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llvm-project/llvm/lib/Transforms/IPO/MergeFunctions.cpp

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//===- MergeFunctions.cpp - Merge identical functions ---------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass looks for equivalent functions that are mergable and folds them.
//
// A hash is computed from the function, based on its type and number of
// basic blocks.
//
// Once all hashes are computed, we perform an expensive equality comparison
// on each function pair. This takes n^2/2 comparisons per bucket, so it's
// important that the hash function be high quality. The equality comparison
// iterates through each instruction in each basic block.
//
// When a match is found the functions are folded. If both functions are
// overridable, we move the functionality into a new internal function and
// leave two overridable thunks to it.
//
//===----------------------------------------------------------------------===//
//
// Future work:
//
// * virtual functions.
//
// Many functions have their address taken by the virtual function table for
// the object they belong to. However, as long as it's only used for a lookup
// and call, this is irrelevant, and we'd like to fold such implementations.
//
// * switch from n^2 pair-wise comparisons to an n-way comparison for each
// bucket.
//
// * be smarter about bitcast.
//
// In order to fold functions, we will sometimes add either bitcast instructions
// or bitcast constant expressions. Unfortunately, this can confound further
// analysis since the two functions differ where one has a bitcast and the
// other doesn't. We should learn to peer through bitcasts without imposing bad
// performance properties.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "mergefunc"
#include "llvm/Transforms/IPO.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/FoldingSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Constants.h"
#include "llvm/InlineAsm.h"
#include "llvm/Instructions.h"
#include "llvm/LLVMContext.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetData.h"
#include <map>
#include <vector>
using namespace llvm;
STATISTIC(NumFunctionsMerged, "Number of functions merged");
namespace {
/// MergeFunctions finds functions which will generate identical machine code,
/// by considering all pointer types to be equivalent. Once identified,
/// MergeFunctions will fold them by replacing a call to one to a call to a
/// bitcast of the other.
///
struct MergeFunctions : public ModulePass {
static char ID; // Pass identification, replacement for typeid
MergeFunctions() : ModulePass(&ID) {}
bool runOnModule(Module &M);
};
}
char MergeFunctions::ID = 0;
INITIALIZE_PASS(MergeFunctions, "mergefunc", "Merge Functions", false, false);
ModulePass *llvm::createMergeFunctionsPass() {
return new MergeFunctions();
}
// ===----------------------------------------------------------------------===
// Comparison of functions
// ===----------------------------------------------------------------------===
namespace {
class FunctionComparator {
public:
FunctionComparator(TargetData *TD, Function *F1, Function *F2)
: F1(F1), F2(F2), TD(TD), IDMap1Count(0), IDMap2Count(0) {}
// Compare - test whether the two functions have equivalent behaviour.
bool Compare();
private:
// Compare - test whether two basic blocks have equivalent behaviour.
bool Compare(const BasicBlock *BB1, const BasicBlock *BB2);
// Enumerate - Assign or look up previously assigned numbers for the two
// values, and return whether the numbers are equal. Numbers are assigned in
// the order visited.
bool Enumerate(const Value *V1, const Value *V2);
// isEquivalentOperation - Compare two Instructions for equivalence, similar
// to Instruction::isSameOperationAs but with modifications to the type
// comparison.
bool isEquivalentOperation(const Instruction *I1,
const Instruction *I2) const;
// isEquivalentGEP - Compare two GEPs for equivalent pointer arithmetic.
bool isEquivalentGEP(const GEPOperator *GEP1, const GEPOperator *GEP2);
bool isEquivalentGEP(const GetElementPtrInst *GEP1,
const GetElementPtrInst *GEP2) {
return isEquivalentGEP(cast<GEPOperator>(GEP1), cast<GEPOperator>(GEP2));
}
// isEquivalentType - Compare two Types, treating all pointer types as equal.
bool isEquivalentType(const Type *Ty1, const Type *Ty2) const;
// The two functions undergoing comparison.
Function *F1, *F2;
TargetData *TD;
typedef DenseMap<const Value *, unsigned long> IDMap;
IDMap Map1, Map2;
unsigned long IDMap1Count, IDMap2Count;
};
}
/// Compute a number which is guaranteed to be equal for two equivalent
/// functions, but is very likely to be different for different functions. This
/// needs to be computed as efficiently as possible.
