This document serves as a high level summary of the optimization features that LLVM provides. Optimizations are implemented as Passes that traverse some portion of a program to either collect information or transform the program. - THe table below divides the passes that LLVM provides into three categories. + The table below divides the passes that LLVM provides into three categories. Analysis passes compute information that other passes can use or for debugging or program visualization purposes. Transform passes can use (or invalidate) the analysis passes. Transform passes all mutate the program in some way. - Utility passes provides ome utility but don't otherwise fit categorization. - For example passes to extract functions to bytecode or write a module to - bytecode are neither analysis nor transform passes. + Utility passes provides some utility but don't otherwise fit categorization. + For example passes to extract functions to bitcode or write a module to + bitcode are neither analysis nor transform passes.
The table below provides a quick summary of each pass and links to the more complete pass description later in the document.
| ANALYSIS PASSES | ||
|---|---|---|
| Option | Name | Directory |
| ANALYSIS PASSES | ||
| Option | Name | |
| -aa-eval | Exhaustive Alias Analysis Precision Evaluator | |
| -anders-aa | Andersen's Interprocedural Alias Analysis | |
| -basicaa | Basic Alias Analysis (default AA impl) | |
| -callgraph | Print a call graph | |
| -callscc | Print SCCs of the Call Graph | |
| -cfgscc | Print SCCs of each function CFG | |
| -codegenprepare | Optimize for code generation | |
| -count-aa | Count Alias Analysis Query Responses | |
| -debug-aa | AA use debugger | |
| -domfrontier | Dominance Frontier Construction | |
| -domset | Dominator Set Construction | |
| -domtree | Dominator Tree Construction | |
| -etforest | ET Forest Construction | |
| -externalfnconstants | Print external fn callsites passed constants | |
| -globalsmodref-aa | Simple mod/ref analysis for globals | |
| -idom | Immediate Dominators Construction | |
| -instcount | Counts the various types of Instructions | |
| -intervals | Interval Partition Construction | |
| -load-vn | Load Value Numbering | |
| -loops | Natural Loop Construction | |
| -memdep | Memory Dependence Analysis | |
| -no-aa | No Alias Analysis (always returns 'may' alias) | |
| -no-profile | No Profile Information | |
| -postdomfrontier | Post-Dominance Frontier Construction | |
| -postdomset | Post-Dominator Set Construction | |
| -postdomtree | Post-Dominator Tree Construction | |
| -postetforest | Post-ET-Forest Construction | |
| -postidom | Immediate Post-Dominators Construction | |
| Print function to stderr | ||
| -print-alias-sets | Alias Set Printer | |
| -print-callgraph | Print Call Graph to 'dot' file | |
| -targetdata | Target Data Layout | |
| TRANSFORM PASSES | ||
| Option | Name | Directory |
| TRANSFORM PASSES | ||
| Option | Name | |
| -adce | Aggressive Dead Code Elimination | |
| -argpromotion | Promote 'by reference' arguments to scalars | |
| -block-placement | Profile Guided Basic Block Placement | |
| -break-crit-edges | Break Critical Edges in CFG | |
| -cee | Correlated Expression Elimination | |
| -break-crit-edges | Break critical edges in CFG | |
| -codegenprepare | Prepare a function for code generation | |
| -condprop | Conditional Propagation | |
| -constmerge | Merge Duplicate Global Constants | |
| -constprop | Simple constant propagation | |
| -gcse | Global Common Subexpression Elimination | |
| -globaldce | Dead Global Elimination | |
| -globalopt | Global Variable Optimizer | |
| -gvn | Global Value Numbering | |
| -gvnpre | Global Value Numbering/Partial Redundancy Elimination | |
| -indmemrem | Indirect Malloc and Free Removal | |
| -indvars | Canonicalize Induction Variables | |
| -inline | Function Integration/Inlining | |
| -internalize | Internalize Global Symbols | |
| -ipconstprop | Interprocedural constant propagation | |
| -ipsccp | Interprocedural Sparse Conditional Constant Propagation | |
| -jump-threading | Thread control through conditional blocks | |
| -lcssa | Loop-Closed SSA Form Pass | |
| -licm | Loop Invariant Code Motion | |
| -loop-deletion | Dead Loop Deletion Pass | |
| -loop-extract | Extract loops into new functions | |
| -loop-extract-single | Extract at most one loop into a new function | |
| -loop-index-split | Index Split Loops | |
| -loop-reduce | Loop Strength Reduction | |
