1 //===-- ArgumentPromotion.cpp - Promote by-reference arguments ------------===//
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 //===----------------------------------------------------------------------===//
10 // This pass promotes "by reference" arguments to be "by value" arguments. In
11 // practice, this means looking for internal functions that have pointer
12 // arguments. If it can prove, through the use of alias analysis, that an
13 // argument is *only* loaded, then it can pass the value into the function
14 // instead of the address of the value. This can cause recursive simplification
15 // of code and lead to the elimination of allocas (especially in C++ template
16 // code like the STL).
18 // This pass also handles aggregate arguments that are passed into a function,
19 // scalarizing them if the elements of the aggregate are only loaded. Note that
20 // it refuses to scalarize aggregates which would require passing in more than
21 // three operands to the function, because passing thousands of operands for a
22 // large array or structure is unprofitable!
24 // Note that this transformation could also be done for arguments that are only
25 // stored to (returning the value instead), but does not currently. This case
26 // would be best handled when and if LLVM begins supporting multiple return
27 // values from functions.
29 //===----------------------------------------------------------------------===//
31 #define DEBUG_TYPE "argpromotion"
32 #include "llvm/Transforms/IPO.h"
33 #include "llvm/Constants.h"
34 #include "llvm/DerivedTypes.h"
35 #include "llvm/Module.h"
36 #include "llvm/CallGraphSCCPass.h"
37 #include "llvm/Instructions.h"
38 #include "llvm/ParameterAttributes.h"
39 #include "llvm/Analysis/AliasAnalysis.h"
40 #include "llvm/Analysis/CallGraph.h"
41 #include "llvm/Target/TargetData.h"
42 #include "llvm/Support/CallSite.h"
43 #include "llvm/Support/CFG.h"
44 #include "llvm/Support/Debug.h"
45 #include "llvm/ADT/DepthFirstIterator.h"
46 #include "llvm/ADT/Statistic.h"
47 #include "llvm/ADT/StringExtras.h"
48 #include "llvm/Support/Compiler.h"
52 STATISTIC(NumArgumentsPromoted , "Number of pointer arguments promoted");
53 STATISTIC(NumAggregatesPromoted, "Number of aggregate arguments promoted");
54 STATISTIC(NumArgumentsDead , "Number of dead pointer args eliminated");
57 /// ArgPromotion - The 'by reference' to 'by value' argument promotion pass.
59 struct VISIBILITY_HIDDEN ArgPromotion : public CallGraphSCCPass {
60 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
61 AU.addRequired<AliasAnalysis>();
62 AU.addRequired<TargetData>();
63 CallGraphSCCPass::getAnalysisUsage(AU);
66 virtual bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
67 static char ID; // Pass identification, replacement for typeid
68 ArgPromotion() : CallGraphSCCPass((intptr_t)&ID) {}
71 bool PromoteArguments(CallGraphNode *CGN);
72 bool isSafeToPromoteArgument(Argument *Arg) const;
73 Function *DoPromotion(Function *F,
74 SmallVectorImpl<Argument*> &ArgsToPromote);
77 char ArgPromotion::ID = 0;
78 RegisterPass<ArgPromotion> X("argpromotion",
79 "Promote 'by reference' arguments to scalars");
82 Pass *llvm::createArgumentPromotionPass() {
83 return new ArgPromotion();
86 bool ArgPromotion::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
87 bool Changed = false, LocalChange;
89 do { // Iterate until we stop promoting from this SCC.
91 // Attempt to promote arguments from all functions in this SCC.
92 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
93 LocalChange |= PromoteArguments(SCC[i]);
94 Changed |= LocalChange; // Remember that we changed something.
95 } while (LocalChange);
100 /// PromoteArguments - This method checks the specified function to see if there
101 /// are any promotable arguments and if it is safe to promote the function (for
102 /// example, all callers are direct). If safe to promote some arguments, it
103 /// calls the DoPromotion method.
105 bool ArgPromotion::PromoteArguments(CallGraphNode *CGN) {
106 Function *F = CGN->getFunction();
108 // Make sure that it is local to this module.
