The LLVM Pass Framework is an important part of the LLVM system, because LLVM passes are where most of the interesting parts of the compiler exist. Passes @@ -113,12 +135,14 @@ build the analysis results that are used by these transformations, and they are, above all, a structuring technique for compiler code.
All LLVM passes are subclasses of the Pass +href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass class, which implement functionality by overriding virtual methods inherited from Pass. Depending on how your pass works, you should inherit from the ModulePass, CallGraphSCCPass, FunctionPass, or LoopPass, or RegionPass, or BasicBlockPass classes, which gives the system more information about what your pass does, and how it can be combined with other passes. One of the main features of the LLVM Pass Framework is that it @@ -132,12 +156,12 @@ more advanced features are discussed.
Here we describe how to write the "hello world" of passes. The "Hello" pass is designed to simply print out the name of non-external functions that exist in @@ -145,45 +169,48 @@ the program being compiled. It does not modify the program at all, it just inspects it. The source code and files for this pass are available in the LLVM source tree in the lib/Transforms/Hello directory.
-First thing you need to do is create a new directory somewhere in the LLVM -source base. For this example, we'll assume that you made -"lib/Transforms/Hello". The first thing you must do is set up a build -script (Makefile) that will compile the source code for the new pass. To do -this, copy this into "Makefile":
++
First, configure and build LLVM. This needs to be done directly inside the + LLVM source tree rather than in a separate objects directory. + Next, you need to create a new directory somewhere in the LLVM source + base. For this example, we'll assume that you made + lib/Transforms/Hello. Finally, you must set up a build script + (Makefile) that will compile the source code for the new pass. To do this, + copy the following into Makefile:
+-
+# Makefile for hello pass -# Path to top level of LLVM heirarchy +# Path to top level of LLVM hierarchy LEVEL = ../../.. # Name of the library to build -LIBRARYNAME = hello +LIBRARYNAME = Hello -# Build a dynamically loadable shared object -SHARED_LIBRARY = 1 +# Make the shared library become a loadable module so the tools can +# dlopen/dlsym on the resulting library. +LOADABLE_MODULE = 1 # Include the makefile implementation stuff include $(LEVEL)/Makefile.common -+This makefile specifies that all of the .cpp files in the current -directory are to be compiled and linked together into a -lib/Debug/libhello.so shared object that can be dynamically loaded by -the opt or analyze tools. If your operating system uses a -suffix other than .so (such as windows or Mac OS/X), the appropriate extension -will be used.
+directory are to be compiled and linked together into a shared object +$(LEVEL)/Debug+Asserts/lib/Hello.so that can be dynamically loaded by +the opt or bugpoint tools via their -load options. +If your operating system uses a suffix other than .so (such as windows or +Mac OS/X), the appropriate extension will be used. + +If you are used CMake to build LLVM, see +Developing an LLVM pass with CMake.
Now that we have the build scripts set up, we just need to write the code for the pass itself.
@@ -191,65 +218,90 @@ the pass itself.
Now that we have a way to compile our new pass, we just have to write it. Start out with:
+-#include "llvm/Pass.h" -#include "llvm/Function.h" +#include "llvm/Pass.h" +#include "llvm/Function.h" +#include "llvm/Support/raw_ostream.h"+
Which are needed because we are writing a Pass, and +href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass, we are operating on Function's.
+href="http://llvm.org/doxygen/classllvm_1_1Function.html">Function's, +and we will be doing some printing.Next we have:
+ +using namespace llvm;+
... which is required because the functions from the include files -live in the llvm namespace. -
+live in the llvm namespace.Next we have:
+
namespace {
+... which starts out an anonymous namespace. Anonymous namespaces are to C++ what the "static" keyword is to C (at global scope). It makes the -things declared inside of the anonymous namespace only visible to the current +things declared inside of the anonymous namespace visible only to the current file. If you're not familiar with them, consult a decent C++ book for more information.
Next, we declare our pass itself:
+struct Hello : public FunctionPass { -
+ +
This declares a "Hello" class that is a subclass of FunctionPass. +href="http://llvm.org/doxygen/classllvm_1_1FunctionPass.html">FunctionPass. The different builtin pass subclasses are described in detail later, but for now, know that FunctionPass's operate a function at a +href="#FunctionPass">FunctionPass's operate on a function at a time.
+
+ static char ID;
+ Hello() : FunctionPass(ID) {}
+
+This declares pass identifier used by LLVM to identify pass. This allows LLVM +to avoid using expensive C++ runtime information.
+ +
virtual bool runOnFunction(Function &F) {
- std::cerr << "Hello: " << F.getName() << "\n";
+ errs() << "Hello: ";
+ errs().write_escaped(F.getName()) << "\n";
return false;
}
}; // end of struct Hello
+} // end of anonymous namespace
+We declare a "runOnFunction" method, which overloads an abstract virtual method inherited from FunctionPass. This is where we are supposed to do our thing, so we just print out our message with the name of each function.
+
- RegisterOpt<Hello> X("hello", "Hello World Pass");
-} // end of anonymous namespace
+char Hello::ID = 0;
+
+We initialize pass ID here. LLVM uses ID's address to identify a pass, so +initialization value is not important.
