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llvm-6502/lib/CodeGen/GlobalMerge.cpp
Ahmed Bougacha 35e8055393 [GlobalMerge] Look at uses to create smaller global sets. 
Instead of merging everything together, look at the users of
GlobalVariables, and try to group them by function, to create
sets of globals used "together".

Using that information, a less-aggressive alternative is to keep merging
everything together *except* globals that are only ever used alone, that
is, those for which it's clearly non-profitable to merge with others.

In my testing, grouping by Function is too aggressive, but grouping by
BasicBlock is too conservative.  Anything in-between isn't trivially
available, so stick with Function grouping for now.

cl::opts are added for testing; both enabled by default.

A few of the testcases aren't testing the merging proper, but just
various edge cases when merging does occur.  Update them to use the
previous grouping behavior. Also, one of the tests is unrelated to
GlobalMerge; change it accordingly.
While there, switch to r234666' flags rather than the brutal -O3.

Differential Revision: http://reviews.llvm.org/D8070


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235249 91177308-0d34-0410-b5e6-96231b3b80d8
2015-04-18 01:21:58 +00:00

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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<bool>
EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
cl::desc("Enable global merge pass on external linkage"),
cl::init(false));
STATISTIC(NumMerged, "Number of globals merged");
namespace {
class GlobalMerge : public FunctionPass {
const TargetMachine *TM;
const DataLayout *DL;
// 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;
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(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)
: FunctionPass(ID), TM(TM), DL(TM->getDataLayout()),
MaxOffset(MaximalOffset) {
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 {
// FIXME: Find better heuristics
std::stable_sort(Globals.begin(), Globals.end(),
[this](const GlobalVariable *GV1, const GlobalVariable *GV2) {
Type *Ty1 = cast<PointerType>(GV1->getType())->getElementType();
Type *Ty2 = cast<PointerType>(GV2->getType())->getElementType();
return (DL->getTypeAllocSize(Ty1) < DL->getTypeAllocSize(Ty2));
});
// 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 occured.
//
// 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())) {
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();
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(SmallVectorImpl<GlobalVariable *> &Globals,
const BitVector &GlobalSet, Module &M, bool isConst,
unsigned AddrSpace) const {
Type *Int32Ty = Type::getInt32Ty(M.getContext());
assert(Globals.size() > 1);
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;
bool HasExternal = false;
GlobalVariable *TheFirstExternal = 0;
for (j = i; j != -1; j = GlobalSet.find_next(j)) {
Type *Ty = Globals[j]->getType()->getElementType();
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;
TheFirstExternal = Globals[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;
StructType *MergedTy = StructType::get(M.getContext(), Tys);
Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
// If merged variables have external linkage, we use symbol name of the
// first variable merged as the suffix of global symbol name. This would
// be able to avoid the link-time naming conflict for globalm symbols.
GlobalVariable *MergedGV = new GlobalVariable(
M, MergedTy, isConst, Linkage, MergedInit,
HasExternal ? "_MergedGlobals_" + TheFirstExternal->getName()
: "_MergedGlobals",
nullptr, GlobalVariable::NotThreadLocal, AddrSpace);
for (ssize_t k = i, idx = 0; k != j; k = GlobalSet.find_next(k)) {
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();
if (Linkage != GlobalValue::InternalLinkage) {
// Generate a new alias...
auto *PTy = cast<PointerType>(GEP->getType());
GlobalAlias::create(PTy->getElementType(), PTy->getAddressSpace(),
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;
DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
BSSGlobals;
bool Changed = false;
setMustKeepGlobalVariables(M);
// Grab all non-const globals.
for (Module::global_iterator I = M.global_begin(),
E = M.global_end(); I != E; ++I) {
// Merge is safe for "normal" internal or external globals only
if (I->isDeclaration() || I->isThreadLocal() || I->hasSection())
continue;
if (!(EnableGlobalMergeOnExternal && I->hasExternalLinkage()) &&
!I->hasInternalLinkage())
continue;
PointerType *PT = dyn_cast<PointerType>(I->getType());
assert(PT && "Global variable is not a pointer!");
unsigned AddressSpace = PT->getAddressSpace();
// Ignore fancy-aligned globals for now.
unsigned Alignment = DL->getPreferredAlignment(I);
Type *Ty = I->getType()->getElementType();
if (Alignment > DL->getABITypeAlignment(Ty))
continue;
// Ignore all 'special' globals.
if (I->getName().startswith("llvm.") ||
I->getName().startswith(".llvm."))
continue;
// Ignore all "required" globals:
if (isMustKeepGlobalVariable(I))
continue;
if (DL->getTypeAllocSize(Ty) < MaxOffset) {
if (TargetLoweringObjectFile::getKindForGlobal(I, *TM).isBSSLocal())
BSSGlobals[AddressSpace].push_back(I);
else if (I->isConstant())
ConstGlobals[AddressSpace].push_back(I);
else
Globals[AddressSpace].push_back(I);
}
}
for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
I = Globals.begin(), E = Globals.end(); I != E; ++I)
if (I->second.size() > 1)
Changed |= doMerge(I->second, M, false, I->first);
for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
I = BSSGlobals.begin(), E = BSSGlobals.end(); I != E; ++I)
if (I->second.size() > 1)
Changed |= doMerge(I->second, M, false, I->first);
if (EnableGlobalMergeOnConst)
for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
I = ConstGlobals.begin(), E = ConstGlobals.end(); I != E; ++I)
if (I->second.size() > 1)
Changed |= doMerge(I->second, M, true, I->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) {
return new GlobalMerge(TM, Offset);
}
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