//===- GlobalMerge.cpp - Internal globals merging -------------------------===// // // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. // See https://llvm.org/LICENSE.txt for license information. // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception // //===----------------------------------------------------------------------===// // // This pass merges globals with internal linkage into one. This way all the // globals which were merged into a biggest one can be addressed using offsets // from the same base pointer (no need for separate base pointer for each of the // global). Such a transformation can significantly reduce the register pressure // when many globals are involved. // // For example, consider the code which touches several global variables at // once: // // static int foo[N], bar[N], baz[N]; // // for (i = 0; i < N; ++i) { // foo[i] = bar[i] * baz[i]; // } // // On ARM the addresses of 3 arrays should be kept in the registers, thus // this code has quite large register pressure (loop body): // // ldr r1, [r5], #4 // ldr r2, [r6], #4 // mul r1, r2, r1 // str r1, [r0], #4 // // Pass converts the code to something like: // // static struct { // int foo[N]; // int bar[N]; // int baz[N]; // } merged; // // for (i = 0; i < N; ++i) { // merged.foo[i] = merged.bar[i] * merged.baz[i]; // } // // and in ARM code this becomes: // // ldr r0, [r5, #40] // ldr r1, [r5, #80] // mul r0, r1, r0 // str r0, [r5], #4 // // note that we saved 2 registers here almostly "for free". // // However, merging globals can have tradeoffs: // - it confuses debuggers, tools, and users // - it makes linker optimizations less useful (order files, LOHs, ...) // - it forces usage of indexed addressing (which isn't necessarily "free") // - it can increase register pressure when the uses are disparate enough. // // We use heuristics to discover the best global grouping we can (cf cl::opts). // // ===---------------------------------------------------------------------===// #include "llvm/ADT/BitVector.h" #include "llvm/ADT/DenseMap.h" #include "llvm/ADT/SmallPtrSet.h" #include "llvm/ADT/SmallVector.h" #include "llvm/ADT/Statistic.h" #include "llvm/ADT/StringRef.h" #include "llvm/ADT/Triple.h" #include "llvm/ADT/Twine.h" #include "llvm/CodeGen/Passes.h" #include "llvm/IR/BasicBlock.h" #include "llvm/IR/Constants.h" #include "llvm/IR/DataLayout.h" #include "llvm/IR/DerivedTypes.h" #include "llvm/IR/Function.h" #include "llvm/IR/GlobalAlias.h" #include "llvm/IR/GlobalValue.h" #include "llvm/IR/GlobalVariable.h" #include "llvm/IR/Instruction.h" #include "llvm/IR/Module.h" #include "llvm/IR/Type.h" #include "llvm/IR/Use.h" #include "llvm/IR/User.h" #include "llvm/InitializePasses.h" #include "llvm/MC/SectionKind.h" #include "llvm/Pass.h" #include "llvm/Support/Casting.h" #include "llvm/Support/CommandLine.h" #include "llvm/Support/Debug.h" #include "llvm/Support/raw_ostream.h" #include "llvm/Target/TargetLoweringObjectFile.h" #include "llvm/Target/TargetMachine.h" #include #include #include #include #include #include using namespace llvm; #define DEBUG_TYPE "global-merge" // FIXME: This is only useful as a last-resort way to disable the pass. static cl::opt EnableGlobalMerge("enable-global-merge", cl::Hidden, cl::desc("Enable the global merge pass"), cl::init(true)); static cl::opt GlobalMergeMaxOffset("global-merge-max-offset", cl::Hidden, cl::desc("Set maximum offset for global merge pass"), cl::init(0)); static cl::opt GlobalMergeGroupByUse( "global-merge-group-by-use", cl::Hidden, cl::desc("Improve global merge pass to look at uses"), cl::init(true)); static cl::opt GlobalMergeIgnoreSingleUse( "global-merge-ignore-single-use", cl::Hidden, cl::desc("Improve global merge pass to ignore globals only used alone"), cl::init(true)); static cl::opt EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden, cl::desc("Enable global merge pass on constants"), cl::init(false)); // FIXME: this could be a transitional option, and we probably need to remove // it if only we are sure this optimization could always benefit all targets. static cl::opt EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden, cl::desc("Enable global merge pass on external linkage")); STATISTIC(NumMerged, "Number of globals merged"); namespace { class GlobalMerge : public FunctionPass { const TargetMachine *TM = nullptr; // FIXME: Infer the maximum possible offset depending on the actual users // (these max offsets are different for the users inside Thumb or ARM // functions), see the code that passes in the offset in the ARM backend // for more information. unsigned MaxOffset; /// Whether we should try to optimize for size only. /// Currently, this applies a dead simple heuristic: only consider globals /// used in minsize functions for merging. /// FIXME: This could learn about optsize, and be used in the cost model. bool OnlyOptimizeForSize = false; /// Whether we should merge global variables that have external linkage. bool MergeExternalGlobals = false; bool IsMachO; bool doMerge(SmallVectorImpl &Globals, Module &M, bool isConst, unsigned AddrSpace) const; /// Merge everything in \p Globals for which the corresponding bit /// in \p GlobalSet is set. bool doMerge(const SmallVectorImpl &Globals, const BitVector &GlobalSet, Module &M, bool isConst, unsigned AddrSpace) const; /// Check if the given variable has been identified as must keep /// \pre setMustKeepGlobalVariables must have been called on the Module that /// contains GV bool isMustKeepGlobalVariable(const GlobalVariable *GV) const { return MustKeepGlobalVariables.count(GV); } /// Collect every variables marked as "used" or used in a landing pad /// instruction for this Module. void setMustKeepGlobalVariables(Module &M); /// Collect every variables marked as "used" void collectUsedGlobalVariables(Module &M, StringRef Name); /// Keep track of the GlobalVariable that must not be merged away SmallPtrSet MustKeepGlobalVariables; public: static char ID; // Pass identification, replacement for typeid. explicit GlobalMerge() : FunctionPass(ID), MaxOffset(GlobalMergeMaxOffset) { initializeGlobalMergePass(*PassRegistry::getPassRegistry()); } explicit GlobalMerge(const TargetMachine *TM, unsigned MaximalOffset, bool OnlyOptimizeForSize, bool MergeExternalGlobals) : FunctionPass(ID), TM(TM), MaxOffset(MaximalOffset), OnlyOptimizeForSize(OnlyOptimizeForSize), MergeExternalGlobals(MergeExternalGlobals) { initializeGlobalMergePass(*PassRegistry::getPassRegistry()); } bool doInitialization(Module &M) override; bool runOnFunction(Function &F) override; bool doFinalization(Module &M) override; StringRef getPassName() const override { return "Merge internal globals"; } void getAnalysisUsage(AnalysisUsage &AU) const override { AU.setPreservesCFG(); FunctionPass::getAnalysisUsage(AU); } }; } // end anonymous namespace char GlobalMerge::ID = 0; INITIALIZE_PASS(GlobalMerge, DEBUG_TYPE, "Merge global variables", false, false) bool GlobalMerge::doMerge(SmallVectorImpl &Globals, Module &M, bool isConst, unsigned AddrSpace) const { auto &DL = M.getDataLayout(); // FIXME: Find better heuristics llvm::stable_sort( Globals, [&DL](const GlobalVariable *GV1, const GlobalVariable *GV2) { // We don't support scalable global variables. return DL.getTypeAllocSize(GV1->getValueType()).getFixedSize() < DL.getTypeAllocSize(GV2->getValueType()).getFixedSize(); }); // If we want to just blindly group all globals together, do so. if (!GlobalMergeGroupByUse) { BitVector AllGlobals(Globals.size()); AllGlobals.set(); return doMerge(Globals, AllGlobals, M, isConst, AddrSpace); } // If we want to be smarter, look at all uses of each global, to try to // discover all sets of globals used together, and how many times each of // these sets occurred. // // Keep this reasonably efficient, by having an append-only list of all sets // discovered so far (UsedGlobalSet), and mapping each "together-ness" unit of // code (currently, a Function) to the set of globals seen so far that are // used together in that unit (GlobalUsesByFunction). // // When we look at the Nth global, we know that any new set is either: // - the singleton set {N}, containing this global only, or // - the union of {N} and a previously-discovered set, containing some // combination of the previous N-1 globals. // Using that knowledge, when looking at the Nth global, we can keep: // - a reference to the singleton set {N} (CurGVOnlySetIdx) // - a list mapping each previous set to its union with {N} (EncounteredUGS), // if it actually occurs. // We keep track of the sets of globals used together "close enough". struct UsedGlobalSet { BitVector Globals; unsigned UsageCount = 1; UsedGlobalSet(size_t Size) : Globals(Size) {} }; // Each set is unique in UsedGlobalSets. std::vector UsedGlobalSets; // Avoid repeating the create-global-set pattern. auto CreateGlobalSet = [&]() -> UsedGlobalSet & { UsedGlobalSets.emplace_back(Globals.size()); return UsedGlobalSets.back(); }; // The first set is the empty set. CreateGlobalSet().UsageCount = 0; // We define "close enough" to be "in the same function". // FIXME: Grouping uses by function is way too aggressive, so we should have // a better metric for distance between uses. // The obvious alternative would be to group by BasicBlock, but that's in // turn too conservative.. // Anything in between wouldn't be trivial to compute, so just stick with // per-function grouping. // The value type is an index into UsedGlobalSets. // The default (0) conveniently points to the empty set. DenseMap GlobalUsesByFunction; // Now, look at each merge-eligible global in turn. // Keep track of the sets we already encountered to which we added the // current global. // Each element matches the same-index element in UsedGlobalSets. // This lets us efficiently tell whether a set has already been expanded to // include the current global. std::vector EncounteredUGS; for (size_t GI = 0, GE = Globals.size(); GI != GE; ++GI) { GlobalVariable *GV = Globals[GI]; // Reset the encountered sets for this global... std::fill(EncounteredUGS.begin(), EncounteredUGS.end(), 0); // ...and grow it in case we created new sets for the previous global. EncounteredUGS.resize(UsedGlobalSets.size()); // We might need to create a set that only consists of the current global. // Keep track of its index into UsedGlobalSets. size_t CurGVOnlySetIdx = 0; // For each global, look at all its Uses. for (auto &U : GV->uses()) { // This Use might be a ConstantExpr. We're interested in Instruction // users, so look through ConstantExpr... Use *UI, *UE; if (ConstantExpr *CE = dyn_cast(U.getUser())) { if (CE->use_empty()) continue; UI = &*CE->use_begin(); UE = nullptr; } else if (isa(U.getUser())) { UI = &U; UE = UI->getNext(); } else { continue; } // ...to iterate on all the instruction users of the global. // Note that we iterate on Uses and not on Users to be able to getNext(). for (; UI != UE; UI = UI->getNext()) { Instruction *I = dyn_cast(UI->getUser()); if (!I) continue; Function *ParentFn = I->getParent()->getParent(); // If we're only optimizing for size, ignore non-minsize functions. if (OnlyOptimizeForSize && !ParentFn->hasMinSize()) continue; size_t UGSIdx = GlobalUsesByFunction[ParentFn]; // If this is the first global the basic block uses, map it to the set // consisting of this global only. if (!UGSIdx) { // If that set doesn't exist yet, create it. if (!CurGVOnlySetIdx) { CurGVOnlySetIdx = UsedGlobalSets.size(); CreateGlobalSet().Globals.set(GI); } else { ++UsedGlobalSets[CurGVOnlySetIdx].UsageCount; } GlobalUsesByFunction[ParentFn] = CurGVOnlySetIdx; continue; } // If we already encountered this BB, just increment the counter. if (UsedGlobalSets[UGSIdx].Globals.test(GI)) { ++UsedGlobalSets[UGSIdx].UsageCount; continue; } // If