static unsigned long ProfileFunction(const Function *F) {
const FunctionType *FTy = F->getFunctionType();
FoldingSetNodeID ID;
ID.AddInteger(F->size());
ID.AddInteger(F->getCallingConv());
ID.AddBoolean(F->hasGC());
ID.AddBoolean(FTy->isVarArg());
ID.AddInteger(FTy->getReturnType()->getTypeID());
for (unsigned i = 0, e = FTy->getNumParams(); i != e; ++i)
ID.AddInteger(FTy->getParamType(i)->getTypeID());
return ID.ComputeHash();
}
/// isEquivalentType - any two pointers in the same address space are
/// equivalent. Otherwise, standard type equivalence rules apply.
bool FunctionComparator::isEquivalentType(const Type *Ty1,
const Type *Ty2) const {
if (Ty1 == Ty2)
return true;
if (Ty1->getTypeID() != Ty2->getTypeID())
return false;
switch(Ty1->getTypeID()) {
default:
llvm_unreachable("Unknown type!");
// Fall through in Release mode.
case Type::IntegerTyID:
case Type::OpaqueTyID:
// Ty1 == Ty2 would have returned true earlier.
return false;
case Type::VoidTyID:
case Type::FloatTyID:
case Type::DoubleTyID:
case Type::X86_FP80TyID:
case Type::FP128TyID:
case Type::PPC_FP128TyID:
case Type::LabelTyID:
case Type::MetadataTyID:
return true;
case Type::PointerTyID: {
const PointerType *PTy1 = cast<PointerType>(Ty1);
const PointerType *PTy2 = cast<PointerType>(Ty2);
return PTy1->getAddressSpace() == PTy2->getAddressSpace();
}
case Type::StructTyID: {
const StructType *STy1 = cast<StructType>(Ty1);
const StructType *STy2 = cast<StructType>(Ty2);
if (STy1->getNumElements() != STy2->getNumElements())
return false;
if (STy1->isPacked() != STy2->isPacked())
return false;
for (unsigned i = 0, e = STy1->getNumElements(); i != e; ++i) {
if (!isEquivalentType(STy1->getElementType(i), STy2->getElementType(i)))
return false;
}
return true;
}
case Type::UnionTyID: {
const UnionType *UTy1 = cast<UnionType>(Ty1);
const UnionType *UTy2 = cast<UnionType>(Ty2);
// TODO: we could be fancy with union(A, union(A, B)) === union(A, B), etc.
if (UTy1->getNumElements() != UTy2->getNumElements())
return false;
for (unsigned i = 0, e = UTy1->getNumElements(); i != e; ++i) {
if (!isEquivalentType(UTy1->getElementType(i), UTy2->getElementType(i)))
return false;
}
return true;
}
case Type::FunctionTyID: {
const FunctionType *FTy1 = cast<FunctionType>(Ty1);
const FunctionType *FTy2 = cast<FunctionType>(Ty2);
if (FTy1->getNumParams() != FTy2->getNumParams() ||
FTy1->isVarArg() != FTy2->isVarArg())
return false;
if (!isEquivalentType(FTy1->getReturnType(), FTy2->getReturnType()))
return false;
for (unsigned i = 0, e = FTy1->getNumParams(); i != e; ++i) {
if (!isEquivalentType(FTy1->getParamType(i), FTy2->getParamType(i)))
return false;
}
return true;
}
case Type::ArrayTyID: {
const ArrayType *ATy1 = cast<ArrayType>(Ty1);
const ArrayType *ATy2 = cast<ArrayType>(Ty2);
return ATy1->getNumElements() == ATy2->getNumElements() &&
isEquivalentType(ATy1->getElementType(), ATy2->getElementType());
}
case Type::VectorTyID: {
const VectorType *VTy1 = cast<VectorType>(Ty1);
const VectorType *VTy2 = cast<VectorType>(Ty2);
return VTy1->getNumElements() == VTy2->getNumElements() &&
isEquivalentType(VTy1->getElementType(), VTy2->getElementType());
}
}
}
/// isEquivalentOperation - determine whether the two operations are the same
/// except that pointer-to-A and pointer-to-B are equivalent. This should be
/// kept in sync with Instruction::isSameOperationAs.
bool FunctionComparator::isEquivalentOperation(const Instruction *I1,
const Instruction *I2) const {
if (I1->getOpcode() != I2->getOpcode() ||
I1->getNumOperands() != I2->getNumOperands() ||
!isEquivalentType(I1->getType(), I2->getType()) ||
!I1->hasSameSubclassOptionalData(I2))
return false;
// We have two instructions of identical opcode and #operands. Check to see
// if all operands are the same type
for (unsigned i = 0, e = I1->getNumOperands(); i != e; ++i)
if (!isEquivalentType(I1->getOperand(i)->getType(),
I2->getOperand(i)->getType()))
return false;
// Check special state that is a part of some instructions.