+ +
+static RegisterPass<Hello> X("hello", "Hello World Pass",
+ false /* Only looks at CFG */,
+ false /* Analysis Pass */);
+Lastly, we register our class Hello, giving it a command line -argument "hello", and a name "Hello World Pass". There are -several different ways of registering your pass, -depending on what it is to be used for. For "optimizations" we use the -RegisterOpt template.
+Lastly, we register our class Hello, +giving it a command line argument "hello", and a name "Hello World +Pass". The last two arguments describe its behavior: if a pass walks CFG +without modifying it then the third argument is set to true; if a pass +is an analysis pass, for example dominator tree pass, then true is +supplied as the fourth argument.
As a whole, the .cpp file looks like:
+-#include "llvm/Pass.h" -#include "llvm/Function.h" +#include "llvm/Pass.h" +#include "llvm/Function.h" +#include "llvm/Support/raw_ostream.h" using namespace llvm; namespace { struct Hello : public FunctionPass { + + static char ID; + Hello() : FunctionPass(ID) {} + virtual bool runOnFunction(Function &F) { - std::cerr << "Hello: " << F.getName() << "\n"; + errs() << "Hello: "; + errs().write_escaped(F.getName()) << '\n'; return false; } + }; - - RegisterOpt<Hello> X("hello", "Hello World Pass"); } + +char Hello::ID = 0; +static RegisterPass<Hello> X("hello", "Hello World Pass", false, false);+
Now that it's all together, compile the file with a simple "gmake" -command in the local directory and you should get a new -"lib/Debug/libhello.so file. Note that everything in this file is -contained in an anonymous namespace: this reflects the fact that passes are self -contained units that do not need external interfaces (although they can have -them) to be useful.
+command in the local directory and you should get a new file +"Debug+Asserts/lib/Hello.so" under the top level directory of the LLVM +source tree (not in the local directory). Note that everything in this file is +contained in an anonymous namespace — this reflects the fact that passes +are self contained units that do not need external interfaces (although they can +have them) to be useful.+ Running a pass with opt +
-Now that you have a brand new shiny shared object file, we can use the opt command to run an LLVM program through your pass. Because you -registered your pass with the RegisterOpt template, you will be able to +registered your pass with RegisterPass, you will be able to use the opt tool to access it, once loaded.
To test it, follow the example at the end of the Getting Started Guide to compile "Hello World" to -LLVM. We can now run the bytecode file (hello.bc) for the program -through our transformation like this (or course, any bytecode file will +LLVM. We can now run the bitcode file (hello.bc) for the program +through our transformation like this (or course, any bitcode file will work):
--$ opt -load ../../../lib/Debug/libhello.so -hello < hello.bc > /dev/null ++$ opt -load ../../../Debug+Asserts/lib/Hello.so -hello < hello.bc > /dev/null Hello: __main Hello: puts Hello: main -+The '-load' option specifies that 'opt' should load your pass as a shared object, which makes '-hello' a valid command line @@ -330,28 +406,28 @@ interesting way, we just throw away the result of opt (sending it to /dev/null).
To see what happened to the other string you registered, try running -opt with the --help option:
+opt with the -help option: --$ opt -load ../../../lib/Debug/libhello.so --help +-+$ opt -load ../../../Debug+Asserts/lib/Hello.so -help OVERVIEW: llvm .bc -> .bc modular optimizer -USAGE: opt [options] <input bytecode> +USAGE: opt [options] <input bitcode> OPTIONS: Optimizations available: ... - -funcresolve - Resolve Functions - -gcse - Global Common Subexpression Elimination - -globaldce - Dead Global Elimination - -hello - Hello World Pass - -indvars - Canonicalize Induction Variables - -inline - Function Integration/Inlining - -instcombine - Combine redundant instructions + -globalopt - Global Variable Optimizer + -globalsmodref-aa - Simple mod/ref analysis for globals + -gvn - Global Value Numbering + -hello - Hello World Pass + -indvars - Induction Variable Simplification + -inline - Function Integration/Inlining + -insert-edge-profiling - Insert instrumentation for edge profiling ... -+The pass name get added as the information string for your pass, giving some +
The pass name gets added as the information string for your pass, giving some documentation to users of opt. Now that you have a working pass, you would go ahead and make it do the cool transformations you want. Once you get it all working and tested, it may become useful to find out how fast your pass @@ -360,8 +436,8 @@ line option (--time-passes) that allows you to get information about the execution time of your pass along with the other passes you queue up. For example:
--$ opt -load ../../../lib/Debug/libhello.so -hello -time-passes < hello.bc > /dev/null ++$ opt -load ../../../Debug+Asserts/lib/Hello.so -hello -time-passes < hello.bc > /dev/null Hello: __main Hello: puts Hello: main @@ -371,12 +447,12 @@ Hello: main Total Execution Time: 0.02 seconds (0.0479059 wall clock) ---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name --- - 0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bytecode Writer + 0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bitcode Writer 0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass 0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL -+As you can see, our implementation above is pretty fast :). The additional passes listed are automatically inserted by the 'opt' tool to verify @@ -388,13 +464,15 @@ about some more details of how they work and how to use them.
