1 //===-- GlobalMerge.cpp - Internal globals merging -----------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
9 // This pass merges globals with internal linkage into one. This way all the
10 // globals which were merged into a biggest one can be addressed using offsets
11 // from the same base pointer (no need for separate base pointer for each of the
12 // global). Such a transformation can significantly reduce the register pressure
13 // when many globals are involved.
15 // For example, consider the code which touches several global variables at
18 // static int foo[N], bar[N], baz[N];
20 // for (i = 0; i < N; ++i) {
21 // foo[i] = bar[i] * baz[i];
24 // On ARM the addresses of 3 arrays should be kept in the registers, thus
25 // this code has quite large register pressure (loop body):
32 // Pass converts the code to something like:
40 // for (i = 0; i < N; ++i) {
41 // merged.foo[i] = merged.bar[i] * merged.baz[i];
44 // and in ARM code this becomes:
51 // note that we saved 2 registers here almostly "for free".
52 // ===---------------------------------------------------------------------===//
54 #define DEBUG_TYPE "global-merge"
55 #include "llvm/Transforms/Scalar.h"
56 #include "llvm/ADT/SmallPtrSet.h"
57 #include "llvm/ADT/Statistic.h"
58 #include "llvm/IR/Attributes.h"
59 #include "llvm/IR/Constants.h"
60 #include "llvm/IR/DataLayout.h"
61 #include "llvm/IR/DerivedTypes.h"
62 #include "llvm/IR/Function.h"
63 #include "llvm/IR/GlobalVariable.h"
64 #include "llvm/IR/Instructions.h"
65 #include "llvm/IR/Intrinsics.h"
66 #include "llvm/IR/Module.h"
67 #include "llvm/Pass.h"
68 #include "llvm/Support/CommandLine.h"
69 #include "llvm/Target/TargetLowering.h"
70 #include "llvm/Target/TargetLoweringObjectFile.h"
74 EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
75 cl::desc("Enable global merge pass on constants"),
78 STATISTIC(NumMerged , "Number of globals merged");
80 class GlobalMerge : public FunctionPass {
81 const TargetMachine *TM;
83 bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
84 Module &M, bool isConst, unsigned AddrSpace) const;
86 /// \brief Check if the given variable has been identified as must keep
87 /// \pre setMustKeepGlobalVariables must have been called on the Module that
89 bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
90 return MustKeepGlobalVariables.count(GV);
93 /// Collect every variables marked as "used" or used in a landing pad
94 /// instruction for this Module.
95 void setMustKeepGlobalVariables(Module &M);
97 /// Collect every variables marked as "used"
98 void collectUsedGlobalVariables(Module &M);
100 /// Keep track of the GlobalVariable that must not be merged away
101 SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;
104 static char ID; // Pass identification, replacement for typeid.
105 explicit GlobalMerge(const TargetMachine *TM = 0)
106 : FunctionPass(ID), TM(TM) {
107 initializeGlobalMergePass(*PassRegistry::getPassRegistry());
110 virtual bool doInitialization(Module &M);
111 virtual bool runOnFunction(Function &F);
112 virtual bool doFinalization(Module &M);
114 const char *getPassName() const {
115 return "Merge internal globals";
118 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
119 AU.setPreservesCFG();
120 FunctionPass::getAnalysisUsage(AU);
124 const DataLayout *TD;
126 GlobalCmp(const DataLayout *td) : TD(td) { }
128 bool operator()(const GlobalVariable *GV1, const GlobalVariable *GV2) {
129 Type *Ty1 = cast<PointerType>(GV1->getType())->getElementType();
130 Type *Ty2 = cast<PointerType>(GV2->getType())->getElementType();
132 return (TD->getTypeAllocSize(Ty1) < TD->getTypeAllocSize(Ty2));
136 } // end anonymous namespace
138 char GlobalMerge::ID = 0;
139 INITIALIZE_PASS(GlobalMerge, "global-merge",
140 "Global Merge", false, false)
143 bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
144 Module &M, bool isConst, unsigned AddrSpace) const {
145 const TargetLowering *TLI = TM->getTargetLowering();
146 const DataLayout *TD = TLI->getDataLayout();
148 // FIXME: Infer the maximum possible offset depending on the actual users
149 // (these max offsets are different for the users inside Thumb or ARM
151 unsigned MaxOffset = TLI->getMaximalGlobalOffset();
153 // FIXME: Find better heuristics
154 std::stable_sort(Globals.begin(), Globals.end(), GlobalCmp(TD));
156 Type *Int32Ty = Type::getInt32Ty(M.getContext());
158 for (size_t i = 0, e = Globals.size(); i != e; ) {
160 uint64_t MergedSize = 0;
161 std::vector<Type*> Tys;
162 std::vector<Constant*> Inits;
163 for (j = i; j != e; ++j) {
164 Type *Ty = Globals[j]->getType()->getElementType();
165 MergedSize += TD->getTypeAllocSize(Ty);
166 if (MergedSize > MaxOffset) {
170 Inits.push_back(Globals[j]->getInitializer());
173 StructType *MergedTy = StructType::get(M.getContext(), Tys);
174 Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
175 GlobalVariable *MergedGV = new GlobalVariable(M, MergedTy, isConst,
176 GlobalValue::InternalLinkage,
177 MergedInit, "_MergedGlobals",
178 0, GlobalVariable::NotThreadLocal,
180 for (size_t k = i; k < j; ++k) {
182 ConstantInt::get(Int32Ty, 0),
183 ConstantInt::get(Int32Ty, k-i)
185 Constant *GEP = ConstantExpr::getInBoundsGetElementPtr(MergedGV, Idx);
186 Globals[k]->replaceAllUsesWith(GEP);
187 Globals[k]->eraseFromParent();
196 void GlobalMerge::collectUsedGlobalVariables(Module &M) {
197 // Extract global variables from llvm.used array
198 const GlobalVariable *GV = M.getGlobalVariable("llvm.used");
199 if (!GV || !GV->hasInitializer()) return;
201 // Should be an array of 'i8*'.