if (const LoadInst *LI = dyn_cast<LoadInst>(I1))
return LI->isVolatile() == cast<LoadInst>(I2)->isVolatile() &&
LI->getAlignment() == cast<LoadInst>(I2)->getAlignment();
if (const StoreInst *SI = dyn_cast<StoreInst>(I1))
return SI->isVolatile() == cast<StoreInst>(I2)->isVolatile() &&
SI->getAlignment() == cast<StoreInst>(I2)->getAlignment();
if (const CmpInst *CI = dyn_cast<CmpInst>(I1))
return CI->getPredicate() == cast<CmpInst>(I2)->getPredicate();
if (const CallInst *CI = dyn_cast<CallInst>(I1))
return CI->isTailCall() == cast<CallInst>(I2)->isTailCall() &&
CI->getCallingConv() == cast<CallInst>(I2)->getCallingConv() &&
CI->getAttributes().getRawPointer() ==
cast<CallInst>(I2)->getAttributes().getRawPointer();
if (const InvokeInst *CI = dyn_cast<InvokeInst>(I1))
return CI->getCallingConv() == cast<InvokeInst>(I2)->getCallingConv() &&
CI->getAttributes().getRawPointer() ==
cast<InvokeInst>(I2)->getAttributes().getRawPointer();
if (const InsertValueInst *IVI = dyn_cast<InsertValueInst>(I1)) {
if (IVI->getNumIndices() != cast<InsertValueInst>(I2)->getNumIndices())
return false;
for (unsigned i = 0, e = IVI->getNumIndices(); i != e; ++i)
if (IVI->idx_begin()[i] != cast<InsertValueInst>(I2)->idx_begin()[i])
return false;
return true;
}
if (const ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(I1)) {
if (EVI->getNumIndices() != cast<ExtractValueInst>(I2)->getNumIndices())
return false;
for (unsigned i = 0, e = EVI->getNumIndices(); i != e; ++i)
if (EVI->idx_begin()[i] != cast<ExtractValueInst>(I2)->idx_begin()[i])
return false;
return true;
}
return true;
}
/// isEquivalentGEP - determine whether two GEP operations perform the same
/// underlying arithmetic.
bool FunctionComparator::isEquivalentGEP(const GEPOperator *GEP1,
const GEPOperator *GEP2) {
// When we have target data, we can reduce the GEP down to the value in bytes
// added to the address.
if (TD && GEP1->hasAllConstantIndices() && GEP2->hasAllConstantIndices()) {
SmallVector<Value *, 8> Indices1(GEP1->idx_begin(), GEP1->idx_end());
SmallVector<Value *, 8> Indices2(GEP2->idx_begin(), GEP2->idx_end());
uint64_t Offset1 = TD->getIndexedOffset(GEP1->getPointerOperandType(),
Indices1.data(), Indices1.size());
uint64_t Offset2 = TD->getIndexedOffset(GEP2->getPointerOperandType(),
Indices2.data(), Indices2.size());
return Offset1 == Offset2;
}
if (GEP1->getPointerOperand()->getType() !=
GEP2->getPointerOperand()->getType())
return false;
if (GEP1->getNumOperands() != GEP2->getNumOperands())
return false;
for (unsigned i = 0, e = GEP1->getNumOperands(); i != e; ++i) {
if (!Enumerate(GEP1->getOperand(i), GEP2->getOperand(i)))
return false;
}
return true;
}
/// Enumerate - Compare two values used by the two functions under pair-wise
/// comparison. If this is the first time the values are seen, they're added to
/// the mapping so that we will detect mismatches on next use.
bool FunctionComparator::Enumerate(const Value *V1, const Value *V2) {
// Check for function @f1 referring to itself and function @f2 referring to
// itself, or referring to each other, or both referring to either of them.
// They're all equivalent if the two functions are otherwise equivalent.
if (V1 == F1 && V2 == F2)
return true;
if (V1 == F2 && V2 == F1)
return true;
// TODO: constant expressions with GEP or references to F1 or F2.
if (isa<Constant>(V1))
return V1 == V2;
if (isa<InlineAsm>(V1) && isa<InlineAsm>(V2)) {
const InlineAsm *IA1 = cast<InlineAsm>(V1);
const InlineAsm *IA2 = cast<InlineAsm>(V2);
return IA1->getAsmString() == IA2->getAsmString() &&
IA1->getConstraintString() == IA2->getConstraintString();
}
unsigned long &ID1 = Map1[V1];
if (!ID1)
ID1 = ++IDMap1Count;
unsigned long &ID2 = Map2[V2];
if (!ID2)
ID2 = ++IDMap2Count;
return ID1 == ID2;
}
// Compare - test whether two basic blocks have equivalent behaviour.
