registers. Just commenting some out is one simple way to avoid them being + used. A more polite way is to explicitly exclude some registers from + the allocation order. See the definition of the GR8 register + class in lib/Target/X86/X86RegisterInfo.td for an example of this. +
+ +Virtual registers are also denoted by integer numbers. Contrary to physical + registers, different virtual registers never share the same number. The + smallest virtual register is normally assigned the number 1024. This may + change, so, in order to know which is the first virtual register, you should + access TargetRegisterInfo::FirstVirtualRegister. Any register whose + number is greater than or equal + to TargetRegisterInfo::FirstVirtualRegister is considered a virtual + register. Whereas physical registers are statically defined in + a TargetRegisterInfo.td file and cannot be created by the + application developer, that is not the case with virtual registers. In order + to create new virtual registers, use the + method MachineRegisterInfo::createVirtualRegister(). This method + will return a virtual register with the highest code.
+ +Before register allocation, the operands of an instruction are mostly virtual + registers, although physical registers may also be used. In order to check if + a given machine operand is a register, use the boolean + function MachineOperand::isRegister(). To obtain the integer code of + a register, use MachineOperand::getReg(). An instruction may define + or use a register. For instance, ADD reg:1026 := reg:1025 reg:1024 + defines the registers 1024, and uses registers 1025 and 1026. Given a + register operand, the method MachineOperand::isUse() informs if that + register is being used by the instruction. The + method MachineOperand::isDef() informs if that registers is being + defined.
+ +We will call physical registers present in the LLVM bitcode before register + allocation pre-colored registers. Pre-colored registers are used in + many different situations, for instance, to pass parameters of functions + calls, and to store results of particular instructions. There are two types + of pre-colored registers: the ones implicitly defined, and + those explicitly defined. Explicitly defined registers are normal + operands, and can be accessed + with MachineInstr::getOperand(int)::getReg(). In order to check + which registers are implicitly defined by an instruction, use + the TargetInstrInfo::get(opcode)::ImplicitDefs, + where opcode is the opcode of the target instruction. One important + difference between explicit and implicit physical registers is that the + latter are defined statically for each instruction, whereas the former may + vary depending on the program being compiled. For example, an instruction + that represents a function call will always implicitly define or use the same + set of physical registers. To read the registers implicitly used by an + instruction, + use TargetInstrInfo::get(opcode)::ImplicitUses. Pre-colored + registers impose constraints on any register allocation algorithm. The + register allocator must make sure that none of them are overwritten by + the values of virtual registers while still alive.
+ +There are two ways to map virtual registers to physical registers (or to + memory slots). The first way, that we will call direct mapping, is + based on the use of methods of the classes TargetRegisterInfo, + and MachineOperand. The second way, that we will call indirect + mapping, relies on the VirtRegMap class in order to insert loads + and stores sending and getting values to and from memory.
+ +The direct mapping provides more flexibility to the developer of the register + allocator; however, it is more error prone, and demands more implementation + work. Basically, the programmer will have to specify where load and store + instructions should be inserted in the target function being compiled in + order to get and store values in memory. To assign a physical register to a + virtual register present in a given operand, + use MachineOperand::setReg(p_reg). To insert a store instruction, + use TargetInstrInfo::storeRegToStackSlot(...), and to insert a + load instruction, use TargetInstrInfo::loadRegFromStackSlot.
+ +The indirect mapping shields the application developer from the complexities + of inserting load and store instructions. In order to map a virtual register + to a physical one, use VirtRegMap::assignVirt2Phys(vreg, preg). In + order to map a certain virtual register to memory, + use VirtRegMap::assignVirt2StackSlot(vreg). This method will return + the stack slot where vreg's value will be located. If it is + necessary to map another virtual register to the same stack slot, + use VirtRegMap::assignVirt2StackSlot(vreg, stack_location). One + important point to consider when using the indirect mapping, is that even if + a virtual register is mapped to memory, it still needs to be mapped to a + physical register. This physical register is the location where the virtual + register is supposed to be found before being stored or after being + reloaded.