| -loop-unroll | Unroll Loops | |
| -loop-unswitch | Unswitch Loops | |
| -loopsimplify | Canonicalize Natural Loops | |
| -lower-packed | Lower Packed Operations | |
| -loop-rotate | Rotate Loops | |
| -loop-unroll | Unroll loops | |
| -loop-unswitch | Unswitch loops | |
| -loopsimplify | Canonicalize natural loops | |
| -lowerallocs | Lower allocations from instructions to calls | |
| -lowergc | Lower GC intrinsics, for GCless code generators | |
| -lowerinvoke | Lower Invoke and Unwind | |
| -lowerselect | Lower Selects To Branches | |
| -lowerinvoke | Lower invoke and unwind, for unwindless code generators | |
| -lowersetjmp | Lower Set Jump | |
| -lowerswitch | Lower SwitchInst's to branches | |
| -mem2reg | Promote Memory to Register | |
| -mergereturn | Unify Function Exit Nodes | |
| -memcpyopt | Optimize use of memcpy and friends | |
| -mergereturn | Unify function exit nodes | |
| -predsimplify | Predicate Simplifier | |
| -prune-eh | Remove unused exception handling info | |
| -raiseallocs | Raise allocations from calls to instructions | |
| -reassociate | Reassociate Expressions | |
| -reg2mem | Demote Values to Memory | |
| -reassociate | Reassociate expressions | |
| -reg2mem | Demote all values to stack slots | |
| -scalarrepl | Scalar Replacement of Aggregates | |
| -sccp | Sparse Conditional Constant Propagation | |
| -simplify-libcalls | Simplify well-known library calls | |
| -simplifycfg | Simplify the CFG | |
| -strip | Strip all symbols from a module | |
| -strip-dead-prototypes | Remove unused function declarations | |
| -sretpromotion | Promote sret arguments | |
| -tailcallelim | Tail Call Elimination | |
| -tailduplicate | Tail Duplication | |
| UTILITY PASSES | ||
| Option | Name | Directory |
| -deadarghaX0r | Dead Argument Hacking (BUGPOINT ONLY) | |
| -extract-blocks | Extract Basic Blocks From Module (BUGPOINT ONLY) | |
| -emitbytecode | Bytecode Writer | |
| UTILITY PASSES | ||
| Option | Name | |
| -deadarghaX0r | Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE) | |
| -extract-blocks | Extract Basic Blocks From Module (for bugpoint use) | |
| -preverify | Preliminary module verification | |
| -verify | Module Verifier | |
| -view-cfg | View CFG of function | |
| -view-cfg-only | View CFG of function (with no function bodies) | |
Yet to be written.
+This is a simple N^2 alias analysis accuracy evaluator. + Basically, for each function in the program, it simply queries to see how the + alias analysis implementation answers alias queries between each pair of + pointers in the function.
+ +This is inspired and adapted from code by: Naveen Neelakantam, Francesco + Spadini, and Wojciech Stryjewski.
Yet to be written.
++ This is an implementation of Andersen's interprocedural alias + analysis +
+ ++ In pointer analysis terms, this is a subset-based, flow-insensitive, + field-sensitive, and context-insensitive algorithm pointer algorithm. +
+ ++ This algorithm is implemented as three stages: +
+ ++ The object identification stage identifies all of the memory objects in the + program, which includes globals, heap allocated objects, and stack allocated + objects. +
+ +
+ The inclusion constraint identification stage finds all inclusion constraints
+ in the program by scanning the program, looking for pointer assignments and
+ other statements that effect the points-to graph. For a statement like
+ A = B, this statement is processed to
+ indicate that A can point to anything that B can point
+ to. Constraints can handle copies, loads, and stores, and address taking.
+
+ The offline constraint graph optimization portion includes offline variable + substitution algorithms intended to computer pointer and location + equivalences. Pointer equivalences are those pointers that will have the + same points-to sets, and location equivalences are those variables that + always appear together in points-to sets. +
+ ++ The inclusion constraint solving phase iteratively propagates the inclusion + constraints until a fixed point is reached. This is an O(n³) + algorithm. +
+ +
+ Function constraints are handled as if they were structs with X
+ fields. Thus, an access to argument X of function Y is
+ an access to node index getNode(Y) + X.
+ This representation allows handling of indirect calls without any issues. To
+ wit, an indirect call Y(a,b) is
+ equivalent to *(Y + 1) = a, *(Y + 2) =
+ b. The return node for a function F is always
+ located at getNode(F) + CallReturnPos. The arguments
+ start at getNode(F) + CallArgPos.