One of the first things that you should do when designing a new pass is to
decide what class you should subclass for your pass. The
When choosing a superclass for your Pass, you should choose the most
specific class possible, while still being able to meet the requirements
listed. This gives the LLVM Pass Infrastructure information necessary to
-optimize how passes are run, so that the resultant compiler isn't unneccesarily
+optimize how passes are run, so that the resultant compiler isn't unnecessarily
slow.
The most plain and boring type of pass is the "ImmutablePass" +href="http://llvm.org/doxygen/classllvm_1_1ImmutablePass.html">ImmutablePass" class. This pass type is used for passes that do not have to be run, do not change state, and never need to be updated. This is not a normal type of transformation or analysis, but can provide information about the current @@ -435,52 +511,60 @@ invalidated, and are never "run".
The "ModulePass" +href="http://llvm.org/doxygen/classllvm_1_1ModulePass.html">ModulePass" class is the most general of all superclasses that you can use. Deriving from ModulePass indicates that your pass uses the entire program as a unit, -refering to function bodies in no predictable order, or adding and removing +referring to function bodies in no predictable order, or adding and removing functions. Because nothing is known about the behavior of ModulePass subclasses, no optimization can be done for their execution.
+A module pass can use function level passes (e.g. dominators) using +the getAnalysis interface +getAnalysis<DominatorTree>(llvm::Function *) to provide the +function to retrieve analysis result for, if the function pass does not require +any module or immutable passes. Note that this can only be done for functions for which the +analysis ran, e.g. in the case of dominators you should only ask for the +DominatorTree for function definitions, not declarations.
+To write a correct ModulePass subclass, derive from ModulePass and overload the runOnModule method with the following signature:
-- virtual bool runOnModule(Module &M) = 0; -+
+virtual bool runOnModule(Module &M) = 0; +
The runOnModule method performs the interesting work of the pass, -and should return true if the module was modified by the transformation, false -otherwise.
+The runOnModule method performs the interesting work of the pass. +It should return true if the module was modified by the transformation and +false otherwise.
+ +The "CallGraphSCCPass" +href="http://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html">CallGraphSCCPass" is used by passes that need to traverse the program bottom-up on the call graph (callees before callers). Deriving from CallGraphSCCPass provides some mechanics for building and traversing the CallGraph, but also allows the system @@ -496,11 +580,9 @@ href="#BasicBlockPass">BasicBlockPass, you should derive from
-
-
- ... not allowed to modify any Functions that are not in -the current SCC. - -
- ... allowed to inspect any Function's other than those in the -current SCC and the direct callees of the SCC. +
- ... not allowed to inspect or modify any Functions other +than those in the current SCC and the direct callers and direct callees of the +SCC.
- ... required to preserve the current CallGraph object, updating it to reflect any changes made to the program. @@ -520,19 +602,18 @@ because it has to handle SCCs with more than one node in it. All of the virtual methods described below should return true if they modified the program, or false if they didn't. -
+ + The doInitialization(CallGraph &) method + +
-- virtual bool doInitialization(Module &M); -+
+virtual bool doInitialization(CallGraph &CG); +
The doIninitialize method is allowed to do most of the things that CallGraphSCCPass's are not allowed to do. They can add and remove @@ -545,15 +626,15 @@ fast).
- virtual bool runOnSCC(const std::vector<CallGraphNode *> &SCCM) = 0; -+
+virtual bool runOnSCC(CallGraphSCC &SCC) = 0; +
The runOnSCC method performs the interesting work of the pass, and should return true if the module was modified by the transformation, false @@ -562,16 +643,17 @@ otherwise.
+ + The doFinalization(CallGraph &) method + +
-In contrast to ModulePass subclasses, FunctionPass +href="http://llvm.org/doxygen/classllvm_1_1Pass.html">FunctionPass subclasses do have a predictable, local behavior that can be expected by the system. All FunctionPass execute on each function in the program independent of all of the other functions in the program. @@ -610,19 +694,18 @@ href="#basiccode">Hello World pass for example). FunctionPass's may overload three virtual methods to do their work. All of these methods should return true if they modified the program, or false if they didn't.
-+ + The doInitialization(Module &) method + +
-- virtual bool doInitialization(Module &M); -+
+virtual bool doInitialization(Module &M); +
The doIninitialize method is allowed to do most of the things that FunctionPass's are not allowed to do. They can add and remove @@ -633,7 +716,7 @@ scheduled to overlap with any other pass executions (thus it should be very fast).
A good example of how this method should be used is the LowerAllocations +href="http://llvm.org/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations pass. This pass converts malloc and free instructions into platform dependent malloc() and free() function calls. It uses the doInitialization method to get a reference to the malloc and @@ -642,15 +725,15 @@ free functions that it needs, adding prototypes to the module if necessary.
- virtual bool runOnFunction(Function &F) = 0; -
+
+virtual bool runOnFunction(Function &F) = 0; +
The runOnFunction method must be implemented by your subclass to do the transformation or analysis work of your pass. As usual, a true value should @@ -659,16 +742,17 @@ be returned if the function is modified.
