llvm-6502/lib/Transforms/IPO/MergeFunctions.cpp
Nick Lewycky 6feb333695 Changes from Duncan's review:
* merge two weak functions by making them both alias a third non-weak fn
 * don't reimplement CallSite::hasArgument
 * whitelist the safe linkage types


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@58568 91177308-0d34-0410-b5e6-96231b3b80d8
2008-11-02 16:46:26 +00:00

378 lines
12 KiB
C++

//===- 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 Function will not be analyzed if:
// * it is overridable at runtime (except for weak linkage), or
// * it is used by anything other than the callee parameter of a call/invoke
//
// 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. We can only fold two
// functions when we know that the definition of one of them is not
// overridable.
// * fold a function marked internal by replacing all of its users.
// * fold extern or weak functions by replacing them with a global alias
//
//===----------------------------------------------------------------------===//
//
// Future work:
//
// * fold vector<T*>::push_back and vector<S*>::push_back.
//
// These two functions have different types, but in a way that doesn't matter
// to us. As long as we never see an S or T itself, using S* and S** is the
// same as using a T* and T**.
//
// * 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.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "mergefunc"
#include "llvm/Transforms/IPO.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Constants.h"
#include "llvm/InlineAsm.h"
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include <map>
#include <vector>
using namespace llvm;
STATISTIC(NumFunctionsMerged, "Number of functions merged");
STATISTIC(NumMergeFails, "Number of identical function pairings not merged");
namespace {
struct VISIBILITY_HIDDEN MergeFunctions : public ModulePass {
static char ID; // Pass identification, replacement for typeid
MergeFunctions() : ModulePass((intptr_t)&ID) {}
bool runOnModule(Module &M);
};
}
char MergeFunctions::ID = 0;
static RegisterPass<MergeFunctions>
X("mergefunc", "Merge Functions");
ModulePass *llvm::createMergeFunctionsPass() {
return new MergeFunctions();
}
static unsigned long hash(const Function *F) {
return F->size() ^ reinterpret_cast<unsigned long>(F->getType());
//return F->size() ^ F->arg_size() ^ F->getReturnType();
}
static bool compare(const Value *V, const Value *U) {
assert(!isa<BasicBlock>(V) && !isa<BasicBlock>(U) &&
"Must not compare basic blocks.");
assert(V->getType() == U->getType() &&
"Two of the same operation have operands of different type.");
// TODO: If the constant is an expression of F, we should accept that it's
// equal to the same expression in terms of G.
if (isa<Constant>(V))
return V == U;
// The caller has ensured that ValueMap[V] != U. Since Arguments are
// pre-loaded into the ValueMap, and Instructions are added as we go, we know
// that this can only be a mis-match.
if (isa<Instruction>(V) || isa<Argument>(V))
return false;
if (isa<InlineAsm>(V) && isa<InlineAsm>(U)) {
const InlineAsm *IAF = cast<InlineAsm>(V);
const InlineAsm *IAG = cast<InlineAsm>(U);
return IAF->getAsmString() == IAG->getAsmString() &&
IAF->getConstraintString() == IAG->getConstraintString();
}
return false;
}
static bool equals(const BasicBlock *BB1, const BasicBlock *BB2,
DenseMap<const Value *, const Value *> &ValueMap,
DenseMap<const Value *, const Value *> &SpeculationMap) {
// Specutively add it anyways. If it's false, we'll notice a difference later, and
// this won't matter.
ValueMap[BB1] = BB2;
BasicBlock::const_iterator FI = BB1->begin(), FE = BB1->end();
BasicBlock::const_iterator GI = BB2->begin(), GE = BB2->end();
do {
if (!FI->isSameOperationAs(const_cast<Instruction *>(&*GI)))
return false;
if (FI->getNumOperands() != GI->getNumOperands())
return false;
if (ValueMap[FI] == GI) {
++FI, ++GI;
continue;
}
if (ValueMap[FI] != NULL)
return false;
for (unsigned i = 0, e = FI->getNumOperands(); i != e; ++i) {
Value *OpF = FI->getOperand(i);
Value *OpG = GI->getOperand(i);
if (ValueMap[OpF] == OpG)
continue;
if (ValueMap[OpF] != NULL)
return false;
assert(OpF->getType() == OpG->getType() &&
"Two of the same operation has operands of different type.");
if (OpF->getValueID() != OpG->getValueID())
return false;
if (isa<PHINode>(FI)) {
if (SpeculationMap[OpF] == NULL)
SpeculationMap[OpF] = OpG;
else if (SpeculationMap[OpF] != OpG)
return false;
continue;
} else if (isa<BasicBlock>(OpF)) {
assert(isa<TerminatorInst>(FI) &&
"BasicBlock referenced by non-Terminator non-PHI");
// This call changes the ValueMap, hence we can't use
