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, bool isByVal) const;
73 Function *DoPromotion(Function *F,
74 SmallPtrSet<Argument*, 8> &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<std::pair<Argument*, unsigned>, 16> PointerArgs;
114 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
115 I != E; ++I, ++ArgNo)
116 if (isa<PointerType>(I->getType()))
117 PointerArgs.push_back(std::pair<Argument*, unsigned>(I, ArgNo));
118 if (PointerArgs.empty()) return false;
120 // Second check: make sure that all callers are direct callers. We can't
121 // transform functions that have indirect callers.
122 for (Value::use_iterator UI = F->use_begin(), E = F->use_end();
124 CallSite CS = CallSite::get(*UI);
125 if (!CS.getInstruction()) // "Taking the address" of the function
128 // Ensure that this call site is CALLING the function, not passing it as
130 if (UI.getOperandNo() != 0)
134 // Check to see which arguments are promotable. If an argument is promotable,
135 // add it to ArgsToPromote.
136 SmallPtrSet<Argument*, 8> ArgsToPromote;
137 for (unsigned i = 0; i != PointerArgs.size(); ++i) {
138 bool isByVal = F->paramHasAttr(PointerArgs[i].second, ParamAttr::ByVal);
139 if (isSafeToPromoteArgument(PointerArgs[i].first, isByVal))
140 ArgsToPromote.insert(PointerArgs[i].first);
143 // No promotable pointer arguments.
144 if (ArgsToPromote.empty()) return false;
146 Function *NewF = DoPromotion(F, ArgsToPromote);
148 // Update the call graph to know that the function has been transformed.
149 getAnalysis<CallGraph>().changeFunction(F, NewF);
153 /// IsAlwaysValidPointer - Return true if the specified pointer is always legal
155 static bool IsAlwaysValidPointer(Value *V) {
156 if (isa<AllocaInst>(V) || isa<GlobalVariable>(V)) return true;
157 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V))
158 return IsAlwaysValidPointer(GEP->getOperand(0));
159 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
160 if (CE->getOpcode() == Instruction::GetElementPtr)
161 return IsAlwaysValidPointer(CE->getOperand(0));
166 /// AllCalleesPassInValidPointerForArgument - Return true if we can prove that
167 /// all callees pass in a valid pointer for the specified function argument.
168 static bool AllCalleesPassInValidPointerForArgument(Argument *Arg) {
169 Function *Callee = Arg->getParent();
171 unsigned ArgNo = std::distance(Callee->arg_begin(),
172 Function::arg_iterator(Arg));
174 // Look at all call sites of the function. At this pointer we know we only
175 // have direct callees.
176 for (Value::use_iterator UI = Callee->use_begin(), E = Callee->use_end();
178 CallSite CS = CallSite::get(*UI);
179 assert(CS.getInstruction() && "Should only have direct calls!");
181 if (!IsAlwaysValidPointer(CS.getArgument(ArgNo)))
188 /// isSafeToPromoteArgument - As you might guess from the name of this method,
189 /// it checks to see if it is both safe and useful to promote the argument.
190 /// This method limits promotion of aggregates to only promote up to three
191 /// elements of the aggregate in order to avoid exploding the number of
192 /// arguments passed in.
193 bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg, bool isByVal) const {
194 // We can only promote this argument if all of the uses are loads, or are GEP
195 // instructions (with constant indices) that are subsequently loaded.
196 bool HasLoadInEntryBlock = false;
197 BasicBlock *EntryBlock = Arg->getParent()->begin();
198 SmallVector<LoadInst*, 16> Loads;
199 std::vector<SmallVector<ConstantInt*, 8> > GEPIndices;
200 for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end();
202 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
203 if (LI->isVolatile()) return false; // Don't hack volatile loads
205 HasLoadInEntryBlock |= LI->getParent() == EntryBlock;
206 } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
207 if (GEP->use_empty()) {
208 // Dead GEP's cause trouble later. Just remove them if we run into
210 getAnalysis<AliasAnalysis>().deleteValue(GEP);
211 GEP->eraseFromParent();
212 return isSafeToPromoteArgument(Arg, isByVal);
214 // Ensure that all of the indices are constants.