+ + The doFinalization(Module &) method + +
-+ The LoopPass class +
+ +All LoopPass execute on each loop in the function independent of +all of the other loops in the function. LoopPass processes loops in +loop nest order such that outer most loop is processed last.
+ +LoopPass subclasses are allowed to update loop nest using +LPPassManager interface. Implementing a loop pass is usually +straightforward. LoopPass's may overload three virtual methods to +do their work. All these methods should return true if they modified the +program, or false if they didn't.
+ + ++ + The doInitialization(Loop *,LPPassManager &) method + +
+ ++virtual bool doInitialization(Loop *, LPPassManager &LPM); +
The doInitialization method is designed to do simple initialization +type of stuff that does not depend on the functions being processed. The +doInitialization method call is not scheduled to overlap with any +other pass executions (thus it should be very fast). LPPassManager +interface should be used to access Function or Module level analysis +information.
+ ++ The runOnLoop method +
+ ++virtual bool runOnLoop(Loop *, LPPassManager &LPM) = 0; +
+ +
The runOnLoop method must be implemented by your subclass to do +the transformation or analysis work of your pass. As usual, a true value should +be returned if the function is modified. LPPassManager interface +should be used to update loop nest.
+ ++ The doFinalization() method +
+ ++virtual bool doFinalization(); +
The doFinalization method is an infrequently used method that is +called when the pass framework has finished calling runOnLoop for every loop in the +program being compiled.
+ ++ The RegionPass class +
+ +RegionPass is similar to LoopPass, +but executes on each single entry single exit region in the function. +RegionPass processes regions in nested order such that the outer most +region is processed last.
+ +RegionPass subclasses are allowed to update the region tree by using +the RGPassManager interface. You may overload three virtual methods of +RegionPass to implement your own region pass. All these +methods should return true if they modified the program, or false if they didn not. +
+ + ++ + The doInitialization(Region *, RGPassManager &) method + +
+ ++virtual bool doInitialization(Region *, RGPassManager &RGM); +
The doInitialization method is designed to do simple initialization +type of stuff that does not depend on the functions being processed. The +doInitialization method call is not scheduled to overlap with any +other pass executions (thus it should be very fast). RPPassManager +interface should be used to access Function or Module level analysis +information.
+ ++ The runOnRegion method +
+ ++virtual bool runOnRegion(Region *, RGPassManager &RGM) = 0; +
+ +
The runOnRegion method must be implemented by your subclass to do +the transformation or analysis work of your pass. As usual, a true value should +be returned if the region is modified. RGPassManager interface +should be used to update region tree.
+ ++ The doFinalization() method +
+ ++virtual bool doFinalization(); +
The doFinalization method is an infrequently used method that is +called when the pass framework has finished calling runOnRegion for every region in the +program being compiled.
++ The BasicBlockPass class +
+ +BasicBlockPass's are just like FunctionPass's, except that they must limit @@ -693,8 +936,8 @@ As such, they are not allowed to do any of the following:
+ + The doInitialization(Function &) method + +
-- virtual bool doInitialization(Function &F); -+
+virtual bool doInitialization(Function &F); +
The doIninitialize method is allowed to do most of the things that BasicBlockPass's are not allowed to do, but that FunctionPass's can. The doInitialization method is designed -to do simple initialization type of stuff that does not depend on the +to do simple initialization that does not depend on the BasicBlocks being processed. The doInitialization method call is not scheduled to overlap with any other pass executions (thus it should be very fast).
@@ -729,15 +971,15 @@ fast).- virtual bool runOnBasicBlock(BasicBlock &BB) = 0; -+
+virtual bool runOnBasicBlock(BasicBlock &BB) = 0; +
Override this function to do the work of the BasicBlockPass. This function is not allowed to inspect or modify basic blocks other than the @@ -747,16 +989,17 @@ if the basic block is modified.
+ + The doFinalization(Function &) method + +
-A MachineFunctionPass is a part of the LLVM code generator that executes on the machine-dependent representation of each LLVM function in the -program. A MachineFunctionPass is also a FunctionPass, so all +program.
+ +Code generator passes are registered and initialized specially by +TargetMachine::addPassesToEmitFile and similar routines, so they +cannot generally be run from the opt or bugpoint +commands.
+ +A MachineFunctionPass is also a FunctionPass, so all the restrictions that apply to a FunctionPass also apply to it. MachineFunctionPasses also have additional restrictions. In particular, MachineFunctionPasses are not allowed to do any of the following:
-
-
- Modify any LLVM Instructions, BasicBlocks or Functions. +
- Modify or create any LLVM IR Instructions, BasicBlocks, Arguments, + Functions, GlobalVariables, GlobalAliases, or Modules.
- Modify a MachineFunction other than the one currently being processed. -
- Add or remove MachineFunctions from the current Module. -
- Add or remove global variables from the current Module.