// Value *& = ValueMap[...]
if (!equals(cast<BasicBlock>(OpF), cast<BasicBlock>(OpG), ValueMap,
SpeculationMap))
return false;
} else {
if (!compare(OpF, OpG))
return false;
}
ValueMap[OpF] = OpG;
}
ValueMap[FI] = GI;
++FI, ++GI;
} while (FI != FE && GI != GE);
return FI == FE && GI == GE;
}
static bool equals(const Function *F, const Function *G) {
// We need to recheck everything, but check the things that weren't included
// in the hash first.
if (F->getAttributes() != G->getAttributes())
return false;
if (F->hasGC() != G->hasGC())
return false;
if (F->hasGC() && F->getGC() != G->getGC())
return false;
if (F->hasSection() != G->hasSection())
return false;
if (F->hasSection() && F->getSection() != G->getSection())
return false;
// TODO: if it's internal and only used in direct calls, we could handle this
// case too.
if (F->getCallingConv() != G->getCallingConv())
return false;
// TODO: We want to permit cases where two functions take T* and S* but
// only load or store them into T** and S**.
if (F->getType() != G->getType())
return false;
DenseMap<const Value *, const Value *> ValueMap;
DenseMap<const Value *, const Value *> SpeculationMap;
ValueMap[F] = G;
assert(F->arg_size() == G->arg_size() &&
"Identical functions have a different number of args.");
for (Function::const_arg_iterator fi = F->arg_begin(), gi = G->arg_begin(),
fe = F->arg_end(); fi != fe; ++fi, ++gi)
ValueMap[fi] = gi;
if (!equals(&F->getEntryBlock(), &G->getEntryBlock(), ValueMap,
SpeculationMap))
return false;
for (DenseMap<const Value *, const Value *>::iterator
I = SpeculationMap.begin(), E = SpeculationMap.end(); I != E; ++I) {
if (ValueMap[I->first] != I->second)
return false;
}
return true;
}
static bool fold(std::vector<Function *> &FnVec, unsigned i, unsigned j) {
if (FnVec[i]->mayBeOverridden() && !FnVec[j]->mayBeOverridden())
std::swap(FnVec[i], FnVec[j]);
Function *F = FnVec[i];
Function *G = FnVec[j];
if (!F->mayBeOverridden()) {
if (G->hasInternalLinkage()) {
F->setAlignment(std::max(F->getAlignment(), G->getAlignment()));
G->replaceAllUsesWith(F);
G->eraseFromParent();
++NumFunctionsMerged;
return true;
}
if (G->hasExternalLinkage() || G->hasWeakLinkage()) {
GlobalAlias *GA = new GlobalAlias(G->getType(), G->getLinkage(), "",
F, G->getParent());
F->setAlignment(std::max(F->getAlignment(), G->getAlignment()));
GA->takeName(G);
GA->setVisibility(G->getVisibility());
G->replaceAllUsesWith(GA);
G->eraseFromParent();
++NumFunctionsMerged;
return true;
}
}
if (F->hasWeakLinkage() && G->hasWeakLinkage()) {
GlobalAlias *GA_F = new GlobalAlias(F->getType(), F->getLinkage(), "",
0, F->getParent());
GA_F->takeName(F);
GA_F->setVisibility(F->getVisibility());
F->setAlignment(std::max(F->getAlignment(), G->getAlignment()));
F->replaceAllUsesWith(GA_F);
F->setName("folded." + GA_F->getName());
F->setLinkage(GlobalValue::ExternalLinkage);
GA_F->setAliasee(F);
GlobalAlias *GA_G = new GlobalAlias(G->getType(), G->getLinkage(), "",
F, G->getParent());
GA_G->takeName(G);
GA_G->setVisibility(G->getVisibility());
G->replaceAllUsesWith(GA_G);
G->eraseFromParent();
++NumFunctionsMerged;
return true;
}
DOUT << "Failed on " << F->getName() << " and " << G->getName() << "\n";
++NumMergeFails;
return false;
}
static bool hasAddressTaken(User *U) {
for (User::use_iterator I = U->use_begin(), E = U->use_end(); I != E; ++I) {
User *Use = *I;
// 'call (bitcast @F to ...)' happens a lot.
while (isa<ConstantExpr>(Use) && Use->hasOneUse()) {
Use = *Use->use_begin();
}
if (isa<ConstantExpr>(Use)) {
if (hasAddressTaken(Use))
return true;
}
if (!isa<CallInst>(Use) && !isa<InvokeInst>(Use))
return true;
// Make sure we aren't passing U as a parameter to call instead of the
// callee.
if (CallSite(cast<Instruction>(Use)).hasArgument(U))
return true;
}
return false;
}
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->isIntrinsic())
continue;
if (!F->hasInternalLinkage() && !F->hasExternalLinkage() &&
!F->hasWeakLinkage())
continue;
if (hasAddressTaken(F))
continue;
FnMap[hash(F)].push_back(F);
}
// TODO: instead of running in a loop, we could also fold functions in callgraph
// order. Constructing the CFG probably isn't cheaper than just running in a loop.
bool LocalChanged;
do {
LocalChanged = false;
for (std::map<unsigned long, std::vector<Function *> >::iterator
I = FnMap.begin(), E = FnMap.end(); I != E; ++I) {
DOUT << "size: " << FnMap.size() << "\n";
std::vector<Function *> &FnVec = I->second;
DOUT << "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 = equals(FnVec[i], FnVec[j]);
DOUT << " " << 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;
}