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+//===-- GlobalMerge.cpp - Internal globals merging -----------------------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// 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/Transforms/Scalar.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallBitVector.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/IR/Attributes.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/GlobalVariable.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Target/TargetLowering.h"
+#include "llvm/Target/TargetLoweringObjectFile.h"
+#include "llvm/Target/TargetSubtargetInfo.h"
+#include <algorithm>
+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<bool>
+EnableGlobalMerge("enable-global-merge", cl::Hidden,
+ cl::desc("Enable the global merge pass"),
+ cl::init(true));
+
+static cl::opt<bool> GlobalMergeGroupByUse(
+ "global-merge-group-by-use", cl::Hidden,
+ cl::desc("Improve global merge pass to look at uses"), cl::init(true));
+
+static cl::opt<bool> 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<bool>
+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<cl::boolOrDefault>
+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;
+ // 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;
+
+ /// Whether we should merge global variables that have external linkage.
+ bool MergeExternalGlobals;
+
+ bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
+ Module &M, bool isConst, unsigned AddrSpace) const;
+ /// \brief Merge everything in \p Globals for which the corresponding bit
+ /// in \p GlobalSet is set.
+ bool doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
+ const BitVector &GlobalSet, Module &M, bool isConst,
+ unsigned AddrSpace) const;
+
+ /// \brief 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);
+
+ /// Keep track of the GlobalVariable that must not be merged away
+ SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;
+
+ public:
+ static char ID; // Pass identification, replacement for typeid.
+ explicit GlobalMerge(const TargetMachine *TM = nullptr,
+ unsigned MaximalOffset = 0,
+ bool OnlyOptimizeForSize = false,
+ bool MergeExternalGlobals = false)
+ : 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;
+
+ const char *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_BEGIN(GlobalMerge, "global-merge", "Merge global variables",
+ false, false)
+INITIALIZE_PASS_END(GlobalMerge, "global-merge", "Merge global variables",
+ false, false)
+
+bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
+ Module &M, bool isConst, unsigned AddrSpace) const {
+ auto &DL = M.getDataLayout();
+ // FIXME: Find better heuristics
+ std::stable_sort(Globals.begin(), Globals.end(),
+ [&DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
+ return DL.getTypeAllocSize(GV1->getValueType()) <
+ DL.getTypeAllocSize(GV2->getValueType());
+ });
+
+ // 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 now 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 {
+ UsedGlobalSet(size_t Size) : Globals(Size), UsageCount(1) {}
+ BitVector Globals;
+ unsigned UsageCount;
+ };
+
+ // Each set is unique in UsedGlobalSets.
+ std::vector<UsedGlobalSet> 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<Function *, size_t /*UsedGlobalSetIdx*/> 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<size_t> 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<ConstantExpr>(U.getUser())) {
+ if (CE->use_empty())
+ continue;
+ UI = &*CE->use_begin();
+ UE = nullptr;
+ } else if (isa<Instruction>(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<Instruction>(UI->getUser());
+ if (!I)
+ continue;
+
+ Function *ParentFn = I->getParent()->getParent();
+
+ // If we're only optimizing for size, ignore non-minsize functions.
+ if (OnlyOptimizeForSize && !ParentFn->optForMinSize())
+ 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.
+ std::sort(UsedGlobalSets.begin(), UsedGlobalSets.end(),
+ [](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<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();
+
+ DEBUG(dbgs() << " Trying to merge set, starts with #"
+ << GlobalSet.find_first() << "\n");
+
+ 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;
+
+ 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());
+ }
+
+ StructType *MergedTy = StructType::get(M.getContext(), Tys);
+ Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
+
+ GlobalVariable *MergedGV = new GlobalVariable(
+ M, MergedTy, isConst, GlobalValue::PrivateLinkage, MergedInit,
+ "_MergedGlobals", nullptr, GlobalVariable::NotThreadLocal, AddrSpace);
+
+ 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();
+
+ 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 ||
+ !TM->getTargetTriple().isOSBinFormatMachO()) {
+ GlobalAlias::create(Tys[idx], AddrSpace, Linkage, Name, GEP, &M);
+ }
+
+ NumMerged++;
+ }
+ i = j;
+ }
+
+ return true;
+}
+
+void GlobalMerge::collectUsedGlobalVariables(Module &M) {
+ // Extract global variables from llvm.used array
+ const GlobalVariable *GV = M.getGlobalVariable("llvm.used");
+ if (!GV || !GV->hasInitializer()) return;
+
+ // Should be an array of 'i8*'.
+ const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer());
+
+ for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
+ if (const GlobalVariable *G =
+ dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
+ MustKeepGlobalVariables.insert(G);
+}
+
+void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
+ collectUsedGlobalVariables(M);
+
+ for (Module::iterator IFn = M.begin(), IEndFn = M.end(); IFn != IEndFn;
+ ++IFn) {
+ for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end();
+ IBB != IEndBB; ++IBB) {
+ // Follow the invoke link to find the landing pad instruction
+ const InvokeInst *II = dyn_cast<InvokeInst>(IBB->getTerminator());
+ if (!II) continue;
+
+ const LandingPadInst *LPInst = II->getUnwindDest()->getLandingPadInst();
+ // Look for globals in the clauses of the landing pad instruction
+ for (unsigned Idx = 0, NumClauses = LPInst->getNumClauses();
+ Idx != NumClauses; ++Idx)
+ if (const GlobalVariable *GV =
+ dyn_cast<GlobalVariable>(LPInst->getClause(Idx)
+ ->stripPointerCasts()))
+ MustKeepGlobalVariables.insert(GV);
+ }
+ }
+}
+
+bool GlobalMerge::doInitialization(Module &M) {
+ if (!EnableGlobalMerge)
+ return false;
+
+ auto &DL = M.getDataLayout();
+ DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > 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.hasSection())
+ continue;
+
+ if (!(MergeExternalGlobals && GV.hasExternalLinkage()) &&
+ !GV.hasInternalLinkage())
+ continue;
+
+ PointerType *PT = dyn_cast<PointerType>(GV.getType());
+ assert(PT && "Global variable is not a pointer!");
+
+ unsigned AddressSpace = PT->getAddressSpace();
+
+ // Ignore fancy-aligned globals for now.
+ unsigned Alignment = DL.getPreferredAlignment(&GV);
+ Type *Ty = GV.getValueType();
+ if (Alignment > DL.getABITypeAlignment(Ty))
+ continue;
+
+ // Ignore all 'special' globals.
+ if (GV.getName().startswith("llvm.") ||
+ GV.getName().startswith(".llvm."))
+ continue;
+
+ // Ignore all "required" globals:
+ if (isMustKeepGlobalVariable(&GV))
+ continue;
+
+ if (DL.getTypeAllocSize(Ty) < MaxOffset) {
+ if (TargetLoweringObjectFile::getKindForGlobal(&GV, *TM).isBSSLocal())
+ BSSGlobals[AddressSpace].push_back(&GV);
+ else if (GV.isConstant())
+ ConstGlobals[AddressSpace].push_back(&GV);
+ else
+ Globals[AddressSpace].push_back(&GV);
+ }
+ }
+
+ for (auto &P : Globals)
+ if (P.second.size() > 1)
+ Changed |= doMerge(P.second, M, false, P.first);
+
+ for (auto &P : BSSGlobals)
+ if (P.second.size() > 1)
+ Changed |= doMerge(P.second, M, false, P.first);
+
+ if (EnableGlobalMergeOnConst)
+ for (auto &P : ConstGlobals)
+ if (P.second.size() > 1)
+ Changed |= doMerge(P.second, M, true, P.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);
+}
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