- Maintain state across invocations of runOnMachineFunction (including global data)
+ + The runOnMachineFunction(MachineFunction &MF) method + +
-- virtual bool runOnMachineFunction(MachineFunction &MF) = 0; -+
+virtual bool runOnMachineFunction(MachineFunction &MF) = 0; +
runOnMachineFunction can be considered the main entry point of a MachineFunctionPass; that is, you should override this method to do the @@ -819,69 +1069,50 @@ remember, you may not modify the LLVM Function or its contents from a
In the Hello World example pass we illustrated how pass registration works, and discussed some of the reasons that it is used and what it does. Here we discuss how and why passes are registered.
-Passes can be registered in several different ways. Depending on the general -classification of the pass, you should use one of the following templates to -register the pass:
- --
-
- RegisterOpt - This template should be used when you are -registering a pass that logically should be available for use in the -'opt' utility. - -
- RegisterAnalysis - This template should be used when you are -registering a pass that logically should be available for use in the -'analyze' utility. - -
- RegisterPass - This is the generic form of the -Register* templates that should be used if you want your pass listed by -multiple or no utilities. This template takes an extra third argument that -specifies which tools it should be listed in. See the PassSupport.h -file for more information. - -
Regardless of how you register your pass, you must specify at least two -parameters. The first parameter is the name of the pass that is to be used on +
As we saw above, passes are registered with the RegisterPass +template. The template parameter is the name of the pass that is to be used on the command line to specify that the pass should be added to a program (for -example opt or analyze). The second argument is the name of -the pass, which is to be used for the --help output of programs, as +example, with opt or bugpoint). The first argument is the +name of the pass, which is to be used for the -help output of +programs, as well as for debug output generated by the --debug-pass option.
-If a pass is registered to be used by the analyze utility, you -should implement the virtual print method:
- -If you want your pass to be easily dumpable, you should +implement the virtual print method:
-The print method -
- virtual void print(std::ostream &O, const Module *M) const; -+
+virtual void print(std::ostream &O, const Module *M) const; +
The print method must be implemented by "analyses" in order to print a human readable version of the analysis results. This is useful for debugging an analysis itself, as well as for other people to figure out how an analysis -works. The analyze tool uses this method to generate its output.
+works. Use the opt -analyze argument to invoke this method. -The ostream parameter specifies the stream to write the results on, +
The llvm::OStream parameter specifies the stream to write the results on, and the Module parameter gives a pointer to the top level module of the program that has been analyzed. Note however that this pointer may be null in certain circumstances (such as calling the Pass::dump() from a @@ -890,15 +1121,17 @@ depended on.
One of the main responsibilities of the PassManager is the make sure +
One of the main responsibilities of the PassManager is to make sure that passes interact with each other correctly. Because PassManager tries to optimize the execution of passes it must know how the passes interact with each other and what dependencies exist between @@ -913,40 +1146,41 @@ specifies. If a pass does not implement the getAnalysisUsage method, it defaults to not having any prerequisite passes, and invalidating all other passes.
-- virtual void getAnalysisUsage(AnalysisUsage &Info) const; -+
+virtual void getAnalysisUsage(AnalysisUsage &Info) const; +
By implementing the getAnalysisUsage method, the required and invalidated sets may be specified for your transformation. The implementation should fill in the AnalysisUsage +href="http://llvm.org/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage object with information about which passes are required and not invalidated. To do this, a pass may call any of the following methods on the AnalysisUsage object:
+ + The AnalysisUsage::addRequired<> + and AnalysisUsage::addRequiredTransitive<> methods + +
+ +-If you pass requires a previous pass to be executed (an analysis for example), +If your pass requires a previous pass to be executed (an analysis for example), it can use one of these methods to arrange for it to be run before your pass. LLVM has many different types of analyses and passes that can be required, -spaning the range from DominatorSet to BreakCriticalEdges. -requiring BreakCriticalEdges, for example, guarantees that there will +spanning the range from DominatorSet to BreakCriticalEdges. +Requiring BreakCriticalEdges, for example, guarantees that there will be no critical edges in the CFG when your pass has been run.
@@ -962,17 +1196,19 @@ pass is.+ + The AnalysisUsage::addPreserved<> method + +
-One of the jobs of the PassManager is to optimize how and when analyses are run. In particular, it attempts to avoid recomputing data unless it needs to. For this reason, passes are allowed to declare that they preserve (i.e., they don't invalidate) an existing analysis if it's available. For example, a simple -constant folding pass would not modify the CFG, so it can't possible effect the +constant folding pass would not modify the CFG, so it can't possibly affect the results of dominator analysis. By default, all passes are assumed to invalidate all others.
@@ -997,39 +1233,33 @@ the fact that it hacks on the CFG.- // This is an example implementation from an analysis, which does not modify - // the program at all, yet has a prerequisite. - void PostDominanceFrontier::getAnalysisUsage(AnalysisUsage &AU) const { - AU.setPreservesAll(); - AU.addRequired<PostDominatorTree>(); - } -- -
and:
- -- // This example modifies the program, but does not modify the CFG - void LICM::getAnalysisUsage(AnalysisUsage &AU) const { - AU.setPreservesCFG(); - AU.addRequired<LoopInfo>(); - } -+
+ + Example implementations of getAnalysisUsage + +
+ ++ + The getAnalysis<> and + getAnalysisIfAvailable<> methods + +
-The Pass::getAnalysis<> method is automatically inherited by your class, providing you with access to the passes that you declared that you @@ -1037,45 +1267,61 @@ required with the getAnalysisUsage method. It takes a single template argument that specifies which pass class you want, and returns a reference to that pass. For example:
-
- bool LICM::runOnFunction(Function &F) {
- LoopInfo &LI = getAnalysis<LoopInfo>();
- ...
