1 //===- GlobalOpt.cpp - Optimize Global Variables --------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass transforms simple global variables that never have their address
11 // taken. If obviously true, it marks read/write globals as constant, deletes
12 // variables only stored to, etc.
14 //===----------------------------------------------------------------------===//
16 #define DEBUG_TYPE "globalopt"
17 #include "llvm/Transforms/IPO.h"
18 #include "llvm/Constants.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Instructions.h"
21 #include "llvm/Module.h"
22 #include "llvm/Pass.h"
23 #include "llvm/Support/Debug.h"
24 #include "llvm/Target/TargetData.h"
25 #include "llvm/Transforms/Utils/Local.h"
26 #include "llvm/ADT/Statistic.h"
27 #include "llvm/ADT/StringExtras.h"
33 Statistic<> NumMarked ("globalopt", "Number of globals marked constant");
34 Statistic<> NumSRA ("globalopt", "Number of aggregate globals broken "
36 Statistic<> NumSubstitute("globalopt",
37 "Number of globals with initializers stored into them");
38 Statistic<> NumDeleted ("globalopt", "Number of globals deleted");
39 Statistic<> NumFnDeleted("globalopt", "Number of functions deleted");
40 Statistic<> NumGlobUses ("globalopt", "Number of global uses devirtualized");
42 struct GlobalOpt : public ModulePass {
43 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
44 AU.addRequired<TargetData>();
47 bool runOnModule(Module &M);
50 bool ProcessInternalGlobal(GlobalVariable *GV, Module::giterator &GVI);
53 RegisterOpt<GlobalOpt> X("globalopt", "Global Variable Optimizer");
56 ModulePass *llvm::createGlobalOptimizerPass() { return new GlobalOpt(); }
58 /// GlobalStatus - As we analyze each global, keep track of some information
59 /// about it. If we find out that the address of the global is taken, none of
60 /// this info will be accurate.
62 /// isLoaded - True if the global is ever loaded. If the global isn't ever
63 /// loaded it can be deleted.
66 /// StoredType - Keep track of what stores to the global look like.
69 /// NotStored - There is no store to this global. It can thus be marked
73 /// isInitializerStored - This global is stored to, but the only thing
74 /// stored is the constant it was initialized with. This is only tracked
75 /// for scalar globals.
78 /// isStoredOnce - This global is stored to, but only its initializer and
79 /// one other value is ever stored to it. If this global isStoredOnce, we
80 /// track the value stored to it in StoredOnceValue below. This is only
81 /// tracked for scalar globals.
84 /// isStored - This global is stored to by multiple values or something else
85 /// that we cannot track.
89 /// StoredOnceValue - If only one value (besides the initializer constant) is
90 /// ever stored to this global, keep track of what value it is.
91 Value *StoredOnceValue;
93 /// isNotSuitableForSRA - Keep track of whether any SRA preventing users of
94 /// the global exist. Such users include GEP instruction with variable
95 /// indexes, and non-gep/load/store users like constant expr casts.
96 bool isNotSuitableForSRA;
98 GlobalStatus() : isLoaded(false), StoredType(NotStored), StoredOnceValue(0),
99 isNotSuitableForSRA(false) {}
104 /// ConstantIsDead - Return true if the specified constant is (transitively)
105 /// dead. The constant may be used by other constants (e.g. constant arrays and
106 /// constant exprs) as long as they are dead, but it cannot be used by anything
108 static bool ConstantIsDead(Constant *C) {
109 if (isa<GlobalValue>(C)) return false;
111 for (Value::use_iterator UI = C->use_begin(), E = C->use_end(); UI != E; ++UI)
112 if (Constant *CU = dyn_cast<Constant>(*UI)) {
113 if (!ConstantIsDead(CU)) return false;
120 /// AnalyzeGlobal - Look at all uses of the global and fill in the GlobalStatus
121 /// structure. If the global has its address taken, return true to indicate we
122 /// can't do anything with it.