215 SmallVector<ConstantInt*, 8> Operands;
216 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i)
217 if (ConstantInt *C = dyn_cast<ConstantInt>(GEP->getOperand(i)))
218 Operands.push_back(C);
220 return false; // Not a constant operand GEP!
222 // Ensure that the only users of the GEP are load instructions.
223 for (Value::use_iterator UI = GEP->use_begin(), E = GEP->use_end();
225 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
226 if (LI->isVolatile()) return false; // Don't hack volatile loads
228 HasLoadInEntryBlock |= LI->getParent() == EntryBlock;
233 // See if there is already a GEP with these indices. If not, check to
234 // make sure that we aren't promoting too many elements. If so, nothing
236 if (std::find(GEPIndices.begin(), GEPIndices.end(), Operands) ==
238 if (GEPIndices.size() == 3) {
239 DOUT << "argpromotion disable promoting argument '"
240 << Arg->getName() << "' because it would require adding more "
241 << "than 3 arguments to the function.\n";
242 // We limit aggregate promotion to only promoting up to three elements
246 GEPIndices.push_back(Operands);
249 return false; // Not a load or a GEP.
252 if (Loads.empty()) return true; // No users, this is a dead argument.
254 // If we decide that we want to promote this argument, the value is going to
255 // be unconditionally loaded in all callees. This is only safe to do if the
256 // pointer was going to be unconditionally loaded anyway (i.e. there is a load
257 // of the pointer in the entry block of the function) or if we can prove that
258 // all pointers passed in are always to legal locations (for example, no null
259 // pointers are passed in, no pointers to free'd memory, etc).
260 if (!HasLoadInEntryBlock && !AllCalleesPassInValidPointerForArgument(Arg))
261 return false; // Cannot prove that this is safe!!
263 // Okay, now we know that the argument is only used by load instructions and
264 // it is safe to unconditionally load the pointer. Use alias analysis to
265 // check to see if the pointer is guaranteed to not be modified from entry of
266 // the function to each of the load instructions.
268 // Because there could be several/many load instructions, remember which
269 // blocks we know to be transparent to the load.
270 std::set<BasicBlock*> TranspBlocks;
272 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
273 TargetData &TD = getAnalysis<TargetData>();
275 for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
276 // Check to see if the load is invalidated from the start of the block to
278 LoadInst *Load = Loads[i];
279 BasicBlock *BB = Load->getParent();
281 const PointerType *LoadTy =
282 cast<PointerType>(Load->getOperand(0)->getType());
283 unsigned LoadSize = (unsigned)TD.getTypeStoreSize(LoadTy->getElementType());
285 if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
286 return false; // Pointer is invalidated!
288 // Now check every path from the entry block to the load for transparency.
289 // To do this, we perform a depth first search on the inverse CFG from the
291 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
292 for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks),
293 E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
294 if (AA.canBasicBlockModify(**I, Arg, LoadSize))
298 // If the path from the entry of the function to each load is free of
299 // instructions that potentially invalidate the load, we can make the
305 /// GEPIdxComparator - Provide a strong ordering for GEP indices. All Value*
306 /// elements are instances of ConstantInt.
308 struct GEPIdxComparator {
309 bool operator()(const std::vector<Value*> &LHS,
310 const std::vector<Value*> &RHS) const {
312 for (; idx < LHS.size() && idx < RHS.size(); ++idx) {
313 if (LHS[idx] != RHS[idx]) {
314 return cast<ConstantInt>(LHS[idx])->getZExtValue() <
315 cast<ConstantInt>(RHS[idx])->getZExtValue();
319 // Return less than if we ran out of stuff in LHS and we didn't run out of
321 return idx == LHS.size() && idx != RHS.size();
327 /// DoPromotion - This method actually performs the promotion of the specified
328 /// arguments, and returns the new function. At this point, we know that it's
330 Function *ArgPromotion::DoPromotion(Function *F,
331 SmallPtrSet<Argument*, 8> &ArgsToPromote) {
333 // Start by computing a new prototype for the function, which is the same as
334 // the old function, but has modified arguments.