+ +If the indirect strategy is used, after all the virtual registers have been + mapped to physical registers or stack slots, it is necessary to use a spiller + object to place load and store instructions in the code. Every virtual that + has been mapped to a stack slot will be stored to memory after been defined + and will be loaded before being used. The implementation of the spiller tries + to recycle load/store instructions, avoiding unnecessary instructions. For an + example of how to invoke the spiller, + see RegAllocLinearScan::runOnMachineFunction + in lib/CodeGen/RegAllocLinearScan.cpp.
+ +With very rare exceptions (e.g., function calls), the LLVM machine code + instructions are three address instructions. That is, each instruction is + expected to define at most one register, and to use at most two registers. + However, some architectures use two address instructions. In this case, the + defined register is also one of the used register. For instance, an + instruction such as ADD %EAX, %EBX, in X86 is actually equivalent + to %EAX = %EAX + %EBX.
+ +In order to produce correct code, LLVM must convert three address + instructions that represent two address instructions into true two address + instructions. LLVM provides the pass TwoAddressInstructionPass for + this specific purpose. It must be run before register allocation takes + place. After its execution, the resulting code may no longer be in SSA + form. This happens, for instance, in situations where an instruction such + as %a = ADD %b %c is converted to two instructions such as:
+ ++%a = MOVE %b +%a = ADD %a %c ++
Notice that, internally, the second instruction is represented as + ADD %a[def/use] %c. I.e., the register operand %a is both + used and defined by the instruction.
+ +An important transformation that happens during register allocation is called + the SSA Deconstruction Phase. The SSA form simplifies many analyses + that are performed on the control flow graph of programs. However, + traditional instruction sets do not implement PHI instructions. Thus, in + order to generate executable code, compilers must replace PHI instructions + with other instructions that preserve their semantics.
+ +There are many ways in which PHI instructions can safely be removed from the + target code. The most traditional PHI deconstruction algorithm replaces PHI + instructions with copy instructions. That is the strategy adopted by + LLVM. The SSA deconstruction algorithm is implemented + in lib/CodeGen/PHIElimination.cpp. In order to invoke this pass, the + identifier PHIEliminationID must be marked as required in the code + of the register allocator.
+ +Instruction folding is an optimization performed during register + allocation that removes unnecessary copy instructions. For instance, a + sequence of instructions such as:
+ ++%EBX = LOAD %mem_address +%EAX = COPY %EBX ++
can be safely substituted by the single instruction:
+ ++%EAX = LOAD %mem_address ++
Instructions can be folded with + the TargetRegisterInfo::foldMemoryOperand(...) method. Care must be + taken when folding instructions; a folded instruction can be quite different + from the original + instruction. See LiveIntervals::addIntervalsForSpills + in lib/CodeGen/LiveIntervalAnalysis.cpp for an example of its + use.
+ +The LLVM infrastructure provides the application developer with three + different register allocators:
+ +-
+
- Linear Scan — The default allocator. This is the + well-know linear scan register allocator. Whereas the + Simple and Local algorithms use a direct mapping + implementation technique, the Linear Scan implementation + uses a spiller in order to place load and stores. + +
- Fast — This register allocator is the default for debug + builds. It allocates registers on a basic block level, attempting to keep + values in registers and reusing registers as appropriate. + +
- PBQP — A Partitioned Boolean Quadratic Programming (PBQP) + based register allocator. This allocator works by constructing a PBQP + problem representing the register allocation problem under consideration, + solving this using a PBQP solver, and mapping the solution back to a + register assignment. + +
The type of register allocator used in llc can be chosen with the + command line option -regalloc=...:
+ ++$ llc -regalloc=linearscan file.bc -o ln.s; +$ llc -regalloc=fast file.bc -o fa.s; +$ llc -regalloc=pbqp file.bc -o pbqp.s; ++
To Be Written
To Be Written
The code emission step of code generation is responsible for lowering from +the code generator abstractions (like MachineFunction, MachineInstr, etc) down +to the abstractions used by the MC layer (MCInst, +MCStreamer, etc). This is +done with a combination of several different classes: the (misnamed) +target-independent AsmPrinter class, target-specific subclasses of AsmPrinter +(such as SparcAsmPrinter), and the TargetLoweringObjectFile class.