124 static bool AnalyzeGlobal(Value *V, GlobalStatus &GS,
125 std::set<PHINode*> &PHIUsers) {
126 for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI)
127 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(*UI)) {
128 if (AnalyzeGlobal(CE, GS, PHIUsers)) return true;
129 if (CE->getOpcode() != Instruction::GetElementPtr)
130 GS.isNotSuitableForSRA = true;
131 else if (!GS.isNotSuitableForSRA) {
132 // Check to see if this ConstantExpr GEP is SRA'able. In particular, we
133 // don't like < 3 operand CE's, and we don't like non-constant integer
135 if (CE->getNumOperands() < 3 || !CE->getOperand(1)->isNullValue())
136 GS.isNotSuitableForSRA = true;
138 for (unsigned i = 1, e = CE->getNumOperands(); i != e; ++i)
139 if (!isa<ConstantInt>(CE->getOperand(i))) {
140 GS.isNotSuitableForSRA = true;
146 } else if (Instruction *I = dyn_cast<Instruction>(*UI)) {
147 if (isa<LoadInst>(I)) {
149 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
150 // Don't allow a store OF the address, only stores TO the address.
151 if (SI->getOperand(0) == V) return true;
153 // If this is a direct store to the global (i.e., the global is a scalar
154 // value, not an aggregate), keep more specific information about
156 if (GS.StoredType != GlobalStatus::isStored)
157 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(SI->getOperand(1))){
158 if (SI->getOperand(0) == GV->getInitializer()) {
159 if (GS.StoredType < GlobalStatus::isInitializerStored)
160 GS.StoredType = GlobalStatus::isInitializerStored;
161 } else if (GS.StoredType < GlobalStatus::isStoredOnce) {
162 GS.StoredType = GlobalStatus::isStoredOnce;
163 GS.StoredOnceValue = SI->getOperand(0);
164 } else if (GS.StoredType == GlobalStatus::isStoredOnce &&
165 GS.StoredOnceValue == SI->getOperand(0)) {
168 GS.StoredType = GlobalStatus::isStored;
171 GS.StoredType = GlobalStatus::isStored;
173 } else if (I->getOpcode() == Instruction::GetElementPtr) {
174 if (AnalyzeGlobal(I, GS, PHIUsers)) return true;
176 // If the first two indices are constants, this can be SRA'd.
177 if (isa<GlobalVariable>(I->getOperand(0))) {
178 if (I->getNumOperands() < 3 || !isa<Constant>(I->getOperand(1)) ||
179 !cast<Constant>(I->getOperand(1))->isNullValue() ||
180 !isa<ConstantInt>(I->getOperand(2)))
181 GS.isNotSuitableForSRA = true;
182 } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(I->getOperand(0))){
183 if (CE->getOpcode() != Instruction::GetElementPtr ||
184 CE->getNumOperands() < 3 || I->getNumOperands() < 2 ||
185 !isa<Constant>(I->getOperand(0)) ||
186 !cast<Constant>(I->getOperand(0))->isNullValue())
187 GS.isNotSuitableForSRA = true;
189 GS.isNotSuitableForSRA = true;
191 } else if (I->getOpcode() == Instruction::Select) {
192 if (AnalyzeGlobal(I, GS, PHIUsers)) return true;
193 GS.isNotSuitableForSRA = true;
194 } else if (PHINode *PN = dyn_cast<PHINode>(I)) {
195 // PHI nodes we can check just like select or GEP instructions, but we
196 // have to be careful about infinite recursion.
197 if (PHIUsers.insert(PN).second) // Not already visited.
198 if (AnalyzeGlobal(I, GS, PHIUsers)) return true;
199 GS.isNotSuitableForSRA = true;
200 } else if (isa<SetCondInst>(I)) {
201 GS.isNotSuitableForSRA = true;
203 return true; // Any other non-load instruction might take address!
205 } else if (Constant *C = dyn_cast<Constant>(*UI)) {
206 // We might have a dead and dangling constant hanging off of here.
207 if (!ConstantIsDead(C))
210 // Otherwise must be a global or some other user.