+
Yet to be written.
++ This is the default implementation of the Alias Analysis interface + that simply implements a few identities (two different globals cannot alias, + etc), but otherwise does no analysis. +
Yet to be written.
@@ -188,18 +298,32 @@Yet to be written.
+
+ This is the default implementation of the ValueNumbering
+ interface. It walks the SSA def-use chains to trivially identify
+ lexically identical expressions. This does not require any ahead of time
+ analysis, so it is a very fast default implementation.
+
+ The ValueNumbering analysis passes are mostly deprecated. They are only used + by the Global Common Subexpression Elimination pass, which + is deprecated by the Global Value Numbering pass (which + does its value numbering on its own). +
Yet to be written.
+
+ This pass, only available in opt, prints the call graph to
+ standard output in a human-readable form.
+
Yet to be written.
+
+ This pass, only available in opt, prints the SCCs of the call
+ graph to standard output in a human-readable form.
+
Yet to be written.
+
+ This pass, only available in opt, prints the SCCs of each
+ function CFG to standard output in a human-readable form.
+
Yet to be written.
++ This pass munges the code in the input function to better prepare it for + SelectionDAG-based code generation. This works around limitations in it's + basic-block-at-a-time approach. It should eventually be removed. +
Yet to be written.
++ A pass which can be used to count how many alias queries + are being made and how the alias analysis implementation being used responds. +
Yet to be written.
++ This simple pass checks alias analysis users to ensure that if they + create a new value, they do not query AA without informing it of the value. + It acts as a shim over any other AA pass you want. +
+ ++ Yes keeping track of every value in the program is expensive, but this is + a debugging pass. +
Yet to be written.
++ This pass is a simple dominator construction algorithm for finding forward + dominator frontiers. +
Yet to be written.
-Yet to be written.
-Yet to be written.
++ This pass is a simple dominator construction algorithm for finding forward + dominators. +
Yet to be written.
+
+ This pass, only available in opt, prints out call sites to
+ external functions that are called with constant arguments. This can be
+ useful when looking for standard library functions we should constant fold
+ or handle in alias analyses.
+
Yet to be written.
-Yet to be written.
++ This simple pass provides alias and mod/ref information for global values + that do not have their address taken, and keeps track of whether functions + read or write memory (are "pure"). For this simple (but very common) case, + we can provide pretty accurate and useful information. +
Yet to be written.
++ This pass collects the count of all instructions and reports them +
Yet to be written.
++ This analysis calculates and represents the interval partition of a function, + or a preexisting interval partition. +
+ ++ In this way, the interval partition may be used to reduce a flow graph down + to its degenerate single node interval partition (unless it is irreducible). +
Yet to be written.
++ This pass value numbers load and call instructions. To do this, it finds + lexically identical load instructions, and uses alias analysis to determine + which loads are guaranteed to produce the same value. To value number call + instructions, it looks for calls to functions that do not write to memory + which do not have intervening instructions that clobber the memory that is + read from. +
+ ++ This pass builds off of another value numbering pass to implement value + numbering for non-load and non-call instructions. It uses Alias Analysis so + that it can disambiguate the load instructions. The more powerful these base + analyses are, the more powerful the resultant value numbering will be. +
Yet to be written.
++ This analysis is used to identify natural loops and determine the loop depth + of various nodes of the CFG. Note that the loops identified may actually be + several natural loops that share the same header node... not just a single + natural loop. +
Yet to be written.
++ An analysis that determines, for a given memory operation, what preceding + memory operations it depends on. It builds on alias analysis information, and + tries to provide a lazy, caching interface to a common kind of alias + information query. +
Yet to be written.
++ Always returns "I don't know" for alias queries. NoAA is unlike other alias + analysis implementations, in that it does not chain to a previous analysis. As + such it doesn't follow many of the rules that other alias analyses must. +
Yet to be written.
+
+ The default "no profile" implementation of the abstract
+ ProfileInfo interface.
+
Yet to be written.
++ This pass is a simple post-dominator construction algorithm for finding + post-dominator frontiers. +
Yet to be written.
-Yet to be written.
-Yet to be written.
++ This pass is a simple post-dominator construction algorithm for finding + post-dominators. +
Yet to be written.
+
+ The PrintFunctionPass class is designed to be pipelined with
+ other FunctionPasses, and prints out the functions of the module
+ as they are processed.
+
Yet to be written.
+
+ This pass, only available in opt, prints the call graph into a
+ .dot graph. This graph can then be processed with the "dot" tool
+ to convert it to postscript or some other suitable format.