+ +Since the MC layer works at the level of abstraction of object files, it +doesn't have a notion of functions, global variables etc. Instead, it thinks +about labels, directives, and instructions. A key class used at this time is +the MCStreamer class. This is an abstract API that is implemented in different +ways (e.g. to output a .s file, output an ELF .o file, etc) that is effectively +an "assembler API". MCStreamer has one method per directive, such as EmitLabel, +EmitSymbolAttribute, SwitchSection, etc, which directly correspond to assembly +level directives. +
+ +If you are interested in implementing a code generator for a target, there +are three important things that you have to implement for your target:
+ +-
+
- First, you need a subclass of AsmPrinter for your target. This class +implements the general lowering process converting MachineFunction's into MC +label constructs. The AsmPrinter base class provides a number of useful methods +and routines, and also allows you to override the lowering process in some +important ways. You should get much of the lowering for free if you are +implementing an ELF, COFF, or MachO target, because the TargetLoweringObjectFile +class implements much of the common logic. + +
- Second, you need to implement an instruction printer for your target. The +instruction printer takes an MCInst and renders it to a +raw_ostream as text. Most of this is automatically generated from the .td file +(when you specify something like "add $dst, $src1, $src2" in the +instructions), but you need to implement routines to print operands. + +
- Third, you need to implement code that lowers a MachineInstr to an MCInst, usually implemented in +"<target>MCInstLower.cpp". This lowering process is often target +specific, and is responsible for turning jump table entries, constant pool +indices, global variable addresses, etc into MCLabels as appropriate. This +translation layer is also responsible for expanding pseudo ops used by the code +generator into the actual machine instructions they correspond to. The MCInsts +that are generated by this are fed into the instruction printer or the encoder. + + +
Finally, at your choosing, you can also implement an subclass of +MCCodeEmitter which lowers MCInst's into machine code bytes and relocations. +This is important if you want to support direct .o file emission, or would like +to implement an assembler for your target.
+ +Though you're probably reading this because you want to write or maintain a +compiler backend, LLVM also fully supports building a native assemblers too. +We've tried hard to automate the generation of the assembler from the .td files +(in particular the instruction syntax and encodings), which means that a large +part of the manual and repetitive data entry can be factored and shared with the +compiler.
+ +To Be Written
Once the instruction is parsed, it enters the MatchInstructionImpl function. +The MatchInstructionImpl function performs alias processing and then does +actual matching.
+ +Alias processing is the phase that canonicalizes different lexical forms of +the same instructions down to one representation. There are several different +kinds of alias that are possible to implement and they are listed below in the +order that they are processed (which is in order from simplest/weakest to most +complex/powerful). Generally you want to use the first alias mechanism that +meets the needs of your instruction, because it will allow a more concise +description.
+ +The first phase of alias processing is simple instruction mnemonic +remapping for classes of instructions which are allowed with two different +mnemonics. This phase is a simple and unconditionally remapping from one input +mnemonic to one output mnemonic. It isn't possible for this form of alias to +look at the operands at all, so the remapping must apply for all forms of a +given mnemonic. Mnemonic aliases are defined simply, for example X86 has: +
+ ++def : MnemonicAlias<"cbw", "cbtw">; +def : MnemonicAlias<"smovq", "movsq">; +def : MnemonicAlias<"fldcww", "fldcw">; +def : MnemonicAlias<"fucompi", "fucomip">; +def : MnemonicAlias<"ud2a", "ud2">; ++
... and many others. With a MnemonicAlias definition, the mnemonic is +remapped simply and directly. Though MnemonicAlias's can't look at any aspect +of the instruction (such as the operands) they can depend on global modes (the +same ones supported by the matcher), through a Requires clause:
+ ++def : MnemonicAlias<"pushf", "pushfq">, Requires<[In64BitMode]>; +def : MnemonicAlias<"pushf", "pushfl">, Requires<[In32BitMode]>; ++
In this example, the mnemonic gets mapped into different a new one depending +on the current instruction set.