109 if (!F || !F->hasInternalLinkage()) return false;
111 // First check: see if there are any pointer arguments! If not, quick exit.
112 SmallVector<Argument*, 16> PointerArgs;
113 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I)
114 if (isa<PointerType>(I->getType()))
115 PointerArgs.push_back(I);
116 if (PointerArgs.empty()) return false;
118 // Second check: make sure that all callers are direct callers. We can't
119 // transform functions that have indirect callers.
120 for (Value::use_iterator UI = F->use_begin(), E = F->use_end();
122 CallSite CS = CallSite::get(*UI);
123 if (!CS.getInstruction()) // "Taking the address" of the function
126 // Ensure that this call site is CALLING the function, not passing it as
128 if (UI.getOperandNo() != 0)
132 // Check to see which arguments are promotable. If an argument is not
133 // promotable, remove it from the PointerArgs vector.
134 for (unsigned i = 0; i != PointerArgs.size(); ++i)
135 if (!isSafeToPromoteArgument(PointerArgs[i])) {
136 std::swap(PointerArgs[i--], PointerArgs.back());
137 PointerArgs.pop_back();
140 // No promotable pointer arguments.
141 if (PointerArgs.empty()) return false;
143 // Okay, promote all of the arguments and rewrite the callees!
144 Function *NewF = DoPromotion(F, PointerArgs);
146 // Update the call graph to know that the function has been transformed.
147 getAnalysis<CallGraph>().changeFunction(F, NewF);
151 /// IsAlwaysValidPointer - Return true if the specified pointer is always legal
153 static bool IsAlwaysValidPointer(Value *V) {
154 if (isa<AllocaInst>(V) || isa<GlobalVariable>(V)) return true;
155 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V))
156 return IsAlwaysValidPointer(GEP->getOperand(0));
157 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
158 if (CE->getOpcode() == Instruction::GetElementPtr)
159 return IsAlwaysValidPointer(CE->getOperand(0));
164 /// AllCalleesPassInValidPointerForArgument - Return true if we can prove that
165 /// all callees pass in a valid pointer for the specified function argument.
166 static bool AllCalleesPassInValidPointerForArgument(Argument *Arg) {
167 Function *Callee = Arg->getParent();
169 unsigned ArgNo = std::distance(Callee->arg_begin(),
170 Function::arg_iterator(Arg));
172 // Look at all call sites of the function. At this pointer we know we only
173 // have direct callees.
174 for (Value::use_iterator UI = Callee->use_begin(), E = Callee->use_end();
176 CallSite CS = CallSite::get(*UI);
177 assert(CS.getInstruction() && "Should only have direct calls!");
179 if (!IsAlwaysValidPointer(CS.getArgument(ArgNo)))
186 /// isSafeToPromoteArgument - As you might guess from the name of this method,
187 /// it checks to see if it is both safe and useful to promote the argument.
188 /// This method limits promotion of aggregates to only promote up to three
189 /// elements of the aggregate in order to avoid exploding the number of
190 /// arguments passed in.
191 bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const {
192 // We can only promote this argument if all of the uses are loads, or are GEP
193 // instructions (with constant indices) that are subsequently loaded.
194 bool HasLoadInEntryBlock = false;
195 BasicBlock *EntryBlock = Arg->getParent()->begin();
196 SmallVector<LoadInst*, 16> Loads;
197 std::vector<SmallVector<ConstantInt*, 8> > GEPIndices;
198 for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end();
200 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
201 if (LI->isVolatile()) return false; // Don't hack volatile loads
203 HasLoadInEntryBlock |= LI->getParent() == EntryBlock;
204 } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
205 if (GEP->use_empty()) {
206 // Dead GEP's cause trouble later. Just remove them if we run into
208 getAnalysis<AliasAnalysis>().deleteValue(GEP);
209 GEP->getParent()->getInstList().erase(GEP);
210 return isSafeToPromoteArgument(Arg);
212 // Ensure that all of the indices are constants.
213 SmallVector<ConstantInt*, 8> Operands;
214 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i)
215 if (ConstantInt *C = dyn_cast<ConstantInt>(GEP->getOperand(i)))
216 Operands.push_back(C);
218 return false; // Not a constant operand GEP!