+
Yet to be written.
+
+ This pass, only available in opt, prints the control flow graph
+ into a .dot graph. This graph can then be processed with the
+ "dot" tool to convert it to postscript or some other suitable format.
+
Yet to be written.
+
+ This pass, only available in opt, prints the control flow graph
+ into a .dot graph, omitting the function bodies. This graph can
+ then be processed with the "dot" tool to convert it to postscript or some
+ other suitable format.
+
Yet to be written.
++ This pass simply prints out the entire module when it is executed. +
Yet to be written.
++ This pass is used to seek out all of the types in use by the program. Note + that this analysis explicitly does not include types only used by the symbol + table.
Yet to be written.
++ A concrete implementation of profiling information that loads the information + from a profile dump file. +
Yet to be written.
+
+ The ScalarEvolution analysis can be used to analyze and
+ catagorize scalar expressions in loops. It specializes in recognizing general
+ induction variables, representing them with the abstract and opaque
+ SCEV class. Given this analysis, trip counts of loops and other
+ important properties can be obtained.
+
+ This analysis is primarily useful for induction variable substitution and + strength reduction. +
Yet to be written.
+Provides other passes access to information on how the size and alignment + required by the the target ABI for various data types.
Yet to be written.
++ This pass promotes "by reference" arguments to be "by value" arguments. In + practice, this means looking for internal functions that have pointer + arguments. If it can prove, through the use of alias analysis, that an + argument is *only* loaded, then it can pass the value into the function + instead of the address of the value. This can cause recursive simplification + of code and lead to the elimination of allocas (especially in C++ template + code like the STL). +
+ ++ This pass also handles aggregate arguments that are passed into a function, + scalarizing them if the elements of the aggregate are only loaded. Note that + it refuses to scalarize aggregates which would require passing in more than + three operands to the function, because passing thousands of operands for a + large array or structure is unprofitable! +
+ ++ Note that this transformation could also be done for arguments that are only + stored to (returning the value instead), but does not currently. This case + would be best handled when and if LLVM starts supporting multiple return + values from functions. +
This pass implements a very simple profile guided basic block placement - algorithm. The idea is to put frequently executed blocks together at the - start of the function, and hopefully increase the number of fall-through - conditional branches. If there is no profile information for a particular - function, this pass basically orders blocks in depth-first order.
-The algorithm implemented here is basically "Algo1" from "Profile Guided - Code Positioning" by Pettis and Hansen, except that it uses basic block - counts instead of edge counts. This could be improved in many ways, but is - very simple for now.
-Basically we "place" the entry block, then loop over all successors in a - DFO, placing the most frequently executed successor until we run out of - blocks. Did we mention that this was extremely simplistic? This is - also much slower than it could be. When it becomes important, this pass - will be rewritten to use a better algorithm, and then we can worry about - efficiency.
+This pass is a very simple profile guided basic block placement algorithm. + The idea is to put frequently executed blocks together at the start of the + function and hopefully increase the number of fall-through conditional + branches. If there is no profile information for a particular function, this + pass basically orders blocks in depth-first order.
Yet to be written.
++ Break all of the critical edges in the CFG by inserting a dummy basic block. + It may be "required" by passes that cannot deal with critical edges. This + transformation obviously invalidates the CFG, but can update forward dominator + (set, immediate dominators, tree, and frontier) information. +
Correlated Expression Elimination propagates information from conditional - branches to blocks dominated by destinations of the branch. It propagates - information from the condition check itself into the body of the branch, - allowing transformations like these for example: -
- if (i == 7) - ... 4*i; // constant propagation - - M = i+1; N = j+1; - if (i == j) - X = M-N; // = M-M == 0; -- -
This is called Correlated Expression Elimination because we eliminate or - simplify expressions that are correlated with the direction of a branch. In - this way we use static information to give us some information about the - dynamic value of a variable.
+ This pass munges the code in the input function to better prepare it for + SelectionDAG-based code generation. This works around limitations in it's + basic-block-at-a-time approach. It should eventually be removed.Yet to be written.
++ Merges duplicate global constants together into a single constant that is + shared. This is useful because some passes (ie TraceValues) insert a lot of + string constants into the program, regardless of whether or not an existing + string is available. +
This file implements constant propagation and merging. It looks for instructions involving only constant operands and replaces them with a - constant value instead of an instruction. For example: -
add i32 1, 2
i32 3+ constant value instead of an instruction. For example: +
+add i32 1, 2
becomes
+i32 3
NOTE: this pass has a habit of making definitions be dead. It is a good idea to to run a DIE (Dead Instruction Elimination) pass sometime after running this pass.
