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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".
// ===---------------------------------------------------------------------===//
#define DEBUG_TYPE "global-merge"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Attributes.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/Intrinsics.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetLoweringObjectFile.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;
STATISTIC(NumMerged , "Number of globals merged");
namespace {
class GlobalMerge : public FunctionPass {
/// TLI - Keep a pointer of a TargetLowering to consult for determining
/// target type sizes.
const TargetLowering *TLI;
bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
Module &M, bool isConst) const;
public:
static char ID; // Pass identification, replacement for typeid.
explicit GlobalMerge(const TargetLowering *tli = 0)
: FunctionPass(ID), TLI(tli) {
initializeGlobalMergePass(*PassRegistry::getPassRegistry());
}
virtual bool doInitialization(Module &M);
virtual bool runOnFunction(Function &F);
const char *getPassName() const {
return "Merge internal globals";
}
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
FunctionPass::getAnalysisUsage(AU);
}
struct GlobalCmp {
const TargetData *TD;
GlobalCmp(const TargetData *td) : TD(td) { }
bool operator()(const GlobalVariable *GV1, const GlobalVariable *GV2) {
Type *Ty1 = cast<PointerType>(GV1->getType())->getElementType();
Type *Ty2 = cast<PointerType>(GV2->getType())->getElementType();
return (TD->getTypeAllocSize(Ty1) < TD->getTypeAllocSize(Ty2));
}
};
};
} // end anonymous namespace
char GlobalMerge::ID = 0;
INITIALIZE_PASS(GlobalMerge, "global-merge",
"Global Merge", false, false)
bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
Module &M, bool isConst) const {
const TargetData *TD = TLI->getTargetData();
// FIXME: Infer the maximum possible offset depending on the actual users
// (these max offsets are different for the users inside Thumb or ARM
// functions)
unsigned MaxOffset = TLI->getMaximalGlobalOffset();
// FIXME: Find better heuristics
std::stable_sort(Globals.begin(), Globals.end(), GlobalCmp(TD));
Type *Int32Ty = Type::getInt32Ty(M.getContext());
for (size_t i = 0, e = Globals.size(); i != e; ) {
size_t j = 0;
uint64_t MergedSize = 0;
std::vector<Type*> Tys;
std::vector<Constant*> Inits;
for (j = i; j != e; ++j) {
Type *Ty = Globals[j]->getType()->getElementType();
MergedSize += TD->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::InternalLinkage,
MergedInit, "_MergedGlobals");
for (size_t k = i; k < j; ++k) {
Constant *Idx[2] = {
ConstantInt::get(Int32Ty, 0),
ConstantInt::get(Int32Ty, k-i)
};
Constant *GEP = ConstantExpr::getInBoundsGetElementPtr(MergedGV, Idx);
Globals[k]->replaceAllUsesWith(GEP);
Globals[k]->eraseFromParent();
NumMerged++;
}
i = j;
}
return true;
}
bool GlobalMerge::doInitialization(Module &M) {
SmallVector<GlobalVariable*, 16> Globals, ConstGlobals, BSSGlobals;
const TargetData *TD = TLI->getTargetData();
unsigned MaxOffset = TLI->getMaximalGlobalOffset();
bool Changed = false;
// 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 globals only
if (!I->hasLocalLinkage() || I->isThreadLocal() || I->hasSection())
continue;
// Ignore fancy-aligned globals for now.
unsigned Alignment = TD->getPreferredAlignment(I);
Type *Ty = I->getType()->getElementType();
if (Alignment > TD->getABITypeAlignment(Ty))
continue;
// Ignore all 'special' globals.
if (I->getName().startswith("llvm.") ||
I->getName().startswith(".llvm."))
continue;
if (TD->getTypeAllocSize(Ty) < MaxOffset) {
if (TargetLoweringObjectFile::getKindForGlobal(I, TLI->getTargetMachine())
.isBSSLocal())
BSSGlobals.push_back(I);
else if (I->isConstant())
ConstGlobals.push_back(I);
else
Globals.push_back(I);
}
}
if (Globals.size() > 1)
Changed |= doMerge(Globals, M, false);
if (BSSGlobals.size() > 1)
Changed |= doMerge(BSSGlobals, M, false);
// FIXME: This currently breaks the EH processing due to way how the
// typeinfo detection works. We might want to detect the TIs and ignore
// them in the future.
// if (ConstGlobals.size() > 1)
// Changed |= doMerge(ConstGlobals, M, true);
return Changed;
}
bool GlobalMerge::runOnFunction(Function &F) {
return false;
}
Pass *llvm::createGlobalMergePass(const TargetLowering *tli) {
return new GlobalMerge(tli);
}
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