bool FunctionComparator::Compare(const BasicBlock *BB1, const BasicBlock *BB2) {
BasicBlock::const_iterator F1I = BB1->begin(), F1E = BB1->end();
BasicBlock::const_iterator F2I = BB2->begin(), F2E = BB2->end();
do {
if (!Enumerate(F1I, F2I))
return false;
if (const GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(F1I)) {
const GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(F2I);
if (!GEP2)
return false;
if (!Enumerate(GEP1->getPointerOperand(), GEP2->getPointerOperand()))
return false;
if (!isEquivalentGEP(GEP1, GEP2))
return false;
} else {
if (!isEquivalentOperation(F1I, F2I))
return false;
assert(F1I->getNumOperands() == F2I->getNumOperands());
for (unsigned i = 0, e = F1I->getNumOperands(); i != e; ++i) {
Value *OpF1 = F1I->getOperand(i);
Value *OpF2 = F2I->getOperand(i);
if (!Enumerate(OpF1, OpF2))
return false;
if (OpF1->getValueID() != OpF2->getValueID() ||
!isEquivalentType(OpF1->getType(), OpF2->getType()))
return false;
}
}
++F1I, ++F2I;
} while (F1I != F1E && F2I != F2E);
return F1I == F1E && F2I == F2E;
}
bool FunctionComparator::Compare() {
// We need to recheck everything, but check the things that weren't included
// in the hash first.
if (F1->getAttributes() != F2->getAttributes())
return false;
if (F1->hasGC() != F2->hasGC())
return false;
if (F1->hasGC() && F1->getGC() != F2->getGC())
return false;
if (F1->hasSection() != F2->hasSection())
return false;
if (F1->hasSection() && F1->getSection() != F2->getSection())
return false;
if (F1->isVarArg() != F2->isVarArg())
return false;
// TODO: if it's internal and only used in direct calls, we could handle this
// case too.
if (F1->getCallingConv() != F2->getCallingConv())
return false;
if (!isEquivalentType(F1->getFunctionType(), F2->getFunctionType()))
return false;
assert(F1->arg_size() == F2->arg_size() &&
"Identical functions have a different number of args.");
// Visit the arguments so that they get enumerated in the order they're
// passed in.
for (Function::const_arg_iterator f1i = F1->arg_begin(),
f2i = F2->arg_begin(), f1e = F1->arg_end(); f1i != f1e; ++f1i, ++f2i) {
if (!Enumerate(f1i, f2i))
llvm_unreachable("Arguments repeat");
}
// We need to do an ordered walk since the actual ordering of the blocks in
// the linked list is immaterial. Our walk starts at the entry block for both
// functions, then takes each block from each terminator in order. As an
// artifact, this also means that unreachable blocks are ignored.
SmallVector<const BasicBlock *, 8> F1BBs, F2BBs;
SmallSet<const BasicBlock *, 128> VisitedBBs; // in terms of F1.
F1BBs.push_back(&F1->getEntryBlock());
F2BBs.push_back(&F2->getEntryBlock());
VisitedBBs.insert(F1BBs[0]);
while (!F1BBs.empty()) {
const BasicBlock *F1BB = F1BBs.pop_back_val();
const BasicBlock *F2BB = F2BBs.pop_back_val();
if (!Enumerate(F1BB, F2BB) || !Compare(F1BB, F2BB))
return false;
const TerminatorInst *F1TI = F1BB->getTerminator();
const TerminatorInst *F2TI = F2BB->getTerminator();
assert(F1TI->getNumSuccessors() == F2TI->getNumSuccessors());
for (unsigned i = 0, e = F1TI->getNumSuccessors(); i != e; ++i) {
if (!VisitedBBs.insert(F1TI->getSuccessor(i)))
continue;
F1BBs.push_back(F1TI->getSuccessor(i));
F2BBs.push_back(F2TI->getSuccessor(i));
}
}
return true;
}
// ===----------------------------------------------------------------------===
// Folding of functions
// ===----------------------------------------------------------------------===
// Cases:
// * F is external strong, G is external strong:
// turn G into a thunk to F
// * F is external strong, G is external weak:
// turn G into a thunk to F
// * F is external weak, G is external weak:
// unfoldable
// * F is external strong, G is internal:
// turn G into a thunk to F
// * F is internal, G is external weak
// turn G into a thunk to F
// * F is internal, G is internal:
// turn G into a thunk to F
//
// external means 'externally visible' linkage != (internal,private)
// internal means linkage == (internal,private)
// weak means linkage mayBeOverridable
/// ThunkGToF - Replace G with a simple tail call to bitcast(F). Also replace
/// direct uses of G with bitcast(F).
static void ThunkGToF(Function *F, Function *G) {
if (!G->mayBeOverridden()) {
// Redirect direct callers of G to F.