@@ -583,7 +798,11 @@ Dead Code EliminationYet to be written.
++ Dead code elimination is similar to dead instruction + elimination, but it rechecks instructions that were used by removed + instructions to see if they are newly dead. +
Yet to be written.
++ This pass deletes dead arguments from internal functions. Dead argument + elimination removes arguments which are directly dead, as well as arguments + only passed into function calls as dead arguments of other functions. This + pass also deletes dead arguments in a similar way. +
+ ++ This pass is often useful as a cleanup pass to run after aggressive + interprocedural passes, which add possibly-dead arguments. +
Yet to be written.
++ This pass is used to cleanup the output of GCC. It eliminate names for types + that are unused in the entire translation unit, using the find used types pass. +
Yet to be written.
++ Dead instruction elimination performs a single pass over the function, + removing instructions that are obviously dead. +
Yet to be written.
++ A trivial dead store elimination that only considers basic-block local + redundant stores. +
Yet to be written.
++ This pass is designed to be a very quick global transformation that + eliminates global common subexpressions from a function. It does this by + using an existing value numbering analysis pass to identify the common + subexpressions, eliminating them when possible. +
++ This pass is deprecated by the Global Value Numbering pass + (which does a better job with its own value numbering). +
Yet to be written.
++ This transform is designed to eliminate unreachable internal globals from the + program. It uses an aggressive algorithm, searching out globals that are + known to be alive. After it finds all of the globals which are needed, it + deletes whatever is left over. This allows it to delete recursive chunks of + the program which are unreachable. +
Yet to be written.
++ This pass transforms simple global variables that never have their address + taken. If obviously true, it marks read/write globals as constant, deletes + variables only stored to, etc. +
++ This pass performs global value numbering to eliminate fully redundant + instructions. It also performs simple dead load elimination. +
++ Note that this pass does the value numbering itself, it does not use the + ValueNumbering analysis passes. +
++ This pass performs a hybrid of global value numbering and partial redundancy + elimination, known as GVN-PRE. It performs partial redundancy elimination on + values, rather than lexical expressions, allowing a more comprehensive view + the optimization. It replaces redundant values with uses of earlier + occurences of the same value. While this is beneficial in that it eliminates + unneeded computation, it also increases register pressure by creating large + live ranges, and should be used with caution on platforms that are very + sensitive to register pressure. +
++ Note that this pass does the value numbering itself, it does not use the + ValueNumbering analysis passes. +
Yet to be written.
++ This pass finds places where memory allocation functions may escape into + indirect land. Some transforms are much easier (aka possible) only if free + or malloc are not called indirectly. +
+ ++ Thus find places where the address of memory functions are taken and construct + bounce functions with direct calls of those functions. +
Yet to be written.
++ This transformation analyzes and transforms the induction variables (and + computations derived from them) into simpler forms suitable for subsequent + analysis and transformation. +
+ ++ This transformation makes the following changes to each loop with an + identifiable induction variable: +
+ ++ If the trip count of a loop is computable, this pass also makes the following + changes: +
+ ++ into +for (i = 7; i*i < 1000; ++i)
for (i = 0; i != 25; ++i)
+ This transformation should be followed by strength reduction after all of the + desired loop transformations have been performed. Additionally, on targets + where it is profitable, the loop could be transformed to count down to zero + (the "do loop" optimization). +
Yet to be written.
++ Bottom-up inlining of functions into callees. +
Yet to be written.
++ This pass instruments the specified program with counters for basic block + profiling, which counts the number of times each basic block executes. This + is the most basic form of profiling, which can tell which blocks are hot, but + cannot reliably detect hot paths through the CFG. +
+ ++ Note that this implementation is very naïve. Control equivalent regions of + the CFG should not require duplicate counters, but it does put duplicate + counters in. +
Yet to be written.
++ This pass instruments the specified program with counters for edge profiling. + Edge profiling can give a reasonable approximation of the hot paths through a + program, and is used for a wide variety of program transformations. +
+ ++ Note that this implementation is very naïve. It inserts a counter for + every edge in the program, instead of using control flow information + to prune the number of counters inserted. +
Yet to be written.
++ This pass instruments the specified program with counters for function + profiling, which counts the number of times each function is called. +
Yet to be written.
+
+ The basic profiler that does nothing. It is the default profiler and thus
+ terminates RSProfiler chains. It is useful for measuring
+ framework overhead.