217 static Constant *getAggregateConstantElement(Constant *Agg, Constant *Idx) {
218 ConstantInt *CI = dyn_cast<ConstantInt>(Idx);
220 uint64_t IdxV = CI->getRawValue();
222 if (ConstantStruct *CS = dyn_cast<ConstantStruct>(Agg)) {
223 if (IdxV < CS->getNumOperands()) return CS->getOperand(IdxV);
224 } else if (ConstantArray *CA = dyn_cast<ConstantArray>(Agg)) {
225 if (IdxV < CA->getNumOperands()) return CA->getOperand(IdxV);
226 } else if (ConstantPacked *CP = dyn_cast<ConstantPacked>(Agg)) {
227 if (IdxV < CP->getNumOperands()) return CP->getOperand(IdxV);
228 } else if (isa<ConstantAggregateZero>(Agg)) {
229 if (const StructType *STy = dyn_cast<StructType>(Agg->getType())) {
230 if (IdxV < STy->getNumElements())
231 return Constant::getNullValue(STy->getElementType(IdxV));
232 } else if (const SequentialType *STy =
233 dyn_cast<SequentialType>(Agg->getType())) {
234 return Constant::getNullValue(STy->getElementType());
236 } else if (isa<UndefValue>(Agg)) {
237 if (const StructType *STy = dyn_cast<StructType>(Agg->getType())) {
238 if (IdxV < STy->getNumElements())
239 return UndefValue::get(STy->getElementType(IdxV));
240 } else if (const SequentialType *STy =
241 dyn_cast<SequentialType>(Agg->getType())) {
242 return UndefValue::get(STy->getElementType());
248 static Constant *TraverseGEPInitializer(User *GEP, Constant *Init) {
249 if (GEP->getNumOperands() == 1 ||
250 !isa<Constant>(GEP->getOperand(1)) ||
251 !cast<Constant>(GEP->getOperand(1))->isNullValue())
254 for (unsigned i = 2, e = GEP->getNumOperands(); i != e; ++i) {
255 ConstantInt *Idx = dyn_cast<ConstantInt>(GEP->getOperand(i));
257 Init = getAggregateConstantElement(Init, Idx);
258 if (Init == 0) return 0;
263 /// CleanupConstantGlobalUsers - We just marked GV constant. Loop over all
264 /// users of the global, cleaning up the obvious ones. This is largely just a
265 /// quick scan over the use list to clean up the easy and obvious cruft. This
266 /// returns true if it made a change.
267 static bool CleanupConstantGlobalUsers(Value *V, Constant *Init) {
268 bool Changed = false;
269 for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E;) {
272 if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
273 // Replace the load with the initializer.
274 LI->replaceAllUsesWith(Init);
275 LI->eraseFromParent();
277 } else if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
278 // Store must be unreachable or storing Init into the global.
279 SI->eraseFromParent();
281 } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U)) {
282 if (CE->getOpcode() == Instruction::GetElementPtr) {
283 if (Constant *SubInit = TraverseGEPInitializer(CE, Init))
284 Changed |= CleanupConstantGlobalUsers(CE, SubInit);
285 if (CE->use_empty()) {
286 CE->destroyConstant();
290 } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
291 if (Constant *SubInit = TraverseGEPInitializer(GEP, Init))
292 Changed |= CleanupConstantGlobalUsers(GEP, SubInit);
294 // If this GEP has variable indexes, we should still be able to delete
295 // any stores through it.
296 for (Value::use_iterator GUI = GEP->use_begin(), E = GEP->use_end();
298 if (StoreInst *SI = dyn_cast<StoreInst>(*GUI++)) {
299 SI->eraseFromParent();
304 if (GEP->use_empty()) {
305 GEP->eraseFromParent();
308 } else if (Constant *C = dyn_cast<Constant>(U)) {
309 // If we have a chain of dead constantexprs or other things dangling from
310 // us, and if they are all dead, nuke them without remorse.
311 if (ConstantIsDead(C)) {
312 C->destroyConstant();
313 // This could have incalidated UI, start over from scratch.x
314 CleanupConstantGlobalUsers(V, Init);
322 /// SRAGlobal - Perform scalar replacement of aggregates on the specified global
323 /// variable. This opens the door for other optimizations by exposing the
324 /// behavior of the program in a more fine-grained way. We have determined that
325 /// this transformation is safe already. We return the first global variable we
326 /// insert so that the caller can reprocess it.