Constant *BitcastF = ConstantExpr::getBitCast(F, G->getType());
for (Value::use_iterator UI = G->use_begin(), UE = G->use_end();
UI != UE;) {
Value::use_iterator TheIter = UI;
++UI;
CallSite CS(*TheIter);
if (CS && CS.isCallee(TheIter))
TheIter.getUse().set(BitcastF);
}
}
// If G was internal then we may have replaced all uses if G with F. If so,
// stop here and delete G. There's no need for a thunk.
if (G->hasLocalLinkage() && G->use_empty()) {
G->eraseFromParent();
return;
}
Function *NewG = Function::Create(G->getFunctionType(), G->getLinkage(), "",
G->getParent());
BasicBlock *BB = BasicBlock::Create(F->getContext(), "", NewG);
SmallVector<Value *, 16> Args;
unsigned i = 0;
const FunctionType *FFTy = F->getFunctionType();
for (Function::arg_iterator AI = NewG->arg_begin(), AE = NewG->arg_end();
AI != AE; ++AI) {
if (FFTy->getParamType(i) == AI->getType()) {
Args.push_back(AI);
} else {
Args.push_back(new BitCastInst(AI, FFTy->getParamType(i), "", BB));
}
++i;
}
CallInst *CI = CallInst::Create(F, Args.begin(), Args.end(), "", BB);
CI->setTailCall();
CI->setCallingConv(F->getCallingConv());
if (NewG->getReturnType()->isVoidTy()) {
ReturnInst::Create(F->getContext(), BB);
} else if (CI->getType() != NewG->getReturnType()) {
Value *BCI = new BitCastInst(CI, NewG->getReturnType(), "", BB);
ReturnInst::Create(F->getContext(), BCI, BB);
} else {
ReturnInst::Create(F->getContext(), CI, BB);
}
NewG->copyAttributesFrom(G);
NewG->takeName(G);
G->replaceAllUsesWith(NewG);
G->eraseFromParent();
}
static bool fold(std::vector<Function *> &FnVec, unsigned i, unsigned j) {
Function *F = FnVec[i];
Function *G = FnVec[j];
if (F->isWeakForLinker() && !G->isWeakForLinker()) {
std::swap(FnVec[i], FnVec[j]);
std::swap(F, G);
}
if (F->isWeakForLinker()) {
assert(G->isWeakForLinker());
// Make them both thunks to the same internal function.
Function *H = Function::Create(F->getFunctionType(), F->getLinkage(), "",
F->getParent());
H->copyAttributesFrom(F);
H->takeName(F);
F->replaceAllUsesWith(H);
ThunkGToF(F, G);
ThunkGToF(F, H);
F->setAlignment(std::max(G->getAlignment(), H->getAlignment()));
F->setLinkage(GlobalValue::InternalLinkage);
} else {
ThunkGToF(F, G);
}
++NumFunctionsMerged;
return true;
}
// ===----------------------------------------------------------------------===
// Pass definition
// ===----------------------------------------------------------------------===
bool MergeFunctions::runOnModule(Module &M) {
bool Changed = false;
std::map<unsigned long, std::vector<Function *> > FnMap;
for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
if (F->isDeclaration() || F->hasAvailableExternallyLinkage())
continue;
FnMap[ProfileFunction(F)].push_back(F);
}
TargetData *TD = getAnalysisIfAvailable<TargetData>();
bool LocalChanged;
do {
LocalChanged = false;
DEBUG(dbgs() << "size: " << FnMap.size() << "\n");
for (std::map<unsigned long, std::vector<Function *> >::iterator
I = FnMap.begin(), E = FnMap.end(); I != E; ++I) {
std::vector<Function *> &FnVec = I->second;
DEBUG(dbgs() << "hash (" << I->first << "): " << FnVec.size() << "\n");
for (int i = 0, e = FnVec.size(); i != e; ++i) {
for (int j = i + 1; j != e; ++j) {
bool isEqual = FunctionComparator(TD, FnVec[i], FnVec[j]).Compare();
DEBUG(dbgs() << " " << FnVec[i]->getName()
<< (isEqual ? " == " : " != ")
<< FnVec[j]->getName() << "\n");
if (isEqual) {
if (fold(FnVec, i, j)) {
LocalChanged = true;
FnVec.erase(FnVec.begin() + j);
--j, --e;
}
}
}
}
}
Changed |= LocalChanged;
} while (LocalChanged);
return Changed;
}
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