- }
-
+
+bool LICM::runOnFunction(Function &F) {
+ LoopInfo &LI = getAnalysis<LoopInfo>();
+ ...
+}
+This method call returns a reference to the pass desired. You may get a runtime assertion failure if you attempt to get an analysis that you did not declare as required in your getAnalysisUsage implementation. This method can be called by your run* method implementation, or by any -other local method invoked by your run* method.
+other local method invoked by your run* method. + +A module level pass can use function level analysis info using this interface. +For example: + +
+bool ModuleLevelPass::runOnModule(Module &M) {
+ ...
+ DominatorTree &DT = getAnalysis<DominatorTree>(Func);
+ ...
+}
+In above example, runOnFunction for DominatorTree is called by pass manager +before returning a reference to the desired pass.
If your pass is capable of updating analyses if they exist (e.g., BreakCriticalEdges, as described above), you can use the -getAnalysisToUpdate method, which returns a pointer to the analysis if -it is active. For example:
+getAnalysisIfAvailable method, which returns a pointer to the analysis +if it is active. For example: -
- ...
- if (DominatorSet *DS = getAnalysisToUpdate<DominatorSet>()) {
- // A DominatorSet is active. This code will update it.
- }
- ...
-
+
+...
+if (DominatorSet *DS = getAnalysisIfAvailable<DominatorSet>()) {
+ // A DominatorSet is active. This code will update it.
+}
+...
+Now that we understand the basics of how passes are defined, how the are +
Now that we understand the basics of how passes are defined, how they are used, and how they are required from other passes, it's time to get a little bit fancier. All of the pass relationships that we have seen so far are very simple: one pass depends on one other specific pass to be run before it can run. @@ -1092,14 +1338,12 @@ between these two extremes for other implementations). To cleanly support situations like this, the LLVM Pass Infrastructure supports the notion of Analysis Groups.
-An Analysis Group is a single simple interface that may be implemented by multiple different passes. Analysis Groups can be given human readable names @@ -1117,22 +1361,22 @@ between passes still apply.
Although Pass Registration is optional for normal passes, all analysis group implementations must be registered, and must use the -RegisterAnalysisGroup template to join the +INITIALIZE_AG_PASS template to join the implementation pool. Also, a default implementation of the interface must be registered with RegisterAnalysisGroup.
As a concrete example of an Analysis Group in action, consider the AliasAnalysis +href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis analysis group. The default implementation of the alias analysis interface (the basicaa +href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">basicaa pass) just does a few simple checks that don't require significant analysis to compute (such as: two different globals can never alias each other, etc). Passes that use the AliasAnalysis +href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis interface (for example the gcse pass), do +href="http://llvm.org/doxygen/structGCSE.html">gcse pass), do not care which implementation of alias analysis is actually provided, they just use the designated interface.
@@ -1146,75 +1390,96 @@ hypothetical example) instead.The RegisterAnalysisGroup template is used to register the analysis -group itself as well as add pass implementations to the analysis group. First, -an analysis should be registered, with a human readable name provided for it. +group itself, while the INITIALIZE_AG_PASS is used to add pass +implementations to the analysis group. First, +an analysis group should be registered, with a human readable name +provided for it. Unlike registration of passes, there is no command line argument to be specified for the Analysis Group Interface itself, because it is "abstract":
-- static RegisterAnalysisGroup<AliasAnalysis> A("Alias Analysis"); -+
+static RegisterAnalysisGroup<AliasAnalysis> A("Alias Analysis"); +
Once the analysis is registered, passes can declare that they are valid implementations of the interface by using the following code:
-+-namespace { - // Analysis Group implementations must be registered normally... - RegisterOpt<FancyAA> - B("somefancyaa", "A more complex alias analysis implementation"); - // Declare that we implement the AliasAnalysis interface - RegisterAnalysisGroup<AliasAnalysis, FancyAA> C; + INITIALIZE_AG_PASS(FancyAA, AliasAnalysis, "somefancyaa", + "A more complex alias analysis implementation", + false, // Is CFG Only? + true, // Is Analysis? + false); // Is default Analysis Group implementation? } -+This just shows a class FancyAA that is registered normally, then -uses the RegisterAnalysisGroup template to "join" the AliasAnalysis +
This just shows a class FancyAA that +uses the INITIALIZE_AG_PASS macro both to register and +to "join" the AliasAnalysis analysis group. Every implementation of an analysis group should join using -this template. A single pass may join multiple different analysis groups with -no problem.