+
Yet to be written.
++ The second stage of the random-sampling instrumentation framework, duplicates + all instructions in a function, ignoring the profiling code, then connects the + two versions together at the entry and at backedges. At each connection point + a choice is made as to whether to jump to the profiled code (take a sample) or + execute the unprofiled code. +
+ ++ After this pass, it is highly recommended to runmem2reg + and adce. instcombine, + load-vn, gdce, and + dse also are good to run afterwards. +
Yet to be written.
++ Combine instructions to form fewer, simple + instructions. This pass does not modify the CFG This pass is where algebraic + simplification happens. +
+ ++ This pass combines things like: +
+ ++ +%Y = add i32 %X, 1 +%Z = add i32 %Y, 1
+ into: +
+ ++ +%Z = add i32 %X, 2
+ This is a simple worklist driven algorithm. +
+ ++ This pass guarantees that the following canonicalizations are performed on + the program: +
+ +ors, then
+ ands, then xors.<,
+ >, â¤, or ⥠to
+ = or â if possible.cmp instructions on boolean values are replaced with
+ logical operations.add X, X is represented as
+ mul X, 2 â shl X, 1Yet to be written.
++ This pass loops over all of the functions in the input module, looking for a + main function. If a main function is found, all other functions and all + global variables with initializers are marked as internal. +
Yet to be written.
++ This pass implements an extremely simple interprocedural constant + propagation pass. It could certainly be improved in many different ways, + like using a worklist. This pass makes arguments dead, but does not remove + them. The existing dead argument elimination pass should be run after this + to clean up the mess. +
Yet to be written.
++ An interprocedural variant of Sparse Conditional Constant + Propagation. +
Yet to be written.
++ Jump threading tries to find distinct threads of control flow running through + a basic block. This pass looks at blocks that have multiple predecessors and + multiple successors. If one or more of the predecessors of the block can be + proven to always cause a jump to one of the successors, we forward the edge + from the predecessor to the successor by duplicating the contents of this + block. +
++ An example of when this can occur is code like this: +
+ +if () { ...
+ X = 4;
+}
+if (X < 3) {
+
+ + In this case, the unconditional branch at the end of the first if can be + revectored to the false side of the second if. +
Yet to be written.
++ This pass transforms loops by placing phi nodes at the end of the loops for + all values that are live across the loop boundary. For example, it turns + the left into the right code: +
+ +for (...) for (...) + if (c) if (c) + X1 = ... X1 = ... + else else + X2 = ... X2 = ... + X3 = phi(X1, X2) X3 = phi(X1, X2) +... = X3 + 4 X4 = phi(X3) + ... = X4 + 4+ +
+ This is still valid LLVM; the extra phi nodes are purely redundant, and will
+ be trivially eliminated by InstCombine. The major benefit of
+ this transformation is that it makes many other loop optimizations, such as
+ LoopUnswitching, simpler.
+
Yet to be written.
++ This pass performs loop invariant code motion, attempting to remove as much + code from the body of a loop as possible. It does this by either hoisting + code into the preheader block, or by sinking code to the exit blocks if it is + safe. This pass also promotes must-aliased memory locations in the loop to + live in registers, thus hoisting and sinking "invariant" loads and stores. +
+ ++ This pass uses alias analysis for two purposes: +
+ ++ This file implements the Dead Loop Deletion Pass. This pass is responsible + for eliminating loops with non-infinite computable trip counts that have no + side effects or volatile instructions, and do not contribute to the + computation of the function's return value. +
Yet to be written.
+
+ A pass wrapper around the ExtractLoop() scalar transformation to
+ extract each top-level loop into its own new function. If the loop is the
+ only loop in a given function, it is not touched. This is a pass most
+ useful for debugging via bugpoint.
+
Yet to be written.
++ Similar to Extract loops into new functions, + this pass extracts one natural loop from the program into a function if it + can. This is used by bugpoint. +
Yet to be written.
++ This pass divides loop's iteration range by spliting loop such that each + individual loop is executed efficiently. +
Yet to be written.
++ This pass performs a strength reduction on array references inside loops that + have as one or more of their components the loop induction variable. This is + accomplished by creating a new value to hold the initial value of the array + access for the first iteration, and then creating a new GEP instruction in + the loop to increment the value by the appropriate amount. +
Yet to be written.
+A simple loop rotation transformation.
Yet to be written.