335 const FunctionType *FTy = F->getFunctionType();
336 std::vector<const Type*> Params;
338 typedef std::set<std::vector<Value*>, GEPIdxComparator> ScalarizeTable;
340 // ScalarizedElements - If we are promoting a pointer that has elements
341 // accessed out of it, keep track of which elements are accessed so that we
342 // can add one argument for each.
344 // Arguments that are directly loaded will have a zero element value here, to
345 // handle cases where there are both a direct load and GEP accesses.
347 std::map<Argument*, ScalarizeTable> ScalarizedElements;
349 // OriginalLoads - Keep track of a representative load instruction from the
350 // original function so that we can tell the alias analysis implementation
351 // what the new GEP/Load instructions we are inserting look like.
352 std::map<std::vector<Value*>, LoadInst*> OriginalLoads;
354 // ParamAttrs - Keep track of the parameter attributes for the arguments
355 // that we are *not* promoting. For the ones that we do promote, the parameter
356 // attributes are lost
357 ParamAttrsVector ParamAttrsVec;
358 const ParamAttrsList *PAL = F->getParamAttrs();
361 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E;
363 if (!ArgsToPromote.count(I)) {
364 Params.push_back(I->getType());
366 unsigned attrs = PAL->getParamAttrs(index);
368 ParamAttrsVec.push_back(ParamAttrsWithIndex::get(Params.size(),
371 } else if (I->use_empty()) {
374 // Okay, this is being promoted. Check to see if there are any GEP uses
376 ScalarizeTable &ArgIndices = ScalarizedElements[I];
377 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
379 Instruction *User = cast<Instruction>(*UI);
380 assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User));
381 std::vector<Value*> Indices(User->op_begin()+1, User->op_end());
382 ArgIndices.insert(Indices);
384 if (LoadInst *L = dyn_cast<LoadInst>(User))
387 OrigLoad = cast<LoadInst>(User->use_back());
388 OriginalLoads[Indices] = OrigLoad;
391 // Add a parameter to the function for each element passed in.
392 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
393 E = ArgIndices.end(); SI != E; ++SI)
394 Params.push_back(GetElementPtrInst::getIndexedType(I->getType(),
398 if (ArgIndices.size() == 1 && ArgIndices.begin()->empty())
399 ++NumArgumentsPromoted;
401 ++NumAggregatesPromoted;
404 const Type *RetTy = FTy->getReturnType();
406 // Recompute the parameter attributes list based on the new arguments for
408 if (ParamAttrsVec.empty())
411 PAL = ParamAttrsList::get(ParamAttrsVec);
413 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
414 // have zero fixed arguments.
415 bool ExtraArgHack = false;
416 if (Params.empty() && FTy->isVarArg()) {
418 Params.push_back(Type::Int32Ty);
421 // Construct the new function type using the new arguments.
422 FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
424 // Create the new function body and insert it into the module...
425 Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
426 NF->setCallingConv(F->getCallingConv());
427 NF->setParamAttrs(PAL);
428 if (F->hasCollector())
429 NF->setCollector(F->getCollector());
430 F->getParent()->getFunctionList().insert(F, NF);
432 // Get the alias analysis information that we need to update to reflect our
434 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
436 // Loop over all of the callers of the function, transforming the call sites
437 // to pass in the loaded pointers.
439 std::vector<Value*> Args;
440 while (!F->use_empty()) {
441 CallSite CS = CallSite::get(F->use_back());
442 Instruction *Call = CS.getInstruction();
444 // Loop over the operands, inserting GEP and loads in the caller as
446 CallSite::arg_iterator AI = CS.arg_begin();
447 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
449 if (!ArgsToPromote.count(I))
450 Args.push_back(*AI); // Unmodified argument
451 else if (!I->use_empty()) {
452 // Non-dead argument: insert GEPs and loads as appropriate.