327 static GlobalVariable *SRAGlobal(GlobalVariable *GV) {
328 assert(GV->hasInternalLinkage() && !GV->isConstant());
329 Constant *Init = GV->getInitializer();
330 const Type *Ty = Init->getType();
332 std::vector<GlobalVariable*> NewGlobals;
333 Module::GlobalListType &Globals = GV->getParent()->getGlobalList();
335 if (const StructType *STy = dyn_cast<StructType>(Ty)) {
336 NewGlobals.reserve(STy->getNumElements());
337 for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
338 Constant *In = getAggregateConstantElement(Init,
339 ConstantUInt::get(Type::UIntTy, i));
340 assert(In && "Couldn't get element of initializer?");
341 GlobalVariable *NGV = new GlobalVariable(STy->getElementType(i), false,
342 GlobalVariable::InternalLinkage,
343 In, GV->getName()+"."+utostr(i));
344 Globals.insert(GV, NGV);
345 NewGlobals.push_back(NGV);
347 } else if (const SequentialType *STy = dyn_cast<SequentialType>(Ty)) {
348 unsigned NumElements = 0;
349 if (const ArrayType *ATy = dyn_cast<ArrayType>(STy))
350 NumElements = ATy->getNumElements();
351 else if (const PackedType *PTy = dyn_cast<PackedType>(STy))
352 NumElements = PTy->getNumElements();
354 assert(0 && "Unknown aggregate sequential type!");
356 if (NumElements > 16 && GV->use_size() > 16) return 0; // It's not worth it.
357 NewGlobals.reserve(NumElements);
358 for (unsigned i = 0, e = NumElements; i != e; ++i) {
359 Constant *In = getAggregateConstantElement(Init,
360 ConstantUInt::get(Type::UIntTy, i));
361 assert(In && "Couldn't get element of initializer?");
363 GlobalVariable *NGV = new GlobalVariable(STy->getElementType(), false,
364 GlobalVariable::InternalLinkage,
365 In, GV->getName()+"."+utostr(i));
366 Globals.insert(GV, NGV);
367 NewGlobals.push_back(NGV);
371 if (NewGlobals.empty())
374 DEBUG(std::cerr << "PERFORMING GLOBAL SRA ON: " << *GV);
376 Constant *NullInt = Constant::getNullValue(Type::IntTy);
378 // Loop over all of the uses of the global, replacing the constantexpr geps,
379 // with smaller constantexpr geps or direct references.
380 while (!GV->use_empty()) {
381 User *GEP = GV->use_back();
382 assert(((isa<ConstantExpr>(GEP) &&
383 cast<ConstantExpr>(GEP)->getOpcode()==Instruction::GetElementPtr)||
384 isa<GetElementPtrInst>(GEP)) && "NonGEP CE's are not SRAable!");
386 // Ignore the 1th operand, which has to be zero or else the program is quite
387 // broken (undefined). Get the 2nd operand, which is the structure or array
389 unsigned Val = cast<ConstantInt>(GEP->getOperand(2))->getRawValue();
390 if (Val >= NewGlobals.size()) Val = 0; // Out of bound array access.
392 Value *NewPtr = NewGlobals[Val];
394 // Form a shorter GEP if needed.
395 if (GEP->getNumOperands() > 3)
396 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(GEP)) {
397 std::vector<Constant*> Idxs;
398 Idxs.push_back(NullInt);
399 for (unsigned i = 3, e = CE->getNumOperands(); i != e; ++i)
400 Idxs.push_back(CE->getOperand(i));
401 NewPtr = ConstantExpr::getGetElementPtr(cast<Constant>(NewPtr), Idxs);
403 GetElementPtrInst *GEPI = cast<GetElementPtrInst>(GEP);
404 std::vector<Value*> Idxs;
405 Idxs.push_back(NullInt);
406 for (unsigned i = 3, e = GEPI->getNumOperands(); i != e; ++i)
407 Idxs.push_back(GEPI->getOperand(i));
408 NewPtr = new GetElementPtrInst(NewPtr, Idxs,
409 GEPI->getName()+"."+utostr(Val), GEPI);
411 GEP->replaceAllUsesWith(NewPtr);
413 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(GEP))
414 GEPI->eraseFromParent();
416 cast<ConstantExpr>(GEP)->destroyConstant();
419 // Delete the old global, now that it is dead.
423 // Loop over the new globals array deleting any globals that are obviously
424 // dead. This can arise due to scalarization of a structure or an array that
425 // has elements that are dead.
426 unsigned FirstGlobal = 0;
427 for (unsigned i = 0, e = NewGlobals.size(); i != e; ++i)
428 if (NewGlobals[i]->use_empty()) {
429 Globals.erase(NewGlobals[i]);
430 if (FirstGlobal == i) ++FirstGlobal;
433 return FirstGlobal != NewGlobals.size() ? NewGlobals[FirstGlobal] : 0;
436 /// AllUsesOfValueWillTrapIfNull - Return true if all users of the specified
437 /// value will trap if the value is dynamically null.
