llvm-6502/lib/Transforms/IPO/ArgumentPromotion.cpp
Chris Lattner 9e7cc2f0d4 Fairly substantial changes to update the alias analysis we are querying as
we make the transformation.  This allows us to use interprocedural alias
analyses successfully.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13691 91177308-0d34-0410-b5e6-96231b3b80d8
2004-05-23 21:21:17 +00:00

515 lines
20 KiB
C++

//===-- 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).
//
// 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
// we refuse to scalarize aggregates which would require passing in more than
// three operands to the function, because we don't want to pass thousands of
// operands for a large array or structure!
//
// 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. This case would be best handled when and if we start supporting
// multiple return values from functions.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "argpromotion"
#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 "Support/StringExtras.h"
#include <set>
using namespace llvm;
namespace {
Statistic<> NumArgumentsPromoted("argpromotion",
"Number of pointer arguments promoted");
Statistic<> NumAggregatesPromoted("argpromotion",
"Number of aggregate 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) && !isa<GlobalVariable>(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;
}
/// PromoteArguments - This method checks the specified function to see if there
/// are any promotable arguments and if it is safe to promote the function (for
/// example, all callers are direct). If safe to promote some arguments, it
/// calls the DoPromotion method.
///
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) {
CallSite CS = CallSite::get(*UI);
if (!CS.getInstruction()) // "Taking the address" of the function
return false;
// Ensure that this call site is CALLING the function, not passing it as
// an argument.
for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end();
AI != E; ++AI)
if (*AI == F) return false; // Passing the function address in!
}
// 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;
}
/// isSafeToPromoteArgument - As you might guess from the name of this method,
/// it checks to see if it is both safe and useful to promote the argument.
/// This method limits promotion of aggregates to only promote up to three
/// elements of the aggregate in order to avoid exploding the number of
/// arguments passed in.
bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const {
// We can only promote this argument if all of the uses are loads, or are GEP
// instructions (with constant indices) that are subsequently loaded.
std::vector<LoadInst*> Loads;
std::vector<std::vector<Constant*> > GEPIndices;
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 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
if (GEP->use_empty()) {
// Dead GEP's cause trouble later. Just remove them if we run into
// them.
getAnalysis<AliasAnalysis>().deleteValue(GEP);
GEP->getParent()->getInstList().erase(GEP);
return isSafeToPromoteArgument(Arg);
}
// Ensure that all of the indices are constants.
std::vector<Constant*> Operands;
for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i)
if (Constant *C = dyn_cast<Constant>(GEP->getOperand(i)))
Operands.push_back(C);
else
return false; // Not a constant operand GEP!
// Ensure that the only users of the GEP are load instructions.
for (Value::use_iterator UI = GEP->use_begin(), E = GEP->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;
}
// See if there is already a GEP with these indices. If not, check to
// make sure that we aren't promoting too many elements. If so, nothing
// to do.
if (std::find(GEPIndices.begin(), GEPIndices.end(), Operands) ==
GEPIndices.end()) {
if (GEPIndices.size() == 3) {
DEBUG(std::cerr << "argpromotion disable promoting argument '"
<< Arg->getName() << "' because it would require adding more "
<< "than 3 arguments to the function.\n");
// We limit aggregate promotion to only promoting up to three elements
// of the aggregate.
return false;
}
GEPIndices.push_back(Operands);
}
} else {
return false; // Not a load or a GEP.
}
if (Loads.empty()) return true; // No users, this is a dead argument.
// Okay, now we know that the argument is only used by load instructions. Use
// alias analysis to 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>();
TargetData &TD = getAnalysis<TargetData>();
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();
const PointerType *LoadTy =
cast<PointerType>(Load->getOperand(0)->getType());
unsigned LoadSize = TD.getTypeSize(LoadTy->getElementType());
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;
}
/// DoPromotion - This method actually performs the promotion of the specified
/// arguments. At this point, we know that it's safe to do so.
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;
// ScalarizedElements - If we are promoting a pointer that has elements
// accessed out of it, keep track of which elements are accessed so that we
// can add one argument for each.
//
// Arguments that are directly loaded will have a zero element value here, to
// handle cases where there are both a direct load and GEP accesses.