not, the previous set wasn't actually used in this function. --UsedGlobalSets[UGSIdx].UsageCount; // If we already expanded the previous set to include this global, just // reuse that expanded set. if (size_t ExpandedIdx = EncounteredUGS[UGSIdx]) { ++UsedGlobalSets[ExpandedIdx].UsageCount; GlobalUsesByFunction[ParentFn] = ExpandedIdx; continue; } // If not, create a new set consisting of the union of the previous set // and this global. Mark it as encountered, so we can reuse it later. GlobalUsesByFunction[ParentFn] = EncounteredUGS[UGSIdx] = UsedGlobalSets.size(); UsedGlobalSet &NewUGS = CreateGlobalSet(); NewUGS.Globals.set(GI); NewUGS.Globals |= UsedGlobalSets[UGSIdx].Globals; } } } // Now we found a bunch of sets of globals used together. We accumulated // the number of times we encountered the sets (i.e., the number of blocks // that use that exact set of globals). // // Multiply that by the size of the set to give us a crude profitability // metric. llvm::stable_sort(UsedGlobalSets, [](const UsedGlobalSet &UGS1, const UsedGlobalSet &UGS2) { return UGS1.Globals.count() * UGS1.UsageCount < UGS2.Globals.count() * UGS2.UsageCount; }); // We can choose to merge all globals together, but ignore globals never used // with another global. This catches the obviously non-profitable cases of // having a single global, but is aggressive enough for any other case. if (GlobalMergeIgnoreSingleUse) { BitVector AllGlobals(Globals.size()); for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) { const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1]; if (UGS.UsageCount == 0) continue; if (UGS.Globals.count() > 1) AllGlobals |= UGS.Globals; } return doMerge(Globals, AllGlobals, M, isConst, AddrSpace); } // Starting from the sets with the best (=biggest) profitability, find a // good combination. // The ideal (and expensive) solution can only be found by trying all // combinations, looking for the one with the best profitability. // Don't be smart about it, and just pick the first compatible combination, // starting with the sets with the best profitability. BitVector PickedGlobals(Globals.size()); bool Changed = false; for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) { const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1]; if (UGS.UsageCount == 0) continue; if (PickedGlobals.anyCommon(UGS.Globals)) continue; PickedGlobals |= UGS.Globals; // If the set only contains one global, there's no point in merging. // Ignore the global for inclusion in other sets though, so keep it in // PickedGlobals. if (UGS.Globals.count() < 2) continue; Changed |= doMerge(Globals, UGS.Globals, M, isConst, AddrSpace); } return Changed; } bool GlobalMerge::doMerge(const SmallVectorImpl &Globals, const BitVector &GlobalSet, Module &M, bool isConst, unsigned AddrSpace) const { assert(Globals.size() > 1); Type *Int32Ty = Type::getInt32Ty(M.getContext()); Type *Int8Ty = Type::getInt8Ty(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 Tys; std::vector Inits; std::vector StructIdxs; bool HasExternal = false; StringRef FirstExternalName; Align MaxAlign; unsigned CurIdx = 0; for (j = i; j != -1; j = GlobalSet.find_next(j)) { Type *Ty = Globals[j]->getValueType(); // Make sure we use the same alignment AsmPrinter would use. Align Alignment = DL.getPreferredAlign(Globals[j]); unsigned Padding = alignTo(MergedSize, Alignment) - MergedSize; MergedSize += Padding; MergedSize += DL.getTypeAllocSize(Ty); if (MergedSize > MaxOffset) { break; } if (Padding) { Tys.push_back(ArrayType::get(Int8Ty, Padding)); Inits.push_back(ConstantAggregateZero::get(Tys.back())); ++CurIdx; } Tys.push_back(Ty); Inits.push_back(Globals[j]->getInitializer()); StructIdxs.push_back(CurIdx++); MaxAlign = std::max(MaxAlign, Alignment); 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; // Use a packed struct so we can control alignment. StructType *MergedTy = StructType::get(M.getContext(), Tys, true); 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); MergedGV->setAlignment(MaxAlign); MergedGV->setSection(Globals[i]->getSection()); 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::VisibilityTypes Visibility = Globals[k]->getVisibility(); 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(StructIdxs[idx])); Constant *Idx[2] = { ConstantInt::get(Int32Ty, 0), ConstantInt::get(Int32Ty, StructIdxs[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[StructIdxs[idx]], AddrSpace, Linkage, Name, GEP, &M); GA->setVisibility(Visibility); GA->setDLLStorageClass(DLLStorage); } NumMerged++; } Changed = true; i = j; } return Changed; } void GlobalMerge::collectUsedGlobalVariables(Module &M, StringRef Name) { // Extract global variables from llvm.used array const GlobalVariable *GV = M.getGlobalVariable(Name); if (!GV || !GV->hasInitializer()) return; // Should be an array of 'i8*'. const ConstantArray *InitList = cast(GV->getInitializer()); for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i) if (const GlobalVariable *G = dyn_cast(InitList->getOperand(i)->stripPointerCasts())) MustKeepGlobalVariables.insert(G); } void GlobalMerge::setMustKeepGlobalVariables(Module &M) { collectUsedGlobalVariables(M, "llvm.used"); collectUsedGlobalVariables(M, "llvm.compiler.used"); for (Function &F : M) { for (BasicBlock &BB : F) { Instruction *Pad = BB.getFirstNonPHI(); if (!Pad->isEHPad()) continue; // Keep globals used by landingpads and catchpads. for (const Use &U : Pad->operands()) { if (const GlobalVariable *GV = dyn_cast(U->stripPointerCasts())) MustKeepGlobalVariables.insert(GV); } } } } bool GlobalMerge::doInitialization(Module &M) { if (!EnableGlobalMerge) return false; IsMachO = Triple(M.getTargetTriple()).isOSBinFormatMachO(); auto &DL = M.getDataLayout(); DenseMap, SmallVector> Globals, ConstGlobals, BSSGlobals; bool Changed = false; setMustKeepGlobalVariables(M); // Grab all non-const globals. for (auto &GV : M.globals()) { // Merge is safe for "normal" internal or external globals only if (GV.isDeclaration() || GV.isThreadLocal() || GV.hasImplicitSection()) continue; // It's not safe to merge globals that may be preempted if (TM && !TM->shouldAssumeDSOLocal(M, &GV)) continue; if (!(MergeExternalGlobals && GV.hasExternalLinkage()) && !GV.hasInternalLinkage()) continue; PointerType *PT = dyn_cast(GV.getType()); assert(PT && "Global variable is not a pointer!"); unsigned AddressSpace = PT->getAddressSpace(); StringRef Section = GV.getSection(); // Ignore all 'special' globals. if (GV.getName().startswith("llvm.") || GV.getName().startswith(".llvm.")) continue; // Ignore all "required" globals: if (isMustKeepGlobalVariable(&GV)) continue; Type *Ty = GV.getValueType(); if (DL.getTypeAllocSize(Ty) < MaxOffset) { if (TM && TargetLoweringObjectFile::getKindForGlobal(&GV, *TM).isBSS()) BSSGlobals[{AddressSpace, Section}].push_back(&GV); else if (GV.isConstant()) ConstGlobals[{AddressSpace, Section}].push_back(&GV); else Globals[{AddressSpace, Section}].push_back(&GV); } } for (auto &P : Globals) if (P.second.size() > 1) Changed |= doMerge(P.second, M, false, P.first.first); for (auto &P : BSSGlobals) if (P.second.size() > 1) Changed |= doMerge(P.second, M, false, P.first.first); if (EnableGlobalMergeOnConst) for (auto &P : ConstGlobals) if (P.second.size() > 1) Changed |= doMerge(P.second, M, true, P.first.first); return Changed; } bool GlobalMerge::runOnFunction(Function &F) { return false; } bool GlobalMerge::doFinalization(Module &M) { MustKeepGlobalVariables.clear(); return false; } Pass *llvm::createGlobalMergePass(const TargetMachine *TM, unsigned Offset, bool OnlyOptimizeForSize, bool MergeExternalByDefault) { bool MergeExternal = (EnableGlobalMergeOnExternal == cl::BOU_UNSET) ? MergeExternalByDefault : (EnableGlobalMergeOnExternal == cl::BOU_TRUE); return new GlobalMerge(TM, Offset, OnlyOptimizeForSize, MergeExternal); }