++ This pass implements a simple loop unroller. It works best when loops have + been canonicalized by the -indvars pass, + allowing it to determine the trip counts of loops easily. +
Yet to be written.
++ This pass transforms loops that contain branches on loop-invariant conditions + to have multiple loops. For example, it turns the left into the right code: +
+ +for (...) if (lic) + A for (...) + if (lic) A; B; C + B else + C for (...) + A; C+ +
+ This can increase the size of the code exponentially (doubling it every time + a loop is unswitched) so we only unswitch if the resultant code will be + smaller than a threshold. +
+ ++ This pass expects LICM to be run before it to hoist invariant conditions out + of the loop, to make the unswitching opportunity obvious. +
Yet to be written.
++ This pass performs several transformations to transform natural loops into a + simpler form, which makes subsequent analyses and transformations simpler and + more effective. +
+ ++ Loop pre-header insertion guarantees that there is a single, non-critical + entry edge from outside of the loop to the loop header. This simplifies a + number of analyses and transformations, such as LICM. +
+ ++ Loop exit-block insertion guarantees that all exit blocks from the loop + (blocks which are outside of the loop that have predecessors inside of the + loop) only have predecessors from inside of the loop (and are thus dominated + by the loop header). This simplifies transformations such as store-sinking + that are built into LICM. +
+ ++ This pass also guarantees that loops will have exactly one backedge. +
+ ++ Note that the simplifycfg pass will clean up blocks which are split out but + end up being unnecessary, so usage of this pass should not pessimize + generated code. +
+ ++ This pass obviously modifies the CFG, but updates loop information and + dominator information. +
Yet to be written.
++ Turn malloc and free instructions into @malloc and + @free calls. +
+ ++ This is a target-dependent tranformation because it depends on the size of + data types and alignment constraints. +
++ This transformation is designed for use by code generators which do not yet + support stack unwinding. This pass supports two models of exception handling + lowering, the 'cheap' support and the 'expensive' support. +
+ ++ 'Cheap' exception handling support gives the program the ability to execute + any program which does not "throw an exception", by turning 'invoke' + instructions into calls and by turning 'unwind' instructions into calls to + abort(). If the program does dynamically use the unwind instruction, the + program will print a message then abort. +
+ ++ 'Expensive' exception handling support gives the full exception handling + support to the program at the cost of making the 'invoke' instruction + really expensive. It basically inserts setjmp/longjmp calls to emulate the + exception handling as necessary. +
+ ++ Because the 'expensive' support slows down programs a lot, and EH is only + used for a subset of the programs, it must be specifically enabled by the + -enable-correct-eh-support option. +
+ ++ Note that after this pass runs the CFG is not entirely accurate (exceptional + control flow edges are not correct anymore) so only very simple things should + be done after the lowerinvoke pass has run (like generation of native code). + This should not be used as a general purpose "my LLVM-to-LLVM pass doesn't + support the invoke instruction yet" lowering pass. +
Yet to be written.
++ Lowers setjmp and longjmp to use the LLVM invoke and unwind + instructions as necessary. +
+ ++ Lowering of longjmp is fairly trivial. We replace the call with a + call to the LLVM library function __llvm_sjljeh_throw_longjmp(). + This unwinds the stack for us calling all of the destructors for + objects allocated on the stack. +
+ ++ At a setjmp call, the basic block is split and the setjmp + removed. The calls in a function that have a setjmp are converted to + invoke where the except part checks to see if it's a longjmp + exception and, if so, if it's handled in the function. If it is, then it gets + the value returned by the longjmp and goes to where the basic block + was split. invoke instructions are handled in a similar fashion with + the original except block being executed if it isn't a longjmp + except that is handled by that function. +
Yet to be written.
++ Rewrites switch instructions with a sequence of branches, which + allows targets to get away with not implementing the switch instruction until + it is convenient. +
Yet to be written.
++ This file promotes memory references to be register references. It promotes + alloca instructions which only have loads and + stores as uses. An alloca is transformed by using dominator + frontiers to place phi nodes, then traversing the function in + depth-first order to rewrite loads and stores as + appropriate. This is just the standard SSA construction algorithm to construct + "pruned" SSA form. +
Yet to be written.
++ This pass performs various transformations related to eliminating memcpy + calls, or transforming sets of stores into memset's. +
Yet to be written.
++ Ensure that functions have at most one ret instruction in them. + Additionally, it keeps track of which node is the new exit node of the CFG. +
Yet to be written.