202 const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer());
204 for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
205 if (const GlobalVariable *G =
206 dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
207 MustKeepGlobalVariables.insert(G);
210 void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
211 collectUsedGlobalVariables(M);
213 for (Module::iterator IFn = M.begin(), IEndFn = M.end(); IFn != IEndFn;
215 for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end();
216 IBB != IEndBB; ++IBB) {
217 // Follow the inwoke link to find the landing pad instruction
218 const InvokeInst *II = dyn_cast<InvokeInst>(IBB->getTerminator());
221 const LandingPadInst *LPInst = II->getUnwindDest()->getLandingPadInst();
222 // Look for globals in the clauses of the landing pad instruction
223 for (unsigned Idx = 0, NumClauses = LPInst->getNumClauses();
224 Idx != NumClauses; ++Idx)
225 if (const GlobalVariable *GV =
226 dyn_cast<GlobalVariable>(LPInst->getClause(Idx)
227 ->stripPointerCasts()))
228 MustKeepGlobalVariables.insert(GV);
233 bool GlobalMerge::doInitialization(Module &M) {
234 DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
236 const TargetLowering *TLI = TM->getTargetLowering();
237 const DataLayout *TD = TLI->getDataLayout();
238 unsigned MaxOffset = TLI->getMaximalGlobalOffset();
239 bool Changed = false;
240 setMustKeepGlobalVariables(M);
242 // Grab all non-const globals.
243 for (Module::global_iterator I = M.global_begin(),
244 E = M.global_end(); I != E; ++I) {
245 // Merge is safe for "normal" internal globals only
246 if (!I->hasLocalLinkage() || I->isThreadLocal() || I->hasSection())
249 PointerType *PT = dyn_cast<PointerType>(I->getType());
250 assert(PT && "Global variable is not a pointer!");
252 unsigned AddressSpace = PT->getAddressSpace();
254 // Ignore fancy-aligned globals for now.
255 unsigned Alignment = TD->getPreferredAlignment(I);
256 Type *Ty = I->getType()->getElementType();
257 if (Alignment > TD->getABITypeAlignment(Ty))
260 // Ignore all 'special' globals.
261 if (I->getName().startswith("llvm.") ||
262 I->getName().startswith(".llvm."))
265 // Ignore all "required" globals:
266 if (isMustKeepGlobalVariable(I))
269 if (TD->getTypeAllocSize(Ty) < MaxOffset) {
270 if (TargetLoweringObjectFile::getKindForGlobal(I, TLI->getTargetMachine())
272 BSSGlobals[AddressSpace].push_back(I);
273 else if (I->isConstant())
274 ConstGlobals[AddressSpace].push_back(I);
276 Globals[AddressSpace].push_back(I);
280 for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
281 I = Globals.begin(), E = Globals.end(); I != E; ++I)
282 if (I->second.size() > 1)
283 Changed |= doMerge(I->second, M, false, I->first);
285 for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
286 I = BSSGlobals.begin(), E = BSSGlobals.end(); I != E; ++I)
287 if (I->second.size() > 1)
288 Changed |= doMerge(I->second, M, false, I->first);
290 if (EnableGlobalMergeOnConst)
291 for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
292 I = ConstGlobals.begin(), E = ConstGlobals.end(); I != E; ++I)
293 if (I->second.size() > 1)
294 Changed |= doMerge(I->second, M, true, I->first);
299 bool GlobalMerge::runOnFunction(Function &F) {
303 bool GlobalMerge::doFinalization(Module &M) {
304 MustKeepGlobalVariables.clear();
308 Pass *llvm::createGlobalMergePass(const TargetMachine *TM) {
309 return new GlobalMerge(TM);