//
std::map<Argument*, std::set<std::vector<Value*> > > ScalarizedElements;
// OriginalLoads - Keep track of a representative load instruction from the
// original function so that we can tell the alias analysis implementation
// what the new GEP/Load instructions we are inserting look like.
std::map<std::vector<Value*>, LoadInst*> OriginalLoads;
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()) {
++NumArgumentsDead;
} else {
// Okay, this is being promoted. Check to see if there are any GEP uses
// of the argument.
std::set<std::vector<Value*> > &ArgIndices = ScalarizedElements[I];
for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
++UI) {
Instruction *User = cast<Instruction>(*UI);
assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User));
std::vector<Value*> Indices(User->op_begin()+1, User->op_end());
ArgIndices.insert(Indices);
LoadInst *OrigLoad;
if (LoadInst *L = dyn_cast<LoadInst>(User))
OrigLoad = L;
else
OrigLoad = cast<LoadInst>(User->use_back());
OriginalLoads[Indices] = OrigLoad;
}
// Add a parameter to the function for each element passed in.
for (std::set<std::vector<Value*> >::iterator SI = ArgIndices.begin(),
E = ArgIndices.end(); SI != E; ++SI)
Params.push_back(GetElementPtrInst::getIndexedType(I->getType(), *SI));
if (ArgIndices.size() == 1 && ArgIndices.begin()->empty())
++NumArgumentsPromoted;
else
++NumAggregatesPromoted;
}
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);
// Get the alias analysis information that we need to update to reflect our
// changes.
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
// 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 now that we promoted
// arguments in a callee of it.
if (Call->getParent()->getParent()->hasInternalLinkage())
WorkList.insert(Call->getParent()->getParent());
// Loop over the operands, inserting GEP and loads in the caller as
// appropriate.
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 argument: insert GEPs and loads as appropriate.
std::set<std::vector<Value*> > &ArgIndices = ScalarizedElements[I];
for (std::set<std::vector<Value*> >::iterator SI = ArgIndices.begin(),
E = ArgIndices.end(); SI != E; ++SI) {
Value *V = *AI;
LoadInst *OrigLoad = OriginalLoads[*SI];
if (!SI->empty()) {
V = new GetElementPtrInst(V, *SI, V->getName()+".idx", Call);
AA.copyValue(OrigLoad->getOperand(0), V);
}
Args.push_back(new LoadInst(V, V->getName()+".val", Call));
AA.copyValue(OrigLoad, Args.back());
}
}
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();
// Update the alias analysis implementation to know that we are replacing
// the old call with a new one.
AA.replaceWithNewValue(Call, New);
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());
AA.replaceWithNewValue(I, I2);
++I2;
} else if (I->use_empty()) {
AA.deleteValue(I);
} else {
// Otherwise, if we promoted this argument, then all users are load
// instructions, and all loads should be using the new argument that we
// added.
std::set<std::vector<Value*> > &ArgIndices = ScalarizedElements[I];
while (!I->use_empty()) {
if (LoadInst *LI = dyn_cast<LoadInst>(I->use_back())) {
assert(ArgIndices.begin()->empty() &&
"Load element should sort to front!");
I2->setName(I->getName()+".val");
LI->replaceAllUsesWith(I2);
AA.replaceWithNewValue(LI, I2);
LI->getParent()->getInstList().erase(LI);
DEBUG(std::cerr << "*** Promoted load of argument '" << I->getName()
<< "' in function '" << F->getName() << "'\n");
} else {
GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->use_back());
std::vector<Value*> Operands(GEP->op_begin()+1, GEP->op_end());
unsigned ArgNo = 0;
Function::aiterator TheArg = I2;
for (std::set<std::vector<Value*> >::iterator It = ArgIndices.begin();
*It != Operands; ++It, ++TheArg) {
assert(It != ArgIndices.end() && "GEP not handled??");
}
std::string NewName = I->getName();
for (unsigned i = 0, e = Operands.size(); i != e; ++i)
if (ConstantInt *CI = dyn_cast<ConstantInt>(Operands[i]))
NewName += "."+itostr((int64_t)CI->getRawValue());
else
NewName += ".x";
TheArg->setName(NewName+".val");
DEBUG(std::cerr << "*** Promoted agg argument '" << TheArg->getName()
<< "' of function '" << F->getName() << "'\n");
// All of the uses must be load instructions. Replace them all with
// the argument specified by ArgNo.
while (!GEP->use_empty()) {
LoadInst *L = cast<LoadInst>(GEP->use_back());
L->replaceAllUsesWith(TheArg);
AA.replaceWithNewValue(L, TheArg);
L->getParent()->getInstList().erase(L);
}
AA.deleteValue(GEP);
GEP->getParent()->getInstList().erase(GEP);
}
}
// If we inserted a new pointer type, it's possible that IT could be
// promoted too. Also, increment I2 past all of the arguments added for
// this promoted pointer.
for (unsigned i = 0, e = ArgIndices.size(); i != e; ++i, ++I2)
if (isa<PointerType>(I2->getType()))
WorkList.insert(NF);
}
// Notify the alias analysis implementation that we inserted a new argument.
if (ExtraArgHack)
AA.copyValue(Constant::getNullValue(Type::IntTy), NF->abegin());
// Tell the alias analysis that the old function is about to disappear.
AA.replaceWithNewValue(F, NF);
// Now that the old function is dead, delete it.
F->getParent()->getFunctionList().erase(F);
}