453 ScalarizeTable &ArgIndices = ScalarizedElements[I];
454 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
455 E = ArgIndices.end(); SI != E; ++SI) {
457 LoadInst *OrigLoad = OriginalLoads[*SI];
459 V = new GetElementPtrInst(V, SI->begin(), SI->end(),
460 V->getName()+".idx", Call);
461 AA.copyValue(OrigLoad->getOperand(0), V);
463 Args.push_back(new LoadInst(V, V->getName()+".val", Call));
464 AA.copyValue(OrigLoad, Args.back());
469 Args.push_back(Constant::getNullValue(Type::Int32Ty));
471 // Push any varargs arguments on the list
472 for (; AI != CS.arg_end(); ++AI)
476 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
477 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
478 Args.begin(), Args.end(), "", Call);
479 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
480 cast<InvokeInst>(New)->setParamAttrs(PAL);
482 New = new CallInst(NF, Args.begin(), Args.end(), "", Call);
483 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
484 cast<CallInst>(New)->setParamAttrs(PAL);
485 if (cast<CallInst>(Call)->isTailCall())
486 cast<CallInst>(New)->setTailCall();
490 // Update the alias analysis implementation to know that we are replacing
491 // the old call with a new one.
492 AA.replaceWithNewValue(Call, New);
494 if (!Call->use_empty()) {
495 Call->replaceAllUsesWith(New);
499 // Finally, remove the old call from the program, reducing the use-count of
501 Call->eraseFromParent();
504 // Since we have now created the new function, splice the body of the old
505 // function right into the new function, leaving the old rotting hulk of the
507 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
509 // Loop over the argument list, transfering uses of the old arguments over to
510 // the new arguments, also transfering over the names as well.
512 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
513 I2 = NF->arg_begin(); I != E; ++I)
514 if (!ArgsToPromote.count(I)) {
515 // If this is an unmodified argument, move the name and users over to the
517 I->replaceAllUsesWith(I2);
519 AA.replaceWithNewValue(I, I2);
521 } else if (I->use_empty()) {
524 // Otherwise, if we promoted this argument, then all users are load
525 // instructions, and all loads should be using the new argument that we
527 ScalarizeTable &ArgIndices = ScalarizedElements[I];
529 while (!I->use_empty()) {
530 if (LoadInst *LI = dyn_cast<LoadInst>(I->use_back())) {
531 assert(ArgIndices.begin()->empty() &&
532 "Load element should sort to front!");
533 I2->setName(I->getName()+".val");
534 LI->replaceAllUsesWith(I2);
535 AA.replaceWithNewValue(LI, I2);
536 LI->eraseFromParent();
537 DOUT << "*** Promoted load of argument '" << I->getName()
538 << "' in function '" << F->getName() << "'\n";
540 GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->use_back());
541 std::vector<Value*> Operands(GEP->op_begin()+1, GEP->op_end());
543 Function::arg_iterator TheArg = I2;
544 for (ScalarizeTable::iterator It = ArgIndices.begin();
545 *It != Operands; ++It, ++TheArg) {
546 assert(It != ArgIndices.end() && "GEP not handled??");
549 std::string NewName = I->getName();
550 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
551 if (ConstantInt *CI = dyn_cast<ConstantInt>(Operands[i]))
552 NewName += "." + CI->getValue().toStringUnsigned(10);
555 TheArg->setName(NewName+".val");
557 DOUT << "*** Promoted agg argument '" << TheArg->getName()
558 << "' of function '" << F->getName() << "'\n";
560 // All of the uses must be load instructions. Replace them all with
561 // the argument specified by ArgNo.
562 while (!GEP->use_empty()) {
563 LoadInst *L = cast<LoadInst>(GEP->use_back());
564 L->replaceAllUsesWith(TheArg);
565 AA.replaceWithNewValue(L, TheArg);
566 L->eraseFromParent();
569 GEP->eraseFromParent();
573 // Increment I2 past all of the arguments added for this promoted pointer.
574 for (unsigned i = 0, e = ArgIndices.size(); i != e; ++i)
578 // Notify the alias analysis implementation that we inserted a new argument.
580 AA.copyValue(Constant::getNullValue(Type::Int32Ty), NF->arg_begin());
583 // Tell the alias analysis that the old function is about to disappear.
584 AA.replaceWithNewValue(F, NF);
586 // Now that the old function is dead, delete it.
587 F->eraseFromParent();