+this macro. -+-namespace { - // Analysis Group implementations must be registered normally... - RegisterOpt<BasicAliasAnalysis> - D("basicaa", "Basic Alias Analysis (default AA impl)"); - // Declare that we implement the AliasAnalysis interface - RegisterAnalysisGroup<AliasAnalysis, BasicAliasAnalysis, true> E; + INITIALIZE_AG_PASS(BasicAA, AliasAnalysis, "basicaa", + "Basic Alias Analysis (default AA impl)", + false, // Is CFG Only? + true, // Is Analysis? + true); // Is default Analysis Group implementation? } -+Here we show how the default implementation is specified (using the extra -argument to the RegisterAnalysisGroup template). There must be exactly +
Here we show how the default implementation is specified (using the final +argument to the INITIALIZE_AG_PASS template). There must be exactly one default implementation available at all times for an Analysis Group to be -used. Here we declare that the BasicAliasAnalysis +used. Only default implementation can derive from ImmutablePass. +Here we declare that the + BasicAliasAnalysis pass is the default implementation for the interface.
+ Pass Statistics +
+ + +The Statistic +class is designed to be an easy way to expose various success +metrics from passes. These statistics are printed at the end of a +run, when the -stats command line option is enabled on the command +line. See the Statistics section in the Programmer's Manual for details. +
+ What PassManager does +
-The PassManager +href="http://llvm.org/doxygen/PassManager_8h-source.html">PassManager class +href="http://llvm.org/doxygen/classllvm_1_1PassManager.html">class takes a list of passes, ensures their prerequisites are set up correctly, and then schedules passes to run efficiently. All of the LLVM tools that run passes use the PassManager for execution of these @@ -1235,7 +1500,7 @@ results as soon as they are no longer needed.
By using the --debug-pass=Structure option, for example, we can see how our Hello World pass interacts with other passes. Lets try it out with the gcse and licm passes:
--$ opt -load ../../../lib/Debug/libhello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null ++$ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null Module Pass Manager Function Pass Manager Dominator Set Construction @@ -1284,9 +1550,9 @@ Module Pass Manager Module Verifier -- Dominator Set Construction -- Module Verifier - Bytecode Writer ---Bytecode Writer -+ Bitcode Writer +--Bitcode Writer +This output shows us when passes are constructed and when the analysis results are known to be dead (prefixed with '--'). Here we see that @@ -1305,8 +1571,8 @@ passes.
Lets see how this changes when we run the Hello World pass in between the two passes:
--$ opt -load ../../../lib/Debug/libhello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null ++$ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null Module Pass Manager Function Pass Manager Dominator Set Construction @@ -1325,29 +1591,29 @@ Module Pass Manager Module Verifier -- Dominator Set Construction -- Module Verifier - Bytecode Writer ---Bytecode Writer + Bitcode Writer +--Bitcode Writer Hello: __main Hello: puts Hello: main -+Here we see that the Hello World pass has killed the Dominator Set pass, even though it doesn't modify the code at all! To fix this, we need to add the following getAnalysisUsage method to our pass:
-- // We don't modify the program, so we preserve all analyses - virtual void getAnalysisUsage(AnalysisUsage &AU) const { - AU.setPreservesAll(); - } -++// We don't modify the program, so we preserve all analyses +virtual void getAnalysisUsage(AnalysisUsage &AU) const { + AU.setPreservesAll(); +} +Now when we run our pass, we get this output:
--$ opt -load ../../../lib/Debug/libhello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null ++$ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null Pass Arguments: -gcse -hello -licm Module Pass Manager Function Pass Manager @@ -1365,28 +1631,26 @@ Module Pass Manager Module Verifier -- Dominator Set Construction -- Module Verifier - Bytecode Writer ---Bytecode Writer + Bitcode Writer +--Bitcode Writer Hello: __main Hello: puts Hello: main -+Which shows that we don't accidentally invalidate dominator information anymore, and therefore do not have to compute it twice.
-
+virtual void releaseMemory(); -+The PassManager automatically determines when to compute analysis results, and how long to keep them around for. Because the lifetime of the pass @@ -1397,19 +1661,154 @@ need some way to free analysis results when they are no longer useful. The
If you are writing an analysis or any other pass that retains a significant amount of state (for use by another pass which "requires" your pass and uses the getAnalysis method) you should implement -releaseMEmory to, well, release the memory allocated to maintain this +releaseMemory to, well, release the memory allocated to maintain this internal state. This method is called after the run* method for the class, before the next call of run* in your pass.
+ Registering dynamically loaded passes +
+ + +Size matters when constructing production quality tools using llvm, +both for the purposes of distribution, and for regulating the resident code size +when running on the target system. Therefore, it becomes desirable to +selectively use some passes, while omitting others and maintain the flexibility +to change configurations later on. You want to be able to do all this, and, +provide feedback to the user. This is where pass registration comes into +play.
+ +The fundamental mechanisms for pass registration are the +MachinePassRegistry class and subclasses of +MachinePassRegistryNode.
+ +An instance of MachinePassRegistry is used to maintain a list of +MachinePassRegistryNode objects. This instance maintains the list and +communicates additions and deletions to the command line interface.
+ +An instance of MachinePassRegistryNode subclass is used to maintain +information provided about a particular pass. This information includes the +command line name, the command help string and the address of the function used +to create an instance of the pass. A global static constructor of one of these +instances registers with a corresponding MachinePassRegistry, +the static destructor unregisters. Thus a pass that is statically linked +in the tool will be registered at start up. A dynamically loaded pass will +register on load and unregister at unload.