++ Path-sensitive optimizer. In a branch where x == y, replace uses of + x with y. Permits further optimization, such as the + elimination of the unreachable call: +
+ +void test(int *p, int *q)
+{
+ if (p != q)
+ return;
+
+ if (*p != *q)
+ foo(); // unreachable
+}Yet to be written.
++ This file implements a simple interprocedural pass which walks the call-graph, + turning invoke instructions into call instructions if and + only if the callee cannot throw an exception. It implements this as a + bottom-up traversal of the call-graph. +
Yet to be written.
++ Converts @malloc and @free calls to malloc and + free instructions. +
Yet to be written.
++ This pass reassociates commutative expressions in an order that is designed + to promote better constant propagation, GCSE, LICM, PRE, etc. +
+ ++ For example: 4 + (x + 5) â x + (4 + 5) +
+ ++ In the implementation of this algorithm, constants are assigned rank = 0, + function arguments are rank = 1, and other values are assigned ranks + corresponding to the reverse post order traversal of current function + (starting at 2), which effectively gives values in deep loops higher rank + than values not in loops. +
Yet to be written.
++ This file demotes all registers to memory references. It is intented to be + the inverse of -mem2reg. By converting to + load instructions, the only values live accross basic blocks are + alloca instructions and load instructions before + phi nodes. It is intended that this should make CFG hacking much + easier. To make later hacking easier, the entry block is split into two, such + that all introduced alloca instructions (and nothing else) are in the + entry block. +
Yet to be written.
++ The well-known scalar replacement of aggregates transformation. This + transform breaks up alloca instructions of aggregate type (structure + or array) into individual alloca instructions for each member if + possible. Then, if possible, it transforms the individual alloca + instructions into nice clean scalar SSA form. +
+ ++ This combines a simple scalar replacement of aggregates algorithm with the mem2reg algorithm because often interact, + especially for C++ programs. As such, iterating between scalarrepl, + then mem2reg until we run out of things to + promote works well. +
Yet to be written.
++ Sparse conditional constant propagation and merging, which can be summarized + as: +
+ ++ Note that this pass has a habit of making definitions be dead. It is a good + idea to to run a DCE pass sometime after running this pass. +
Yet to be written.
++ Applies a variety of small optimizations for calls to specific well-known + function calls (e.g. runtime library functions). For example, a call + exit(3) that occurs within the main() function can be + transformed into simply return 3. +
Yet to be written.
++ Performs dead code elimination and basic block merging. Specifically: +
+ +Yet to be written.
++ Performs code stripping. This transformation can delete: +
+ ++ Note that this transformation makes code much less readable, so it should + only be used in situations where the strip utility would be used, + such as reducing code size or making it harder to reverse engineer code. +
++ This pass loops over all of the functions in the input module, looking for + dead declarations and removes them. Dead declarations are declarations of + functions for which no implementation is available (i.e., declarations for + unused library functions). +
++ This pass finds functions that return a struct (using a pointer to the struct + as the first argument of the function, marked with the 'sret' attribute) and + replaces them with a new function that simply returns each of the elements of + that struct (using multiple return values). +
+ ++ This pass works under a number of conditions: +
+ +Yet to be written.
++ This file transforms calls of the current function (self recursion) followed + by a return instruction with a branch to the entry of the function, creating + a loop. This pass also implements the following extensions to the basic + algorithm: +
+ +Yet to be written.
++ This pass performs a limited form of tail duplication, intended to simplify + CFGs by removing some unconditional branches. This pass is necessary to + straighten out loops created by the C front-end, but also is capable of + making other code nicer. After this pass is run, the CFG simplify pass + should be run to clean up the mess. +
Yet to be written.
++ Same as dead argument elimination, but deletes arguments to functions which + are external. This is only for use by bugpoint.
Yet to be written.
++ This pass is used by bugpoint to extract all blocks from the module into their + own functions.
Yet to be written.
++ Ensures that the module is in the form required by the Module Verifier pass. +
+ ++ Running the verifier runs this pass automatically, so there should be no need + to use it directly. +
Yet to be written.
++ Verifies an LLVM IR code. This is useful to run after an optimization which is + undergoing testing. Note that llvm-as verifies its input before + emitting bitcode, and also that malformed bitcode is likely to make LLVM + crash. All language front-ends are therefore encouraged to verify their output + before performing optimizing transformations. +
+ ++ Note that this does not provide full security verification (like Java), but + instead just tries to ensure that code is well-formed. +
++ Displays the control flow graph using the GraphViz tool. +
++ Displays the control flow graph using the GraphViz tool, but omitting function + bodies. +