438 static bool AllUsesOfValueWillTrapIfNull(Value *V) {
439 for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI)
440 if (isa<LoadInst>(*UI)) {
442 } else if (StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
443 if (SI->getOperand(0) == V) {
444 //std::cerr << "NONTRAPPING USE: " << **UI;
445 return false; // Storing the value.
447 } else if (CallInst *CI = dyn_cast<CallInst>(*UI)) {
448 if (CI->getOperand(0) != V) {
449 //std::cerr << "NONTRAPPING USE: " << **UI;
450 return false; // Not calling the ptr
452 } else if (InvokeInst *II = dyn_cast<InvokeInst>(*UI)) {
453 if (II->getOperand(0) != V) {
454 //std::cerr << "NONTRAPPING USE: " << **UI;
455 return false; // Not calling the ptr
457 } else if (CastInst *CI = dyn_cast<CastInst>(*UI)) {
458 if (!AllUsesOfValueWillTrapIfNull(CI)) return false;
459 } else if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(*UI)) {
460 if (!AllUsesOfValueWillTrapIfNull(GEPI)) return false;
461 } else if (isa<SetCondInst>(*UI) &&
462 isa<ConstantPointerNull>(UI->getOperand(1))) {
463 // Ignore setcc X, null
465 //std::cerr << "NONTRAPPING USE: " << **UI;
471 /// AllUsesOfLoadedValueWillTrapIfNull - Return true if all uses of any loads
472 /// from GV will trap if the loaded value is null. Note that this also permits
473 /// comparisons of the loaded value against null, as a special case.
474 static bool AllUsesOfLoadedValueWillTrapIfNull(GlobalVariable *GV) {
475 for (Value::use_iterator UI = GV->use_begin(), E = GV->use_end(); UI!=E; ++UI)
476 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
477 if (!AllUsesOfValueWillTrapIfNull(LI))
479 } else if (isa<StoreInst>(*UI)) {
480 // Ignore stores to the global.
482 // We don't know or understand this user, bail out.
483 //std::cerr << "UNKNOWN USER OF GLOBAL!: " << **UI;
490 static bool OptimizeAwayTrappingUsesOfValue(Value *V, Constant *NewV) {
491 bool Changed = false;
492 for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ) {
493 Instruction *I = cast<Instruction>(*UI++);
494 if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
495 LI->setOperand(0, NewV);
497 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
498 if (SI->getOperand(1) == V) {
499 SI->setOperand(1, NewV);
502 } else if (isa<CallInst>(I) || isa<InvokeInst>(I)) {
503 if (I->getOperand(0) == V) {
504 // Calling through the pointer! Turn into a direct call, but be careful
505 // that the pointer is not also being passed as an argument.
506 I->setOperand(0, NewV);
508 bool PassedAsArg = false;
509 for (unsigned i = 1, e = I->getNumOperands(); i != e; ++i)
510 if (I->getOperand(i) == V) {
512 I->setOperand(i, NewV);
516 // Being passed as an argument also. Be careful to not invalidate UI!
520 } else if (CastInst *CI = dyn_cast<CastInst>(I)) {
521 Changed |= OptimizeAwayTrappingUsesOfValue(CI,
522 ConstantExpr::getCast(NewV, CI->getType()));
523 if (CI->use_empty()) {
525 CI->eraseFromParent();
527 } else if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(I)) {
528 // Should handle GEP here.
529 std::vector<Constant*> Indices;
530 Indices.reserve(GEPI->getNumOperands()-1);
531 for (unsigned i = 1, e = GEPI->getNumOperands(); i != e; ++i)
532 if (Constant *C = dyn_cast<Constant>(GEPI->getOperand(i)))
533 Indices.push_back(C);
536 if (Indices.size() == GEPI->getNumOperands()-1)
537 Changed |= OptimizeAwayTrappingUsesOfValue(GEPI,
538 ConstantExpr::getGetElementPtr(NewV, Indices));
539 if (GEPI->use_empty()) {
541 GEPI->eraseFromParent();
550 /// OptimizeAwayTrappingUsesOfLoads - The specified global has only one non-null
551 /// value stored into it. If there are uses of the loaded value that would trap
552 /// if the loaded value is dynamically null, then we know that they cannot be
553 /// reachable with a null optimize away the load.