+ + ++ Using existing registries +
+ +There are predefined registries to track instruction scheduling +(RegisterScheduler) and register allocation (RegisterRegAlloc) +machine passes. Here we will describe how to register a register +allocator machine pass.
+ +Implement your register allocator machine pass. In your register allocator +.cpp file add the following include;
+ ++#include "llvm/CodeGen/RegAllocRegistry.h" +
Also in your register allocator .cpp file, define a creator function in the +form;
+ +
+FunctionPass *createMyRegisterAllocator() {
+ return new MyRegisterAllocator();
+}
+Note that the signature of this function should match the type of +RegisterRegAlloc::FunctionPassCtor. In the same file add the +"installing" declaration, in the form;
+ +
+static RegisterRegAlloc myRegAlloc("myregalloc",
+ "my register allocator help string",
+ createMyRegisterAllocator);
+Note the two spaces prior to the help string produces a tidy result on the +-help query.
+ ++$ llc -help + ... + -regalloc - Register allocator to use (default=linearscan) + =linearscan - linear scan register allocator + =local - local register allocator + =simple - simple register allocator + =myregalloc - my register allocator help string + ... +
And that's it. The user is now free to use -regalloc=myregalloc as +an option. Registering instruction schedulers is similar except use the +RegisterScheduler class. Note that the +RegisterScheduler::FunctionPassCtor is significantly different from +RegisterRegAlloc::FunctionPassCtor.
+ +To force the load/linking of your register allocator into the llc/lli tools, +add your creator function's global declaration to "Passes.h" and add a "pseudo" +call line to llvm/Codegen/LinkAllCodegenComponents.h.
++ Creating new registries +
+ +The easiest way to get started is to clone one of the existing registries; we +recommend llvm/CodeGen/RegAllocRegistry.h. The key things to modify +are the class name and the FunctionPassCtor type.
+ +Then you need to declare the registry. Example: if your pass registry is +RegisterMyPasses then define;
+ ++MachinePassRegistry RegisterMyPasses::Registry; +
And finally, declare the command line option for your passes. Example:
+ +
+cl::opt<RegisterMyPasses::FunctionPassCtor, false,
+ RegisterPassParser<RegisterMyPasses> >
+MyPassOpt("mypass",
+ cl::init(&createDefaultMyPass),
+ cl::desc("my pass option help"));
+Here the command option is "mypass", with createDefaultMyPass as the default +creator.
+ ++ Using GDB with dynamically loaded passes +
-Unfortunately, using GDB with dynamically loaded passes is not as easy as it should be. First of all, you can't set a breakpoint in a shared object that has @@ -1421,18 +1820,16 @@ GDB.
transformation invoked by opt, although nothing described here depends on that. -First thing you do is start gdb on the opt process:
-+$ gdb opt GNU gdb 5.0 Copyright 2000 Free Software Foundation, Inc. @@ -1442,7 +1839,7 @@ Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "sparc-sun-solaris2.6"... (gdb) -+Note that opt has a lot of debugging information in it, so it takes time to load. Be patient. Since we cannot set a breakpoint in our pass yet @@ -1452,15 +1849,15 @@ object. The most foolproof way of doing this is to set a breakpoint in PassManager::run and then run the process with the arguments you want:
--(gdb) break PassManager::run ++(gdb) break llvm::PassManager::run Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70. -(gdb) run test.bc -load $(LLVMTOP)/llvm/lib/Debug/[libname].so -[passoption] -Starting program: opt test.bc -load $(LLVMTOP)/llvm/lib/Debug/[libname].so -[passoption] +(gdb) run test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption] +Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption] Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70 70 bool PassManager::run(Module &M) { return PM->run(M); } (gdb) -+Once the opt stops in the PassManager::run method you are now free to set breakpoints in your pass so that you can trace through execution @@ -1469,11 +1866,11 @@ or do other standard debugging stuff.
Once you have the basics down, there are a couple of problems that GDB has, some with solutions, some without.
@@ -1501,26 +1898,26 @@ href="mailto:sabre@nondot.org">Chris.Although the LLVM Pass Infrastructure is very capable as it stands, and does some nifty stuff, there are things we'd like to add in the future. Here is where we are going:
-Multiple CPU machines are becoming more common and compilation can never be fast enough: obviously we should allow for a multithreaded compiler. Because of @@ -1538,39 +1935,18 @@ Despite that, we have kept the LLVM passes SMP ready, and you should too.
Currently it is illegal for a ModulePass -to require a FunctionPass. This is because -there is only one instance of the FunctionPass object ever created, thus nowhere -to store information for all of the functions in the program at the same time. -Although this has come up a couple of times before, this has always been worked -around by factoring one big complicated pass into a global and an -interprocedural part, both of which are distinct. In the future, it would be -nice to have this though.
- -Note that it is no problem for a FunctionPass to require the results of a ModulePass, only the other way around.
-
+ src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS">
- The LLVM Compiler Infrastructure
+ The LLVM Compiler Infrastructure
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