+ +The most general phase of alias processing occurs while matching is +happening: it provides new forms for the matcher to match along with a specific +instruction to generate. An instruction alias has two parts: the string to +match and the instruction to generate. For example: +
+ ++def : InstAlias<"movsx $src, $dst", (MOVSX16rr8W GR16:$dst, GR8 :$src)>; +def : InstAlias<"movsx $src, $dst", (MOVSX16rm8W GR16:$dst, i8mem:$src)>; +def : InstAlias<"movsx $src, $dst", (MOVSX32rr8 GR32:$dst, GR8 :$src)>; +def : InstAlias<"movsx $src, $dst", (MOVSX32rr16 GR32:$dst, GR16 :$src)>; +def : InstAlias<"movsx $src, $dst", (MOVSX64rr8 GR64:$dst, GR8 :$src)>; +def : InstAlias<"movsx $src, $dst", (MOVSX64rr16 GR64:$dst, GR16 :$src)>; +def : InstAlias<"movsx $src, $dst", (MOVSX64rr32 GR64:$dst, GR32 :$src)>; ++
This shows a powerful example of the instruction aliases, matching the +same mnemonic in multiple different ways depending on what operands are present +in the assembly. The result of instruction aliases can include operands in a +different order than the destination instruction, and can use an input +multiple times, for example:
+ ++def : InstAlias<"clrb $reg", (XOR8rr GR8 :$reg, GR8 :$reg)>; +def : InstAlias<"clrw $reg", (XOR16rr GR16:$reg, GR16:$reg)>; +def : InstAlias<"clrl $reg", (XOR32rr GR32:$reg, GR32:$reg)>; +def : InstAlias<"clrq $reg", (XOR64rr GR64:$reg, GR64:$reg)>; ++
This example also shows that tied operands are only listed once. In the X86 +backend, XOR8rr has two input GR8's and one output GR8 (where an input is tied +to the output). InstAliases take a flattened operand list without duplicates +for tied operands. The result of an instruction alias can also use immediates +and fixed physical registers which are added as simple immediate operands in the +result, for example:
+ ++// Fixed Immediate operand. +def : InstAlias<"aad", (AAD8i8 10)>; + +// Fixed register operand. +def : InstAlias<"fcomi", (COM_FIr ST1)>; + +// Simple alias. +def : InstAlias<"fcomi $reg", (COM_FIr RST:$reg)>; ++
Instruction aliases can also have a Requires clause to make them +subtarget specific.
+ +To Be Written
This section of the document explains features or design decisions that are + specific to the code generator for a particular target. First we start + with a table that summarizes what features are supported by each target.
+ +Note that this table does not include the C backend or Cpp backends, since +they do not use the target independent code generator infrastructure. It also +doesn't list features that are not supported fully by any target yet. It +considers a feature to be supported if at least one subtarget supports it. A +feature being supported means that it is useful and works for most cases, it +does not indicate that there are zero known bugs in the implementation. Here +is the key:
+ + +| Unknown | +No support | +Partial Support | +Complete Support | +
|---|---|---|---|
| + | + | + | + |
Here is the table:
+ +| + | Target | +||||||||||||
| Feature | +ARM | +Alpha | +Blackfin | +CellSPU | +MBlaze | +MSP430 | +Mips | +PTX | +PowerPC | +Sparc | +SystemZ | +X86 | +XCore | +
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| is generally reliable | ++ | + | + | + | + | + | + | + | + | + | + | + | + |
| assembly parser | ++ | + | + | + | + | + | + | + | + | + | + | + | + |
| disassembler | ++ | + | + | + | + | + | + | + | + | + | + | + | + |
| inline asm | ++ | + | + | + | + | + | + | + | + | + | + | * | ++ |
| jit | +* | ++ | + | + | + | + | + | + | + | + | + | + | + |
| .o file writing | ++ | + | + | + | + | + | + | + | + | + | + | + | + |
| tail calls | ++ | + | + | + | + | + | + | + | + | + | + | + | + |
This box indicates whether the target is considered to be production quality. +This indicates that the target has been used as a static compiler to +compile large amounts of code by a variety of different people and is in +continuous use.