220 // Ensure that the only users of the GEP are load instructions.
221 for (Value::use_iterator UI = GEP->use_begin(), E = GEP->use_end();
223 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
224 if (LI->isVolatile()) return false; // Don't hack volatile loads
226 HasLoadInEntryBlock |= LI->getParent() == EntryBlock;
231 // See if there is already a GEP with these indices. If not, check to
232 // make sure that we aren't promoting too many elements. If so, nothing
234 if (std::find(GEPIndices.begin(), GEPIndices.end(), Operands) ==
236 if (GEPIndices.size() == 3) {
237 DOUT << "argpromotion disable promoting argument '"
238 << Arg->getName() << "' because it would require adding more "
239 << "than 3 arguments to the function.\n";
240 // We limit aggregate promotion to only promoting up to three elements
244 GEPIndices.push_back(Operands);
247 return false; // Not a load or a GEP.
250 if (Loads.empty()) return true; // No users, this is a dead argument.
252 // If we decide that we want to promote this argument, the value is going to
253 // be unconditionally loaded in all callees. This is only safe to do if the
254 // pointer was going to be unconditionally loaded anyway (i.e. there is a load
255 // of the pointer in the entry block of the function) or if we can prove that
256 // all pointers passed in are always to legal locations (for example, no null
257 // pointers are passed in, no pointers to free'd memory, etc).
258 if (!HasLoadInEntryBlock && !AllCalleesPassInValidPointerForArgument(Arg))
259 return false; // Cannot prove that this is safe!!
261 // Okay, now we know that the argument is only used by load instructions and
262 // it is safe to unconditionally load the pointer. Use alias analysis to
263 // check to see if the pointer is guaranteed to not be modified from entry of
264 // the function to each of the load instructions.
266 // Because there could be several/many load instructions, remember which
267 // blocks we know to be transparent to the load.
268 std::set<BasicBlock*> TranspBlocks;
270 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
271 TargetData &TD = getAnalysis<TargetData>();
273 for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
274 // Check to see if the load is invalidated from the start of the block to
276 LoadInst *Load = Loads[i];
277 BasicBlock *BB = Load->getParent();
279 const PointerType *LoadTy =
280 cast<PointerType>(Load->getOperand(0)->getType());
281 unsigned LoadSize = (unsigned)TD.getTypeStoreSize(LoadTy->getElementType());
283 if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
284 return false; // Pointer is invalidated!
286 // Now check every path from the entry block to the load for transparency.
287 // To do this, we perform a depth first search on the inverse CFG from the
289 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
290 for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks),
291 E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
292 if (AA.canBasicBlockModify(**I, Arg, LoadSize))
296 // If the path from the entry of the function to each load is free of
297 // instructions that potentially invalidate the load, we can make the
303 /// GEPIdxComparator - Provide a strong ordering for GEP indices. All Value*
304 /// elements are instances of ConstantInt.
306 struct GEPIdxComparator {
307 bool operator()(const std::vector<Value*> &LHS,
308 const std::vector<Value*> &RHS) const {
310 for (; idx < LHS.size() && idx < RHS.size(); ++idx) {
311 if (LHS[idx] != RHS[idx]) {
312 return cast<ConstantInt>(LHS[idx])->getZExtValue() <
313 cast<ConstantInt>(RHS[idx])->getZExtValue();
317 // Return less than if we ran out of stuff in LHS and we didn't run out of
319 return idx == LHS.size() && idx != RHS.size();
325 /// DoPromotion - This method actually performs the promotion of the specified
326 /// arguments, and returns the new function. At this point, we know that it's
328 Function *ArgPromotion::DoPromotion(Function *F,
329 SmallVectorImpl<Argument*> &Args2Prom) {
330 SmallPtrSet<Argument*, 8> ArgsToPromote(Args2Prom.begin(), Args2Prom.end());
332 // Start by computing a new prototype for the function, which is the same as
333 // the old function, but has modified arguments.