554 static bool OptimizeAwayTrappingUsesOfLoads(GlobalVariable *GV, Constant *LV) {
555 std::vector<LoadInst*> Loads;
556 bool Changed = false;
558 // Replace all uses of loads with uses of uses of the stored value.
559 for (Value::use_iterator GUI = GV->use_begin(), E = GV->use_end();
561 if (LoadInst *LI = dyn_cast<LoadInst>(*GUI)) {
563 Changed |= OptimizeAwayTrappingUsesOfValue(LI, LV);
565 assert(isa<StoreInst>(*GUI) && "Only expect load and stores!");
569 DEBUG(std::cerr << "OPTIMIZED LOADS FROM STORED ONCE POINTER: " << *GV);
573 // Delete all of the loads we can, keeping track of whether we nuked them all!
574 bool AllLoadsGone = true;
575 while (!Loads.empty()) {
576 LoadInst *L = Loads.back();
577 if (L->use_empty()) {
578 L->eraseFromParent();
581 AllLoadsGone = false;
586 // If we nuked all of the loads, then none of the stores are needed either,
587 // nor is the global.
589 DEBUG(std::cerr << " *** GLOBAL NOW DEAD!\n");
590 CleanupConstantGlobalUsers(GV, 0);
591 if (GV->use_empty()) {
592 GV->eraseFromParent();
600 /// ConstantPropUsersOf - Walk the use list of V, constant folding all of the
601 /// instructions that are foldable.
602 static void ConstantPropUsersOf(Value *V) {
603 for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; )
604 if (Instruction *I = dyn_cast<Instruction>(*UI++))
605 if (Constant *NewC = ConstantFoldInstruction(I)) {
606 I->replaceAllUsesWith(NewC);
608 // Back up UI to avoid invalidating it!
609 bool AtBegin = false;
610 if (UI == V->use_begin())
614 I->eraseFromParent();
622 /// OptimizeGlobalAddressOfMalloc - This function takes the specified global
623 /// variable, and transforms the program as if it always contained the result of
624 /// the specified malloc. Because it is always the result of the specified
625 /// malloc, there is no reason to actually DO the malloc. Instead, turn the
626 /// malloc into a global, and any laods of GV as uses of the new global.
627 static GlobalVariable *OptimizeGlobalAddressOfMalloc(GlobalVariable *GV,
629 DEBUG(std::cerr << "PROMOTING MALLOC GLOBAL: " << *GV << " MALLOC = " <<*MI);
630 ConstantInt *NElements = cast<ConstantInt>(MI->getArraySize());
632 if (NElements->getRawValue() != 1) {
633 // If we have an array allocation, transform it to a single element
634 // allocation to make the code below simpler.
635 Type *NewTy = ArrayType::get(MI->getAllocatedType(),
636 NElements->getRawValue());
638 new MallocInst(NewTy, Constant::getNullValue(Type::UIntTy),
640 std::vector<Value*> Indices;
641 Indices.push_back(Constant::getNullValue(Type::IntTy));
642 Indices.push_back(Indices[0]);
643 Value *NewGEP = new GetElementPtrInst(NewMI, Indices,
644 NewMI->getName()+".el0", MI);
645 MI->replaceAllUsesWith(NewGEP);
646 MI->eraseFromParent();
650 // Create the new global variable. The contents of the malloc'd memory is
651 // undefined, so initialize with an undef value.
652 Constant *Init = UndefValue::get(MI->getAllocatedType());
653 GlobalVariable *NewGV = new GlobalVariable(MI->getAllocatedType(), false,
654 GlobalValue::InternalLinkage, Init,
655 GV->getName()+".body");
656 GV->getParent()->getGlobalList().insert(GV, NewGV);
658 // Anything that used the malloc now uses the global directly.
659 MI->replaceAllUsesWith(NewGV);
661 Constant *RepValue = NewGV;
662 if (NewGV->getType() != GV->getType()->getElementType())
663 RepValue = ConstantExpr::getCast(RepValue, GV->getType()->getElementType());
665 // If there is a comparison against null, we will insert a global bool to
666 // keep track of whether the global was initialized yet or not.
667 GlobalVariable *InitBool = 0;
669 // Loop over all uses of GV, processing them in turn.
670 while (!GV->use_empty())
671 if (LoadInst *LI = dyn_cast<LoadInst>(GV->use_back())) {
672 while (!LI->use_empty()) {
673 // FIXME: the iterator should expose a getUse() method.
