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New LLVM pass: argument promotion. This version only handles simple scalar

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llvm-svn: 12193
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Chris Lattner 2004-03-07 21:29:54 +00:00
parent f4fa087828
commit 4415950211
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lib/Transforms/IPO/ArgumentPromotion.cpp Normal file
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//===-- ArgumentPromotion.cpp - Promote 'by reference' arguments ----------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass promotes "by reference" arguments to be "by value" arguments. In
// practice, this means looking for internal functions that have pointer
// arguments. If we can prove, through the use of alias analysis, that that an
// argument is *only* loaded, then we 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.
//
// Note that this transformation could also be done for arguments that are only
// stored to (returning the value instead), but we do not currently handle that
// case.
//
// Note that we should be able to promote pointers to structures that are only
// loaded from as well. The danger is creating way to many arguments, so this
// transformation should be limited to 3 element structs or something.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/CFG.h"
#include "Support/Debug.h"
#include "Support/DepthFirstIterator.h"
#include "Support/Statistic.h"
#include <set>
using namespace llvm;
namespace {
Statistic<> NumArgumentsPromoted("argpromotion",
"Number of pointer arguments promoted");
Statistic<> NumArgumentsDead("argpromotion",
"Number of dead pointer args eliminated");
/// ArgPromotion - The 'by reference' to 'by value' argument promotion pass.
///
class ArgPromotion : public Pass {
// WorkList - The set of internal functions that we have yet to process. As
// we eliminate arguments from a function, we push all callers into this set
// so that the by reference argument can be bubbled out as far as possible.
// This set contains only internal functions.
std::set<Function*> WorkList;
public:
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<AliasAnalysis>();
AU.addRequired<TargetData>();
}
virtual bool run(Module &M);
private:
bool PromoteArguments(Function *F);
bool isSafeToPromoteArgument(Argument *Arg) const;
void DoPromotion(Function *F, std::vector<Argument*> &ArgsToPromote);
};
RegisterOpt<ArgPromotion> X("argpromotion",
"Promote 'by reference' arguments to scalars");
}
Pass *llvm::createArgumentPromotionPass() {
return new ArgPromotion();
}
bool ArgPromotion::run(Module &M) {
bool Changed = false;
for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
if (I->hasInternalLinkage()) {
WorkList.insert(I);
// If there are any constant pointer refs pointing to this function,
// eliminate them now if possible.
ConstantPointerRef *CPR = 0;
for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
++UI)
if ((CPR = dyn_cast<ConstantPointerRef>(*UI)))
break; // Found one!
if (CPR) {
// See if we can transform all users to use the function directly.
while (!CPR->use_empty()) {
User *TheUser = CPR->use_back();
if (!isa<Constant>(TheUser)) {
Changed = true;
TheUser->replaceUsesOfWith(CPR, I);
} else {
// We won't be able to eliminate all users. :(
WorkList.erase(I); // Minor efficiency win.
break;
}
}
// If we nuked all users of the CPR, kill the CPR now!
if (CPR->use_empty()) {
CPR->destroyConstant();
Changed = true;
}
}
}
while (!WorkList.empty()) {
Function *F = *WorkList.begin();
WorkList.erase(WorkList.begin());
if (PromoteArguments(F)) // Attempt to promote an argument.
Changed = true; // Remember that we changed something.
}
return Changed;
}
bool ArgPromotion::PromoteArguments(Function *F) {
assert(F->hasInternalLinkage() && "We can only process internal functions!");
// First check: see if there are any pointer arguments! If not, quick exit.
std::vector<Argument*> PointerArgs;
for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
if (isa<PointerType>(I->getType()))
PointerArgs.push_back(I);
if (PointerArgs.empty()) return false;
// Second check: make sure that all callers are direct callers. We can't
// transform functions that have indirect callers.
for (Value::use_iterator UI = F->use_begin(), E = F->use_end();
UI != E; ++UI)
// What about CPRs?
if (!CallSite::get(*UI).getInstruction())
return false; // Cannot promote an indirect call!
// Check to see which arguments are promotable. If an argument is not
// promotable, remove it from the PointerArgs vector.
for (unsigned i = 0; i != PointerArgs.size(); ++i)
if (!isSafeToPromoteArgument(PointerArgs[i])) {
std::swap(PointerArgs[i--], PointerArgs.back());
PointerArgs.pop_back();
}
// No promotable pointer arguments.
if (PointerArgs.empty()) return false;
// Okay, promote all of the arguments are rewrite the callees!
