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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) {
//[i] =[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/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 <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <string>
#include <vector>
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"),
static cl::opt<unsigned>
GlobalMergeMaxOffset("global-merge-max-offset", cl::Hidden,
cl::desc("Set maximum offset for global merge pass"),
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"),
static cl::opt<bool>
EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
cl::desc("Enable global merge pass on constants"),
// 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 = 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<GlobalVariable*> &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<GlobalVariable *> &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<const GlobalVariable *, 16> MustKeepGlobalVariables;
static char ID; // Pass identification, replacement for typeid.
explicit GlobalMerge()
: FunctionPass(ID), MaxOffset(GlobalMergeMaxOffset) {
explicit GlobalMerge(const TargetMachine *TM, unsigned MaximalOffset,
bool OnlyOptimizeForSize, bool MergeExternalGlobals)
: FunctionPass(ID), TM(TM), MaxOffset(MaximalOffset),
MergeExternalGlobals(MergeExternalGlobals) {
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 {
} // end anonymous namespace
char GlobalMerge::ID = 0;
INITIALIZE_PASS(GlobalMerge, DEBUG_TYPE, "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()) <
// If we want to just blindly group all globals together, do so.
if (!GlobalMergeGroupByUse) {
BitVector AllGlobals(Globals.size());
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<UsedGlobalSet> UsedGlobalSets;
// Avoid repeating the create-global-set pattern.
auto CreateGlobalSet = [&]() -> UsedGlobalSet & {
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.
// 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())
UI = &*CE->use_begin();
UE = nullptr;
} else if (isa<Instruction>(U.getUser())) {
UI = &U;
UE = UI->getNext();
} else {
// 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)
Function *ParentFn = I->getParent()->getParent();
// If we're only optimizing for size, ignore non-minsize functions.
if (OnlyOptimizeForSize && !ParentFn->optForMinSize())
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();
} else {
GlobalUsesByFunction[ParentFn] = CurGVOnlySetIdx;
// If we already encountered this BB, just increment the counter.
if (UsedGlobalSets[UGSIdx].Globals.test(GI)) {
// If not, the previous set wasn't actually used in this function.
// If we already expanded the previous set to include this global, just
// reuse that expanded set.
if (size_t ExpandedIdx = EncounteredUGS[UGSIdx]) {
GlobalUsesByFunction[ParentFn] = ExpandedIdx;
// 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] =
UsedGlobalSet &NewUGS = CreateGlobalSet();
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::stable_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)
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)
if (PickedGlobals.anyCommon(UGS.Globals))
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)
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());
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<Type*> Tys;
std::vector<Constant*> Inits;
std::vector<unsigned> StructIdxs;
bool HasExternal = false;
StringRef FirstExternalName;
unsigned MaxAlign = 1;
unsigned CurIdx = 0;
for (j = i; j != -1; j = GlobalSet.find_next(j)) {
Type *Ty = Globals[j]->getValueType();
unsigned Align = DL.getPreferredAlignment(Globals[j]);
unsigned Padding = alignTo(MergedSize, Align) - MergedSize;
MergedSize += Padding;
MergedSize += DL.getTypeAllocSize(Ty);
if (MergedSize > MaxOffset) {
if (Padding) {
Tys.push_back(ArrayType::get(Int8Ty, Padding));
MaxAlign = std::max(MaxAlign, Align);
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;
// 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);
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 =
// Copy metadata while adjusting any debug info metadata by the original
// global's offset within the merged global.
Constant *Idx[2] = {
ConstantInt::get(Int32Ty, 0),
ConstantInt::get(Int32Ty, StructIdxs[idx]),
Constant *GEP =
ConstantExpr::getInBoundsGetElementPtr(MergedTy, MergedGV, Idx);
// 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);
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<ConstantArray>(GV->getInitializer());
for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
if (const GlobalVariable *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())
// Keep globals used by landingpads and catchpads.
for (const Use &U : Pad->operands()) {
if (const GlobalVariable *GV =
bool GlobalMerge::doInitialization(Module &M) {
if (!EnableGlobalMerge)
return false;
IsMachO = Triple(M.getTargetTriple()).isOSBinFormatMachO();
auto &DL = M.getDataLayout();
DenseMap<unsigned, SmallVector<GlobalVariable *, 16>> Globals, ConstGlobals,
bool Changed = false;
// 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() || GV.hasImplicitSection())
// It's not safe to merge globals that may be preempted
if (TM && !TM->shouldAssumeDSOLocal(M, &GV))
if (!(MergeExternalGlobals && GV.hasExternalLinkage()) &&
PointerType *PT = dyn_cast<PointerType>(GV.getType());
assert(PT && "Global variable is not a pointer!");
unsigned AddressSpace = PT->getAddressSpace();
// Ignore all 'special' globals.
if (GV.getName().startswith("llvm.") ||
// Ignore all "required" globals:
if (isMustKeepGlobalVariable(&GV))
Type *Ty = GV.getValueType();
if (DL.getTypeAllocSize(Ty) < MaxOffset) {
if (TM &&
TargetLoweringObjectFile::getKindForGlobal(&GV, *TM).isBSSLocal())
else if (GV.isConstant())
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) {
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);