674 Use &LoadUse = *(const iplist<Use>::iterator&)LI->use_begin();
675 if (!isa<SetCondInst>(LoadUse.getUser()))
679 InitBool = new GlobalVariable(Type::BoolTy, false,
680 GlobalValue::InternalLinkage,
682 GV->getName()+".init");
683 GV->getParent()->getGlobalList().insert(GV, InitBool);
684 // The global is initialized when the malloc is run.
685 new StoreInst(ConstantBool::True, InitBool, MI);
687 // Replace the setcc X, 0 with a use of the bool value.
688 SetCondInst *SCI = cast<SetCondInst>(LoadUse.getUser());
689 Value *LV = new LoadInst(InitBool, InitBool->getName()+".val", SCI);
690 switch (SCI->getOpcode()) {
691 default: assert(0 && "Unknown opcode!");
692 case Instruction::SetLT:
693 LV = ConstantBool::False; // X < null -> always false
695 case Instruction::SetEQ:
696 case Instruction::SetLE:
697 LV = BinaryOperator::createNot(LV, "notinit", SCI);
699 case Instruction::SetNE:
700 case Instruction::SetGE:
701 case Instruction::SetGT:
704 SCI->replaceAllUsesWith(LV);
705 SCI->eraseFromParent();
708 LI->eraseFromParent();
710 StoreInst *SI = cast<StoreInst>(GV->use_back());
711 SI->eraseFromParent();
714 // Now the GV is dead, nuke it and the malloc.
715 GV->eraseFromParent();
716 MI->eraseFromParent();
718 // To further other optimizations, loop over all users of NewGV and try to
719 // constant prop them. This will promote GEP instructions with constant
720 // indices into GEP constant-exprs, which will allow global-opt to hack on it.
721 ConstantPropUsersOf(NewGV);
722 if (RepValue != NewGV)
723 ConstantPropUsersOf(RepValue);
728 // OptimizeOnceStoredGlobal - Try to optimize globals based on the knowledge
729 // that only one value (besides its initializer) is ever stored to the global.
730 static bool OptimizeOnceStoredGlobal(GlobalVariable *GV, Value *StoredOnceVal,
731 Module::giterator &GVI, TargetData &TD) {
732 if (CastInst *CI = dyn_cast<CastInst>(StoredOnceVal))
733 StoredOnceVal = CI->getOperand(0);
734 else if (GetElementPtrInst *GEPI =dyn_cast<GetElementPtrInst>(StoredOnceVal)){
735 // "getelementptr Ptr, 0, 0, 0" is really just a cast.
736 bool IsJustACast = true;
737 for (unsigned i = 1, e = GEPI->getNumOperands(); i != e; ++i)
738 if (!isa<Constant>(GEPI->getOperand(i)) ||
739 !cast<Constant>(GEPI->getOperand(i))->isNullValue()) {
744 StoredOnceVal = GEPI->getOperand(0);
747 // If we are dealing with a pointer global that is initialized to null and
748 // only has one (non-null) value stored into it, then we can optimize any
749 // users of the loaded value (often calls and loads) that would trap if the
751 if (isa<PointerType>(GV->getInitializer()->getType()) &&
752 GV->getInitializer()->isNullValue()) {
753 if (Constant *SOVC = dyn_cast<Constant>(StoredOnceVal)) {
754 if (GV->getInitializer()->getType() != SOVC->getType())
755 SOVC = ConstantExpr::getCast(SOVC, GV->getInitializer()->getType());
757 // Optimize away any trapping uses of the loaded value.
758 if (OptimizeAwayTrappingUsesOfLoads(GV, SOVC))
760 } else if (MallocInst *MI = dyn_cast<MallocInst>(StoredOnceVal)) {
761 // If we have a global that is only initialized with a fixed size malloc,
762 // and if all users of the malloc trap, and if the malloc'd address is not
763 // put anywhere else, transform the program to use global memory instead
764 // of malloc'd memory. This eliminates dynamic allocation (good) and
765 // exposes the resultant global to further GlobalOpt (even better). Note
766 // that we restrict this transformation to only working on small
767 // allocations (2048 bytes currently), as we don't want to introduce a 16M
768 // global or something.
