llvm-6502/lib/CodeGen/GlobalMerge.cpp
Eric Christopher 308458a98b Rewrite the global merge pass to be subprogram agnostic for now.
It was previously using the subtarget to get values for the global
offset without actually checking each function as it was generating
code. Go ahead and solidify the current behavior and make the
existing FIXMEs more prominent.

As a note the ARM backend previously had a thumb1 and non-thumb1
set of defaults. Only the former was tested so I've changed the
behavior to only use that for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230245 91177308-0d34-0410-b5e6-96231b3b80d8
2015-02-23 19:28:45 +00:00

362 lines
13 KiB
C++

//===-- 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".
// ===---------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.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/Target/TargetLowering.h"
#include "llvm/Target/TargetLoweringObjectFile.h"
#include "llvm/Target/TargetSubtargetInfo.h"
using namespace llvm;
#define DEBUG_TYPE "global-merge"
static cl::opt<bool>
EnableGlobalMerge("enable-global-merge", cl::Hidden,
cl::desc("Enable global merge pass"),
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 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));
});
Type *Int32Ty = Type::getInt32Ty(M.getContext());
assert(Globals.size() > 1);
// FIXME: This simple solution merges globals all together as maximum as
// possible. However, with this solution it would be hard to remove dead
// global symbols at link-time. An alternative solution could be checking
// global symbols references function by function, and make the symbols
// being referred in the same function merged and we would probably need
// to introduce heuristic algorithm to solve the merge conflict from
// different functions.
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;
bool HasExternal = false;
GlobalVariable *TheFirstExternal = 0;
for (j = i; j != e; ++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 (size_t k = i; k < j; ++k) {
GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
std::string Name = Globals[k]->getName();
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();
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);
}