334 const FunctionType *FTy = F->getFunctionType();
335 std::vector<const Type*> Params;
337 typedef std::set<std::vector<Value*>, GEPIdxComparator> ScalarizeTable;
339 // ScalarizedElements - If we are promoting a pointer that has elements
340 // accessed out of it, keep track of which elements are accessed so that we
341 // can add one argument for each.
343 // Arguments that are directly loaded will have a zero element value here, to
344 // handle cases where there are both a direct load and GEP accesses.
346 std::map<Argument*, ScalarizeTable> ScalarizedElements;
348 // OriginalLoads - Keep track of a representative load instruction from the
349 // original function so that we can tell the alias analysis implementation
350 // what the new GEP/Load instructions we are inserting look like.
351 std::map<std::vector<Value*>, LoadInst*> OriginalLoads;
353 // ParamAttrs - Keep track of the parameter attributes for the arguments
354 // that we are *not* promoting. For the ones that we do promote, the parameter
355 // attributes are lost
356 ParamAttrsVector ParamAttrsVec;
357 const ParamAttrsList *PAL = F->getParamAttrs();
360 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E;
362 if (!ArgsToPromote.count(I)) {
363 Params.push_back(I->getType());
365 unsigned attrs = PAL->getParamAttrs(index);
367 ParamAttrsVec.push_back(ParamAttrsWithIndex::get(Params.size(),
370 } else if (I->use_empty()) {
373 // Okay, this is being promoted. Check to see if there are any GEP uses
375 ScalarizeTable &ArgIndices = ScalarizedElements[I];
376 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
378 Instruction *User = cast<Instruction>(*UI);
379 assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User));
380 std::vector<Value*> Indices(User->op_begin()+1, User->op_end());
381 ArgIndices.insert(Indices);
383 if (LoadInst *L = dyn_cast<LoadInst>(User))
386 OrigLoad = cast<LoadInst>(User->use_back());
387 OriginalLoads[Indices] = OrigLoad;
390 // Add a parameter to the function for each element passed in.
391 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
392 E = ArgIndices.end(); SI != E; ++SI)
393 Params.push_back(GetElementPtrInst::getIndexedType(I->getType(),
397 if (ArgIndices.size() == 1 && ArgIndices.begin()->empty())
398 ++NumArgumentsPromoted;
400 ++NumAggregatesPromoted;
403 const Type *RetTy = FTy->getReturnType();
405 // Recompute the parameter attributes list based on the new arguments for
407 if (ParamAttrsVec.empty())
410 PAL = ParamAttrsList::get(ParamAttrsVec);
412 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
413 // have zero fixed arguments.
414 bool ExtraArgHack = false;
415 if (Params.empty() && FTy->isVarArg()) {
417 Params.push_back(Type::Int32Ty);
420 // Construct the new function type using the new arguments.
421 FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
423 // Create the new function body and insert it into the module...
424 Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
425 NF->setCallingConv(F->getCallingConv());
426 NF->setParamAttrs(PAL);
427 if (F->hasCollector())
428 NF->setCollector(F->getCollector());
429 F->getParent()->getFunctionList().insert(F, NF);
431 // Get the alias analysis information that we need to update to reflect our
433 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
435 // Loop over all of the callers of the function, transforming the call sites
436 // to pass in the loaded pointers.
438 std::vector<Value*> Args;
439 while (!F->use_empty()) {
440 CallSite CS = CallSite::get(F->use_back());
441 Instruction *Call = CS.getInstruction();
443 // Loop over the operands, inserting GEP and loads in the caller as
445 CallSite::arg_iterator AI = CS.arg_begin();
446 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
448 if (!ArgsToPromote.count(I))
449 Args.push_back(*AI); // Unmodified argument
450 else if (!I->use_empty()) {
451 // Non-dead argument: insert GEPs and loads as appropriate.
