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/InlineAsm.h"
65 #include "llvm/IR/Instructions.h"
66 #include "llvm/IR/Intrinsics.h"
67 #include "llvm/IR/IntrinsicInst.h"
68 #include "llvm/IR/Module.h"
69 #include "llvm/Pass.h"
70 #include "llvm/Support/CommandLine.h"
71 #include "llvm/Target/TargetLowering.h"
72 #include "llvm/Target/TargetLoweringObjectFile.h"
76 EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
77 cl::desc("Enable global merge pass on constants"),
80 STATISTIC(NumMerged , "Number of globals merged");
82 class GlobalMerge : public FunctionPass {
83 /// TLI - Keep a pointer of a TargetLowering to consult for determining
84 /// target type sizes.
85 const TargetLowering *TLI;
87 bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
88 Module &M, bool isConst, unsigned AddrSpace) const;
90 /// \brief Check if the given variable has been identified as must keep
91 /// \pre setMustKeepGlobalVariables must have been called on the Module that
93 bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
94 return MustKeepGlobalVariables.count(GV);
97 /// Collect every variables marked as "used" or used in a landing pad
98 /// instruction for this Module.
99 void setMustKeepGlobalVariables(Module &M);
101 /// Collect every variables marked as "used"
102 void collectUsedGlobalVariables(Module &M);
104 /// Keep track of the GlobalVariable that are marked as "used"
105 SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;
108 static char ID; // Pass identification, replacement for typeid.
109 explicit GlobalMerge(const TargetLowering *tli = 0)
110 : FunctionPass(ID), TLI(tli) {
111 initializeGlobalMergePass(*PassRegistry::getPassRegistry());
114 virtual bool doInitialization(Module &M);
115 virtual bool runOnFunction(Function &F);
117 const char *getPassName() const {
118 return "Merge internal globals";
121 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
122 AU.setPreservesCFG();
123 FunctionPass::getAnalysisUsage(AU);
127 const DataLayout *TD;
129 GlobalCmp(const DataLayout *td) : TD(td) { }
131 bool operator()(const GlobalVariable *GV1, const GlobalVariable *GV2) {
132 Type *Ty1 = cast<PointerType>(GV1->getType())->getElementType();
133 Type *Ty2 = cast<PointerType>(GV2->getType())->getElementType();
135 return (TD->getTypeAllocSize(Ty1) < TD->getTypeAllocSize(Ty2));
139 } // end anonymous namespace
141 char GlobalMerge::ID = 0;
142 INITIALIZE_PASS(GlobalMerge, "global-merge",
143 "Global Merge", false, false)
146 bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
147 Module &M, bool isConst, unsigned AddrSpace) const {
148 const DataLayout *TD = TLI->getDataLayout();
150 // FIXME: Infer the maximum possible offset depending on the actual users
151 // (these max offsets are different for the users inside Thumb or ARM
153 unsigned MaxOffset = TLI->getMaximalGlobalOffset();
155 // FIXME: Find better heuristics
156 std::stable_sort(Globals.begin(), Globals.end(), GlobalCmp(TD));
158 Type *Int32Ty = Type::getInt32Ty(M.getContext());
160 for (size_t i = 0, e = Globals.size(); i != e; ) {
162 uint64_t MergedSize = 0;
163 std::vector<Type*> Tys;
164 std::vector<Constant*> Inits;
165 for (j = i; j != e; ++j) {
166 Type *Ty = Globals[j]->getType()->getElementType();
167 MergedSize += TD->getTypeAllocSize(Ty);
168 if (MergedSize > MaxOffset) {
172 Inits.push_back(Globals[j]->getInitializer());
175 StructType *MergedTy = StructType::get(M.getContext(), Tys);
176 Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
177 GlobalVariable *MergedGV = new GlobalVariable(M, MergedTy, isConst,
178 GlobalValue::InternalLinkage,
179 MergedInit, "_MergedGlobals",
180 0, GlobalVariable::NotThreadLocal,
182 for (size_t k = i; k < j; ++k) {
184 ConstantInt::get(Int32Ty, 0),
185 ConstantInt::get(Int32Ty, k-i)
187 Constant *GEP = ConstantExpr::getInBoundsGetElementPtr(MergedGV, Idx);
188 Globals[k]->replaceAllUsesWith(GEP);
189 Globals[k]->eraseFromParent();
198 void GlobalMerge::collectUsedGlobalVariables(Module &M) {
199 // Extract global variables from llvm.used array
200 const GlobalVariable *GV = M.getGlobalVariable("llvm.used");
201 if (!GV || !GV->hasInitializer()) return;
203 // Should be an array of 'i8*'.