+This box indicates whether the target supports parsing target specific .s +files by implementing the MCAsmParser interface. This is required for llvm-mc +to be able to act as a native assembler and is required for inline assembly +support in the native .o file writer.
+ +This box indicates whether the target supports the MCDisassembler API for +disassembling machine opcode bytes into MCInst's.
+ +This box indicates whether the target supports most popular inline assembly +constraints and modifiers.
+ +X86 lacks reliable support for inline assembly +constraints relating to the X86 floating point stack.
+ +This box indicates whether the target supports the JIT compiler through +the ExecutionEngine interface.
+ +The ARM backend has basic support for integer code +in ARM codegen mode, but lacks NEON and full Thumb support.
+ +This box indicates whether the target supports writing .o files (e.g. MachO, +ELF, and/or COFF) files directly from the target. Note that the target also +must include an assembly parser and general inline assembly support for full +inline assembly support in the .o writer.
+ +Targets that don't support this feature can obviously still write out .o +files, they just rely on having an external assembler to translate from a .s +file to a .o file (as is the case for many C compilers).
+ +This box indicates whether the target supports guaranteed tail calls. These +are calls marked "tail" and use the fastcc +calling convention. Please see the tail call section +more more details.
+ +Tail call optimization, callee reusing the stack of the caller, is currently + supported on x86/x86-64 and PowerPC. It is performed if:
+ +-
+
- Caller and callee have the calling convention fastcc or + cc 10 (GHC call convention). + +
- The call is a tail call - in tail position (ret immediately follows call + and ret uses value of call or is void). + +
- Option -tailcallopt is enabled. + +
- Platform specific constraints are met. +
x86/x86-64 constraints:
+ +-
+
- No variable argument lists are used. + +
- On x86-64 when generating GOT/PIC code only module-local calls (visibility + = hidden or protected) are supported. +
PowerPC constraints:
+ +-
+
- No variable argument lists are used. + +
- No byval parameters are used. + +
- On ppc32/64 GOT/PIC only module-local calls (visibility = hidden or protected) are supported. +
Example:
+ +Call as llc -tailcallopt test.ll.
+ +
+declare fastcc i32 @tailcallee(i32 inreg %a1, i32 inreg %a2, i32 %a3, i32 %a4)
+
+define fastcc i32 @tailcaller(i32 %in1, i32 %in2) {
+ %l1 = add i32 %in1, %in2
+ %tmp = tail call fastcc i32 @tailcallee(i32 %in1 inreg, i32 %in2 inreg, i32 %in1, i32 %l1)
+ ret i32 %tmp
+}
+
+Implications of -tailcallopt:
+ +To support tail call optimization in situations where the callee has more + arguments than the caller a 'callee pops arguments' convention is used. This + currently causes each fastcc call that is not tail call optimized + (because one or more of above constraints are not met) to be followed by a + readjustment of the stack. So performance might be worse in such cases.
+ +Sibling call optimization is a restricted form of tail call optimization. + Unlike tail call optimization described in the previous section, it can be + performed automatically on any tail calls when -tailcallopt option + is not specified.
+ +Sibling call optimization is currently performed on x86/x86-64 when the + following constraints are met:
+ +-
+
- Caller and callee have the same calling convention. It can be either + c or fastcc. + +
- The call is a tail call - in tail position (ret immediately follows call + and ret uses value of call or is void). + +
- Caller and callee have matching return type or the callee result is not + used. + +
- If any of the callee arguments are being passed in stack, they must be + available in caller's own incoming argument stack and the frame offsets + must be the same. +
Example:
+
+declare i32 @bar(i32, i32)
+
+define i32 @foo(i32 %a, i32 %b, i32 %c) {
+entry:
+ %0 = tail call i32 @bar(i32 %a, i32 %b)
+ ret i32 %0
+}
+
+The X86 code generator lives in the lib/Target/X86 directory. This + code generator is capable of targeting a variety of x86-32 and x86-64 + processors, and includes support for ISA extensions such as MMX and SSE.
+ +The following are the known target triples that are supported by the X86 + backend. This is not an exhaustive list, and it would be useful to add those + that people test.