452 ScalarizeTable &ArgIndices = ScalarizedElements[I];
453 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
454 E = ArgIndices.end(); SI != E; ++SI) {
456 LoadInst *OrigLoad = OriginalLoads[*SI];
458 V = new GetElementPtrInst(V, SI->begin(), SI->end(),
459 V->getName()+".idx", Call);
460 AA.copyValue(OrigLoad->getOperand(0), V);
462 Args.push_back(new LoadInst(V, V->getName()+".val", Call));
463 AA.copyValue(OrigLoad, Args.back());
468 Args.push_back(Constant::getNullValue(Type::Int32Ty));
470 // Push any varargs arguments on the list
471 for (; AI != CS.arg_end(); ++AI)
475 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
476 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
477 Args.begin(), Args.end(), "", Call);
478 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
479 cast<InvokeInst>(New)->setParamAttrs(PAL);
481 New = new CallInst(NF, Args.begin(), Args.end(), "", Call);
482 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
483 cast<CallInst>(New)->setParamAttrs(PAL);
484 if (cast<CallInst>(Call)->isTailCall())
485 cast<CallInst>(New)->setTailCall();
489 // Update the alias analysis implementation to know that we are replacing
490 // the old call with a new one.
491 AA.replaceWithNewValue(Call, New);
493 if (!Call->use_empty()) {
494 Call->replaceAllUsesWith(New);
498 // Finally, remove the old call from the program, reducing the use-count of
500 Call->getParent()->getInstList().erase(Call);
503 // Since we have now created the new function, splice the body of the old
504 // function right into the new function, leaving the old rotting hulk of the
506 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
508 // Loop over the argument list, transfering uses of the old arguments over to
509 // the new arguments, also transfering over the names as well.
511 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
512 I2 = NF->arg_begin(); I != E; ++I)
513 if (!ArgsToPromote.count(I)) {
514 // If this is an unmodified argument, move the name and users over to the
516 I->replaceAllUsesWith(I2);
518 AA.replaceWithNewValue(I, I2);
520 } else if (I->use_empty()) {
523 // Otherwise, if we promoted this argument, then all users are load
524 // instructions, and all loads should be using the new argument that we
526 ScalarizeTable &ArgIndices = ScalarizedElements[I];
528 while (!I->use_empty()) {
529 if (LoadInst *LI = dyn_cast<LoadInst>(I->use_back())) {
530 assert(ArgIndices.begin()->empty() &&
531 "Load element should sort to front!");
532 I2->setName(I->getName()+".val");
533 LI->replaceAllUsesWith(I2);
534 AA.replaceWithNewValue(LI, I2);
535 LI->getParent()->getInstList().erase(LI);
536 DOUT << "*** Promoted load of argument '" << I->getName()
537 << "' in function '" << F->getName() << "'\n";
539 GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->use_back());
540 std::vector<Value*> Operands(GEP->op_begin()+1, GEP->op_end());
542 Function::arg_iterator TheArg = I2;
543 for (ScalarizeTable::iterator It = ArgIndices.begin();
544 *It != Operands; ++It, ++TheArg) {
545 assert(It != ArgIndices.end() && "GEP not handled??");
548 std::string NewName = I->getName();
549 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
550 if (ConstantInt *CI = dyn_cast<ConstantInt>(Operands[i]))
551 NewName += "." + CI->getValue().toStringUnsigned(10);
554 TheArg->setName(NewName+".val");
556 DOUT << "*** Promoted agg argument '" << TheArg->getName()
557 << "' of function '" << F->getName() << "'\n";
559 // All of the uses must be load instructions. Replace them all with
560 // the argument specified by ArgNo.
561 while (!GEP->use_empty()) {
562 LoadInst *L = cast<LoadInst>(GEP->use_back());
563 L->replaceAllUsesWith(TheArg);
564 AA.replaceWithNewValue(L, TheArg);
565 L->getParent()->getInstList().erase(L);
568 GEP->getParent()->getInstList().erase(GEP);
572 // Increment I2 past all of the arguments added for this promoted pointer.
573 for (unsigned i = 0, e = ArgIndices.size(); i != e; ++i)
577 // Notify the alias analysis implementation that we inserted a new argument.
579 AA.copyValue(Constant::getNullValue(Type::Int32Ty), NF->arg_begin());
582 // Tell the alias analysis that the old function is about to disappear.
583 AA.replaceWithNewValue(F, NF);
585 // Now that the old function is dead, delete it.
586 F->getParent()->getFunctionList().erase(F);