bool GlobalMerge::doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
const BitVector &GlobalSet, Module &M, bool isConst,
unsigned AddrSpace) const {
assert(Globals.size() > 1);
Type *Int32Ty = Type::getInt32Ty(M.getContext());
auto &DL = M.getDataLayout();
LLVM_DEBUG(dbgs() << " Trying to merge set, starts with #"
<< GlobalSet.find_first() << "\n");
bool Changed = false;
ssize_t i = GlobalSet.find_first();
while (i != -1) {
ssize_t j = 0;
uint64_t MergedSize = 0;
std::vector<Type*> Tys;
std::vector<Constant*> Inits;
bool HasExternal = false;
StringRef FirstExternalName;
for (j = i; j != -1; j = GlobalSet.find_next(j)) {
Type *Ty = Globals[j]->getValueType();
MergedSize += DL.getTypeAllocSize(Ty);
if (MergedSize > MaxOffset) {
break;
}
Tys.push_back(Ty);
Inits.push_back(Globals[j]->getInitializer());
if (Globals[j]->hasExternalLinkage() && !HasExternal) {
HasExternal = true;
FirstExternalName = Globals[j]->getName();
}
}
// Exit early if there is only one global to merge.
if (Tys.size() < 2) {
i = j;
continue;
}
// If merged variables doesn't have external linkage, we needn't to expose
// the symbol after merging.
GlobalValue::LinkageTypes Linkage = HasExternal
? GlobalValue::ExternalLinkage
: GlobalValue::InternalLinkage;
StructType *MergedTy = StructType::get(M.getContext(), Tys);
Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
// On Darwin external linkage needs to be preserved, otherwise
// dsymutil cannot preserve the debug info for the merged
// variables. If they have external linkage, use the symbol name
// of the first variable merged as the suffix of global symbol
// name. This avoids a link-time naming conflict for the
// _MergedGlobals symbols.
Twine MergedName =
(IsMachO && HasExternal)
? "_MergedGlobals_" + FirstExternalName
: "_MergedGlobals";
auto MergedLinkage = IsMachO ? Linkage : GlobalValue::PrivateLinkage;
auto *MergedGV = new GlobalVariable(
M, MergedTy, isConst, MergedLinkage, MergedInit, MergedName, nullptr,
GlobalVariable::NotThreadLocal, AddrSpace);
const StructLayout *MergedLayout = DL.getStructLayout(MergedTy);
for (ssize_t k = i, idx = 0; k != j; k = GlobalSet.find_next(k), ++idx) {
GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
std::string Name = Globals[k]->getName();
GlobalValue::DLLStorageClassTypes DLLStorage =
Globals[k]->getDLLStorageClass();
// Copy metadata while adjusting any debug info metadata by the original
// global's offset within the merged global.
MergedGV->copyMetadata(Globals[k], MergedLayout->getElementOffset(idx));
Constant *Idx[2] = {
ConstantInt::get(Int32Ty, 0),
ConstantInt::get(Int32Ty, idx),
};
Constant *GEP =
ConstantExpr::getInBoundsGetElementPtr(MergedTy, MergedGV, Idx);
Globals[k]->replaceAllUsesWith(GEP);
Globals[k]->eraseFromParent();
// When the linkage is not internal we must emit an alias for the original
// variable name as it may be accessed from another object. On non-Mach-O
// we can also emit an alias for internal linkage as it's safe to do so.
// It's not safe on Mach-O as the alias (and thus the portion of the
// MergedGlobals variable) may be dead stripped at link time.
if (Linkage != GlobalValue::InternalLinkage || !IsMachO) {
GlobalAlias *GA =
GlobalAlias::create(Tys[idx], AddrSpace, Linkage, Name, GEP, &M);
GA->setDLLStorageClass(DLLStorage);
}
NumMerged++;
}
Changed = true;
i = j;
}
return Changed;
}