204 const ConstantArray *InitList = dyn_cast<ConstantArray>(GV->getInitializer());
205 if (InitList == 0) return;
207 for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
208 if (const GlobalVariable *G =
209 dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
210 MustKeepGlobalVariables.insert(G);
213 void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
214 // If we already processed a Module, UsedGlobalVariables may have been
215 // populated. Reset the information for this module.
216 MustKeepGlobalVariables.clear();
217 collectUsedGlobalVariables(M);
219 for (Module::iterator IFn = M.begin(), IEndFn = M.end(); IFn != IEndFn;
221 for (Function::iterator IBB = IFn->begin(), IEndBB = IFn->end();
222 IBB != IEndBB; ++IBB) {
223 // Follow the inwoke link to find the landing pad instruction
224 const InvokeInst *II = dyn_cast<InvokeInst>(IBB->getTerminator());
227 const LandingPadInst *LPInst = II->getUnwindDest()->getLandingPadInst();
228 // Look for globals in the clauses of the landing pad instruction
229 for (unsigned Idx = 0, NumClauses = LPInst->getNumClauses();
230 Idx != NumClauses; ++Idx)
231 if (const GlobalVariable *GV =
232 dyn_cast<GlobalVariable>(LPInst->getClause(Idx)
233 ->stripPointerCasts()))
234 MustKeepGlobalVariables.insert(GV);
239 bool GlobalMerge::doInitialization(Module &M) {
240 DenseMap<unsigned, SmallVector<GlobalVariable*, 16> > Globals, ConstGlobals,
242 const DataLayout *TD = TLI->getDataLayout();
243 unsigned MaxOffset = TLI->getMaximalGlobalOffset();
244 bool Changed = false;
245 setMustKeepGlobalVariables(M);
247 // Grab all non-const globals.
248 for (Module::global_iterator I = M.global_begin(),
249 E = M.global_end(); I != E; ++I) {
250 // Merge is safe for "normal" internal globals only
251 if (!I->hasLocalLinkage() || I->isThreadLocal() || I->hasSection())
254 PointerType *PT = dyn_cast<PointerType>(I->getType());
255 assert(PT && "Global variable is not a pointer!");
257 unsigned AddressSpace = PT->getAddressSpace();
259 // Ignore fancy-aligned globals for now.
260 unsigned Alignment = TD->getPreferredAlignment(I);
261 Type *Ty = I->getType()->getElementType();
262 if (Alignment > TD->getABITypeAlignment(Ty))
265 // Ignore all 'special' globals.
266 if (I->getName().startswith("llvm.") ||
267 I->getName().startswith(".llvm."))
270 // Ignore all "required" globals:
271 // - the ones used for EH
272 // - the ones marked with "used" attribute
273 if (isMustKeepGlobalVariable(I))
276 if (TD->getTypeAllocSize(Ty) < MaxOffset) {
277 if (TargetLoweringObjectFile::getKindForGlobal(I, TLI->getTargetMachine())
279 BSSGlobals[AddressSpace].push_back(I);
280 else if (I->isConstant())
281 ConstGlobals[AddressSpace].push_back(I);
283 Globals[AddressSpace].push_back(I);
287 for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
288 I = Globals.begin(), E = Globals.end(); I != E; ++I)
289 if (I->second.size() > 1)
290 Changed |= doMerge(I->second, M, false, I->first);
292 for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
293 I = BSSGlobals.begin(), E = BSSGlobals.end(); I != E; ++I)
294 if (I->second.size() > 1)
295 Changed |= doMerge(I->second, M, false, I->first);
297 if (EnableGlobalMergeOnConst)
298 for (DenseMap<unsigned, SmallVector<GlobalVariable*, 16> >::iterator
299 I = ConstGlobals.begin(), E = ConstGlobals.end(); I != E; ++I)
300 if (I->second.size() > 1)
301 Changed |= doMerge(I->second, M, true, I->first);
306 bool GlobalMerge::runOnFunction(Function &F) {
310 Pass *llvm::createGlobalMergePass(const TargetLowering *tli) {
311 return new GlobalMerge(tli);