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//===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the interface to a pass that merges duplicate global
// constants together into a single constant that is shared. This is useful
// because some passes (ie TraceValues) insert a lot of string constants into
// the program, regardless of whether or not an existing string is available.
//
// Algorithm: ConstantMerge is designed to build up a map of available constants
// and eliminate duplicates when it is initialized.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO/ConstantMerge.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Transforms/IPO.h"
#include <algorithm>
#include <cassert>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "constmerge"
STATISTIC(NumMerged, "Number of global constants merged");
/// Find values that are marked as llvm.used.
static void FindUsedValues(GlobalVariable *LLVMUsed,
SmallPtrSetImpl<const GlobalValue*> &UsedValues) {
if (!LLVMUsed) return;
ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i) {
Value *Operand = Inits->getOperand(i)->stripPointerCastsNoFollowAliases();
GlobalValue *GV = cast<GlobalValue>(Operand);
UsedValues.insert(GV);
}
}
// True if A is better than B.
static bool IsBetterCanonical(const GlobalVariable &A,
const GlobalVariable &B) {
if (!A.hasLocalLinkage() && B.hasLocalLinkage())
return true;
if (A.hasLocalLinkage() && !B.hasLocalLinkage())
return false;
return A.hasGlobalUnnamedAddr();
}
static bool hasMetadataOtherThanDebugLoc(const GlobalVariable *GV) {
SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
GV->getAllMetadata(MDs);
for (const auto &V : MDs)
if (V.first != LLVMContext::MD_dbg)
return true;
return false;
}
static void copyDebugLocMetadata(const GlobalVariable *From,
GlobalVariable *To) {
SmallVector<DIGlobalVariableExpression *, 1> MDs;
From->getDebugInfo(MDs);
for (auto MD : MDs)
To->addDebugInfo(MD);
}
static unsigned getAlignment(GlobalVariable *GV) {
unsigned Align = GV->getAlignment();
if (Align)
return Align;
return GV->getParent()->getDataLayout().getPreferredAlignment(GV);
}
static bool mergeConstants(Module &M) {
// Find all the globals that are marked "used". These cannot be merged.
SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
// Map unique constants to globals.
DenseMap<Constant *, GlobalVariable *> CMap;
// Replacements - This vector contains a list of replacements to perform.
SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
bool MadeChange = false;
// XXX EMSCRIPTEN: mark @__init_array_start as not to be touched
const GlobalValue *InitArrayStart = M.getNamedGlobal("__init_array_start");
// Iterate constant merging while we are still making progress. Merging two
// constants together may allow us to merge other constants together if the
// second level constants have initializers which point to the globals that
// were just merged.
while (true) {
// First: Find the canonical constants others will be merged with.
for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
GVI != E; ) {
GlobalVariable *GV = &*GVI++;
// XXX EMSCRIPTEN: mark @__init_array_start as not to be touched
if (GV == InitArrayStart)
continue;
// If this GV is dead, remove it.
GV->removeDeadConstantUsers();
if (GV->use_empty() && GV->hasLocalLinkage()) {
GV->eraseFromParent();
continue;
}
// Only process constants with initializers in the default address space.
if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
// Don't touch values marked with attribute(used).
UsedGlobals.count(GV))
continue;
// This transformation is legal for weak ODR globals in the sense it
// doesn't change semantics, but we really don't want to perform it
// anyway; it's likely to pessimize code generation, and some tools
// (like the Darwin linker in cases involving CFString) don't expect it.
if (GV->isWeakForLinker())
continue;
// Don't touch globals with metadata other then !dbg.
if (hasMetadataOtherThanDebugLoc(GV))
continue;
Constant *Init = GV->getInitializer();
// Check to see if the initializer is already known.
GlobalVariable *&Slot = CMap[Init];
// If this is the first constant we find or if the old one is local,
// replace with the current one. If the current is externally visible
// it cannot be replace, but can be the canonical constant we merge with.
if (!Slot || IsBetterCanonical(*GV, *Slot))
Slot = GV;
}
// Second: identify all globals that can be merged together, filling in
// the Replacements vector. We cannot do the replacement in this pass
// because doing so may cause initializers of other globals to be rewritten,
// invalidating the Constant* pointers in CMap.
for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
GVI != E; ) {
GlobalVariable *GV = &*GVI++;
// Only process constants with initializers in the default address space.
if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
// Don't touch values marked with attribute(used).
UsedGlobals.count(GV))
continue;
// We can only replace constant with local linkage.
if (!GV->hasLocalLinkage())
continue;
Constant *Init = GV->getInitializer();
// Check to see if the initializer is already known.
GlobalVariable *Slot = CMap[Init];
if (!Slot || Slot == GV)
continue;
if (!Slot->hasGlobalUnnamedAddr() && !GV->hasGlobalUnnamedAddr())
continue;
if (hasMetadataOtherThanDebugLoc(GV))
continue;
if (!GV->hasGlobalUnnamedAddr())
Slot->setUnnamedAddr(GlobalValue::UnnamedAddr::None);
// Make all uses of the duplicate constant use the canonical version.
Replacements.push_back(std::make_pair(GV, Slot));
}
if (Replacements.empty())
return MadeChange;
CMap.clear();
// Now that we have figured out which replacements must be made, do them all
// now. This avoid invalidating the pointers in CMap, which are unneeded
// now.
for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
// Bump the alignment if necessary.
if (Replacements[i].first->getAlignment() ||
Replacements[i].second->getAlignment()) {
Replacements[i].second->setAlignment(
std::max(getAlignment(Replacements[i].first),
getAlignment(Replacements[i].second)));
}
copyDebugLocMetadata(Replacements[i].first, Replacements[i].second);
// Eliminate any uses of the dead global.
Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
// Delete the global value from the module.
assert(Replacements[i].first->hasLocalLinkage() &&
"Refusing to delete an externally visible global variable.");
Replacements[i].first->eraseFromParent();
}
NumMerged += Replacements.size();
Replacements.clear();
}
}
PreservedAnalyses ConstantMergePass::run(Module &M, ModuleAnalysisManager &) {
if (!mergeConstants(M))
return PreservedAnalyses::all();
return PreservedAnalyses::none();
}
namespace {
struct ConstantMergeLegacyPass : public ModulePass {
static char ID; // Pass identification, replacement for typeid
ConstantMergeLegacyPass() : ModulePass(ID) {
initializeConstantMergeLegacyPassPass(*PassRegistry::getPassRegistry());
}
// For this pass, process all of the globals in the module, eliminating
// duplicate constants.
bool runOnModule(Module &M) override {
if (skipModule(M))
return false;
return mergeConstants(M);
}
};
} // end anonymous namespace
char ConstantMergeLegacyPass::ID = 0;
INITIALIZE_PASS(ConstantMergeLegacyPass, "constmerge",
"Merge Duplicate Global Constants", false, false)
ModulePass *llvm::createConstantMergePass() {
return new ConstantMergeLegacyPass();
}