DoPromotion(F, PointerArgs);
return true;
}
bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const {
// We can only promote this argument if all of the uses are loads...
std::vector<LoadInst*> Loads;
for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end();
UI != E; ++UI)
if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
if (LI->isVolatile()) return false; // Don't hack volatile loads
Loads.push_back(LI);
} else
return false;
if (Loads.empty()) return true; // No users, dead argument.
const Type *LoadTy = cast<PointerType>(Arg->getType())->getElementType();
unsigned LoadSize = getAnalysis<TargetData>().getTypeSize(LoadTy);
// Okay, now we know that the argument is only used by load instructions.
// Check to see if the pointer is guaranteed to not be modified from entry of
// the function to each of the load instructions.
Function &F = *Arg->getParent();
// Because there could be several/many load instructions, remember which
// blocks we know to be transparent to the load.
std::set<BasicBlock*> TranspBlocks;
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
// Check to see if the load is invalidated from the start of the block to
// the load itself.
LoadInst *Load = Loads[i];
BasicBlock *BB = Load->getParent();
if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
return false; // Pointer is invalidated!
// Now check every path from the entry block to the load for transparency.
// To do this, we perform a depth first search on the inverse CFG from the
// loading block.
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks),
E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
if (AA.canBasicBlockModify(**I, Arg, LoadSize))
return false;
}
// If the path from the entry of the function to each load is free of
// instructions that potentially invalidate the load, we can make the
// transformation!
return true;
}
void ArgPromotion::DoPromotion(Function *F, std::vector<Argument*> &Args2Prom) {
std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end());
// Start by computing a new prototype for the function, which is the same as
// the old function, but has modified arguments.
const FunctionType *FTy = F->getFunctionType();
std::vector<const Type*> Params;
for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
if (!ArgsToPromote.count(I)) {
Params.push_back(I->getType());
} else if (!I->use_empty()) {
Params.push_back(cast<PointerType>(I->getType())->getElementType());
++NumArgumentsPromoted;
} else {
++NumArgumentsDead;
}
const Type *RetTy = FTy->getReturnType();
// Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
// have zero fixed arguments.
bool ExtraArgHack = false;
if (Params.empty() && FTy->isVarArg()) {
ExtraArgHack = true;
Params.push_back(Type::IntTy);
}
FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
// Create the new function body and insert it into the module...
Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
F->getParent()->getFunctionList().insert(F, NF);
// Loop over all of the callers of the function, transforming the call sites
// to pass in the loaded pointers.
//
std::vector<Value*> Args;
while (!F->use_empty()) {
CallSite CS = CallSite::get(F->use_back());
Instruction *Call = CS.getInstruction();
// Make sure the caller of this function is revisited.
if (Call->getParent()->getParent()->hasInternalLinkage())
WorkList.insert(Call->getParent()->getParent());
// Loop over the operands, deleting dead ones...
CallSite::arg_iterator AI = CS.arg_begin();
for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I, ++AI)
if (!ArgsToPromote.count(I))
Args.push_back(*AI); // Unmodified argument
else if (!I->use_empty()) {
// Non-dead instruction
Args.push_back(new LoadInst(*AI, (*AI)->getName()+".val", Call));
}
if (ExtraArgHack)
Args.push_back(Constant::getNullValue(Type::IntTy));
// Push any varargs arguments on the list
for (; AI != CS.arg_end(); ++AI)
Args.push_back(*AI);
Instruction *New;
if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
Args, "", Call);
} else {
New = new CallInst(NF, Args, "", Call);
}
Args.clear();
if (!Call->use_empty()) {
Call->replaceAllUsesWith(New);
std::string Name = Call->getName();
Call->setName("");
New->setName(Name);
}
// Finally, remove the old call from the program, reducing the use-count of
// F.
Call->getParent()->getInstList().erase(Call);
}
// Since we have now created the new function, splice the body of the old
// function right into the new function, leaving the old rotting hulk of the
// function empty.
NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
// Loop over the argument list, transfering uses of the old arguments over to
// the new arguments, also transfering over the names as well.
//
for (Function::aiterator I = F->abegin(), E = F->aend(), I2 = NF->abegin();
I != E; ++I)
if (!ArgsToPromote.count(I)) {
// If this is an unmodified argument, move the name and users over to the
// new version.
I->replaceAllUsesWith(I2);
I2->setName(I->getName());
++I2;
} else if (!I->use_empty()) {
// Otherwise, if we promoted this argument, then all users are load
// instructions, and all loads should be using the new argument that we
// added.
/*DEBUG*/(std::cerr << "*** Promoted argument '" << I->getName()
<< "' of function '" << F->getName() << "'\n");
I2->setName(I->getName()+".val");
while (!I->use_empty()) {
LoadInst *LI = cast<LoadInst>(I->use_back());
LI->replaceAllUsesWith(I2);
LI->getParent()->getInstList().erase(LI);
}
++I2;
}
// Now that the old function is dead, delete it.
F->getParent()->getFunctionList().erase(F);
}