+ +-
+
- i686-pc-linux-gnu — Linux + +
- i386-unknown-freebsd5.3 — FreeBSD 5.3 + +
- i686-pc-cygwin — Cygwin on Win32 + +
- i686-pc-mingw32 — MingW on Win32 + +
- i386-pc-mingw32msvc — MingW crosscompiler on Linux + +
- i686-apple-darwin* — Apple Darwin on X86 + +
- x86_64-unknown-linux-gnu — Linux +
The following target-specific calling conventions are known to backend:
+ +-
+
- x86_StdCall — stdcall calling convention seen on Microsoft + Windows platform (CC ID = 64). + +
- x86_FastCall — fastcall calling convention seen on Microsoft + Windows platform (CC ID = 65). +
The x86 has a very flexible way of accessing memory. It is capable of + forming memory addresses of the following expression directly in integer + instructions (which use ModR/M addressing):
+ ++SegmentReg: Base + [1,2,4,8] * IndexReg + Disp32 ++
In order to represent this, LLVM tracks no less than 5 operands for each + memory operand of this form. This means that the "load" form of + 'mov' has the following MachineOperands in this order:
+ ++Index: 0 | 1 2 3 4 5 +Meaning: DestReg, | BaseReg, Scale, IndexReg, Displacement Segment +OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg, SignExtImm PhysReg ++
Stores, and all other instructions, treat the four memory operands in the + same way and in the same order. If the segment register is unspecified + (regno = 0), then no segment override is generated. "Lea" operations do not + have a segment register specified, so they only have 4 operands for their + memory reference.
+ +x86 has an experimental feature which provides + the ability to perform loads and stores to different address spaces + via the x86 segment registers. A segment override prefix byte on an + instruction causes the instruction's memory access to go to the specified + segment. LLVM address space 0 is the default address space, which includes + the stack, and any unqualified memory accesses in a program. Address spaces + 1-255 are currently reserved for user-defined code. The GS-segment is + represented by address space 256, while the FS-segment is represented by + address space 257. Other x86 segments have yet to be allocated address space + numbers.
+ +While these address spaces may seem similar to TLS via the + thread_local keyword, and often use the same underlying hardware, + there are some fundamental differences.
+ +The thread_local keyword applies to global variables and + specifies that they are to be allocated in thread-local memory. There are + no type qualifiers involved, and these variables can be pointed to with + normal pointers and accessed with normal loads and stores. + The thread_local keyword is target-independent at the LLVM IR + level (though LLVM doesn't yet have implementations of it for some + configurations).
+ +
Special address spaces, in contrast, apply to static types. Every + load and store has a particular address space in its address operand type, + and this is what determines which address space is accessed. + LLVM ignores these special address space qualifiers on global variables, + and does not provide a way to directly allocate storage in them. + At the LLVM IR level, the behavior of these special address spaces depends + in part on the underlying OS or runtime environment, and they are specific + to x86 (and LLVM doesn't yet handle them correctly in some cases).
+ +Some operating systems and runtime environments use (or may in the future + use) the FS/GS-segment registers for various low-level purposes, so care + should be taken when considering them.
+ +An instruction name consists of the base name, a default operand size, and a + a character per operand with an optional special size. For example:
+ ++ADD8rr -> add, 8-bit register, 8-bit register +IMUL16rmi -> imul, 16-bit register, 16-bit memory, 16-bit immediate +IMUL16rmi8 -> imul, 16-bit register, 16-bit memory, 8-bit immediate +MOVSX32rm16 -> movsx, 32-bit register, 16-bit memory ++
The PowerPC code generator lives in the lib/Target/PowerPC directory. The + code generation is retargetable to several variations or subtargets of + the PowerPC ISA; including ppc32, ppc64 and altivec.
+ +LLVM follows the AIX PowerPC ABI, with two deviations. LLVM uses a PC + relative (PIC) or static addressing for accessing global values, so no TOC + (r2) is used. Second, r31 is used as a frame pointer to allow dynamic growth + of a stack frame. LLVM takes advantage of having no TOC to provide space to + save the frame pointer in the PowerPC linkage area of the caller frame. + Other details of PowerPC ABI can be found at PowerPC ABI. Note: This link describes the 32 bit ABI. The 64 bit ABI + is similar except space for GPRs are 8 bytes wide (not 4) and r13 is reserved + for system use.