769 if (ConstantInt *NElements = dyn_cast<ConstantInt>(MI->getArraySize()))
770 if (MI->getAllocatedType()->isSized() &&
771 NElements->getRawValue()*
772 TD.getTypeSize(MI->getAllocatedType()) < 2048 &&
773 AllUsesOfLoadedValueWillTrapIfNull(GV)) {
774 // FIXME: do more correctness checking to make sure the result of the
775 // malloc isn't squirrelled away somewhere.
776 GVI = OptimizeGlobalAddressOfMalloc(GV, MI);
785 /// ProcessInternalGlobal - Analyze the specified global variable and optimize
786 /// it if possible. If we make a change, return true.
787 bool GlobalOpt::ProcessInternalGlobal(GlobalVariable *GV,
788 Module::giterator &GVI) {
789 std::set<PHINode*> PHIUsers;
792 GV->removeDeadConstantUsers();
794 if (GV->use_empty()) {
795 DEBUG(std::cerr << "GLOBAL DEAD: " << *GV);
796 GV->eraseFromParent();
801 if (!AnalyzeGlobal(GV, GS, PHIUsers)) {
802 // If the global is never loaded (but may be stored to), it is dead.
805 DEBUG(std::cerr << "GLOBAL NEVER LOADED: " << *GV);
807 // Delete any stores we can find to the global. We may not be able to
808 // make it completely dead though.
809 bool Changed = CleanupConstantGlobalUsers(GV, GV->getInitializer());
811 // If the global is dead now, delete it.
812 if (GV->use_empty()) {
813 GV->eraseFromParent();
819 } else if (GS.StoredType <= GlobalStatus::isInitializerStored) {
820 DEBUG(std::cerr << "MARKING CONSTANT: " << *GV);
821 GV->setConstant(true);
823 // Clean up any obviously simplifiable users now.
824 CleanupConstantGlobalUsers(GV, GV->getInitializer());
826 // If the global is dead now, just nuke it.
827 if (GV->use_empty()) {
828 DEBUG(std::cerr << " *** Marking constant allowed us to simplify "
829 "all users and delete global!\n");
830 GV->eraseFromParent();
836 } else if (!GS.isNotSuitableForSRA &&
837 !GV->getInitializer()->getType()->isFirstClassType()) {
838 if (GlobalVariable *FirstNewGV = SRAGlobal(GV)) {
839 GVI = FirstNewGV; // Don't skip the newly produced globals!
842 } else if (GS.StoredType == GlobalStatus::isStoredOnce) {
843 // If the initial value for the global was an undef value, and if only one
844 // other value was stored into it, we can just change the initializer to
845 // be an undef value, then delete all stores to the global. This allows
846 // us to mark it constant.
847 if (isa<UndefValue>(GV->getInitializer()) &&
848 isa<Constant>(GS.StoredOnceValue)) {
849 // Change the initial value here.
850 GV->setInitializer(cast<Constant>(GS.StoredOnceValue));
852 // Clean up any obviously simplifiable users now.
853 CleanupConstantGlobalUsers(GV, GV->getInitializer());
855 if (GV->use_empty()) {
856 DEBUG(std::cerr << " *** Substituting initializer allowed us to "
857 "simplify all users and delete global!\n");
858 GV->eraseFromParent();
867 // Try to optimize globals based on the knowledge that only one value
868 // (besides its initializer) is ever stored to the global.
869 if (OptimizeOnceStoredGlobal(GV, GS.StoredOnceValue, GVI,
870 getAnalysis<TargetData>()))
878 bool GlobalOpt::runOnModule(Module &M) {
879 bool Changed = false;
881 // As a prepass, delete functions that are trivially dead.
882 bool LocalChange = true;
883 while (LocalChange) {
885 for (Module::iterator FI = M.begin(), E = M.end(); FI != E; ) {
887 F->removeDeadConstantUsers();
888 if (F->use_empty() && (F->hasInternalLinkage() ||
889 F->hasLinkOnceLinkage())) {
890 M.getFunctionList().erase(F);
895 Changed |= LocalChange;
899 while (LocalChange) {
901 for (Module::giterator GVI = M.gbegin(), E = M.gend(); GVI != E;) {
902 GlobalVariable *GV = GVI++;
903 if (!GV->isConstant() && GV->hasInternalLinkage() &&
904 GV->hasInitializer())
905 LocalChange |= ProcessInternalGlobal(GV, GVI);
907 Changed |= LocalChange;