+ +The size of a PowerPC frame is usually fixed for the duration of a + function's invocation. Since the frame is fixed size, all references + into the frame can be accessed via fixed offsets from the stack pointer. The + exception to this is when dynamic alloca or variable sized arrays are + present, then a base pointer (r31) is used as a proxy for the stack pointer + and stack pointer is free to grow or shrink. A base pointer is also used if + llvm-gcc is not passed the -fomit-frame-pointer flag. The stack pointer is + always aligned to 16 bytes, so that space allocated for altivec vectors will + be properly aligned.
+ +An invocation frame is laid out as follows (low memory at top);
+ +| Linkage |
+
| Parameter area |
+
| Dynamic area |
+
| Locals area |
+
| Saved registers area |
+
| Previous Frame |
+
The linkage area is used by a callee to save special registers prior + to allocating its own frame. Only three entries are relevant to LLVM. The + first entry is the previous stack pointer (sp), aka link. This allows + probing tools like gdb or exception handlers to quickly scan the frames in + the stack. A function epilog can also use the link to pop the frame from the + stack. The third entry in the linkage area is used to save the return + address from the lr register. Finally, as mentioned above, the last entry is + used to save the previous frame pointer (r31.) The entries in the linkage + area are the size of a GPR, thus the linkage area is 24 bytes long in 32 bit + mode and 48 bytes in 64 bit mode.
+ +32 bit linkage area
+ +| 0 | +Saved SP (r1) | +
| 4 | +Saved CR | +
| 8 | +Saved LR | +
| 12 | +Reserved | +
| 16 | +Reserved | +
| 20 | +Saved FP (r31) | +
64 bit linkage area
+ +| 0 | +Saved SP (r1) | +
| 8 | +Saved CR | +
| 16 | +Saved LR | +
| 24 | +Reserved | +
| 32 | +Reserved | +
| 40 | +Saved FP (r31) | +
The parameter area is used to store arguments being passed to a callee + function. Following the PowerPC ABI, the first few arguments are actually + passed in registers, with the space in the parameter area unused. However, + if there are not enough registers or the callee is a thunk or vararg + function, these register arguments can be spilled into the parameter area. + Thus, the parameter area must be large enough to store all the parameters for + the largest call sequence made by the caller. The size must also be + minimally large enough to spill registers r3-r10. This allows callees blind + to the call signature, such as thunks and vararg functions, enough space to + cache the argument registers. Therefore, the parameter area is minimally 32 + bytes (64 bytes in 64 bit mode.) Also note that since the parameter area is + a fixed offset from the top of the frame, that a callee can access its spilt + arguments using fixed offsets from the stack pointer (or base pointer.)
+ +Combining the information about the linkage, parameter areas and alignment. A + stack frame is minimally 64 bytes in 32 bit mode and 128 bytes in 64 bit + mode.
+ +The dynamic area starts out as size zero. If a function uses dynamic + alloca then space is added to the stack, the linkage and parameter areas are + shifted to top of stack, and the new space is available immediately below the + linkage and parameter areas. The cost of shifting the linkage and parameter + areas is minor since only the link value needs to be copied. The link value + can be easily fetched by adding the original frame size to the base pointer. + Note that allocations in the dynamic space need to observe 16 byte + alignment.
+ +The locals area is where the llvm compiler reserves space for local + variables.
+ +The saved registers area is where the llvm compiler spills callee + saved registers on entry to the callee.
+ +The llvm prolog and epilog are the same as described in the PowerPC ABI, with + the following exceptions. Callee saved registers are spilled after the frame + is created. This allows the llvm epilog/prolog support to be common with + other targets. The base pointer callee saved register r31 is saved in the + TOC slot of linkage area. This simplifies allocation of space for the base + pointer and makes it convenient to locate programatically and during + debugging.
+ +TODO - More to come.
+ +
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- The LLVM Compiler Infrastructure
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