1 //===-- ArgumentPromotion.cpp - Promote by-reference arguments ------------===//
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 promotes "by reference" arguments to be "by value" arguments. In
11 // practice, this means looking for internal functions that have pointer
12 // arguments. If we can prove, through the use of alias analysis, that an
13 // argument is *only* loaded, then we 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 // we refuse to scalarize aggregates which would require passing in more than
21 // three operands to the function, because we don't want to pass thousands of
22 // operands for a large array or structure!
24 // Note that this transformation could also be done for arguments that are only
25 // stored to (returning the value instead), but we do not currently handle that
26 // case. This case would be best handled when and if we start supporting
27 // multiple return 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/Analysis/AliasAnalysis.h"
39 #include "llvm/Analysis/CallGraph.h"
40 #include "llvm/Target/TargetData.h"
41 #include "llvm/Support/CallSite.h"
42 #include "llvm/Support/CFG.h"
43 #include "llvm/Support/Debug.h"
44 #include "llvm/ADT/DepthFirstIterator.h"
45 #include "llvm/ADT/Statistic.h"
46 #include "llvm/ADT/StringExtras.h"
52 Statistic<> NumArgumentsPromoted("argpromotion",
53 "Number of pointer arguments promoted");
54 Statistic<> NumAggregatesPromoted("argpromotion",
55 "Number of aggregate arguments promoted");
56 Statistic<> NumArgumentsDead("argpromotion",
57 "Number of dead pointer args eliminated");
59 /// ArgPromotion - The 'by reference' to 'by value' argument promotion pass.
61 struct ArgPromotion : public CallGraphSCCPass {
62 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
63 AU.addRequired<AliasAnalysis>();
64 AU.addRequired<TargetData>();
65 CallGraphSCCPass::getAnalysisUsage(AU);
68 virtual bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
70 bool PromoteArguments(CallGraphNode *CGN);
71 bool isSafeToPromoteArgument(Argument *Arg) const;
72 Function *DoPromotion(Function *F, std::vector<Argument*> &ArgsToPromote);
75 RegisterPass<ArgPromotion> X("argpromotion",
76 "Promote 'by reference' arguments to scalars");
79 ModulePass *llvm::createArgumentPromotionPass() {
80 return new ArgPromotion();
83 bool ArgPromotion::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
84 bool Changed = false, LocalChange;
86 do { // Iterate until we stop promoting from this SCC.
88 // Attempt to promote arguments from all functions in this SCC.
89 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
90 LocalChange |= PromoteArguments(SCC[i]);
91 Changed |= LocalChange; // Remember that we changed something.
92 } while (LocalChange);
97 /// PromoteArguments - This method checks the specified function to see if there
98 /// are any promotable arguments and if it is safe to promote the function (for
99 /// example, all callers are direct). If safe to promote some arguments, it
100 /// calls the DoPromotion method.
102 bool ArgPromotion::PromoteArguments(CallGraphNode *CGN) {
103 Function *F = CGN->getFunction();
105 // Make sure that it is local to this module.
106 if (!F || !F->hasInternalLinkage()) return false;
108 // First check: see if there are any pointer arguments! If not, quick exit.
109 std::vector<Argument*> PointerArgs;
110 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I)
111 if (isa<PointerType>(I->getType()))
112 PointerArgs.push_back(I);
113 if (PointerArgs.empty()) return false;
115 // Second check: make sure that all callers are direct callers. We can't
116 // transform functions that have indirect callers.
117 for (Value::use_iterator UI = F->use_begin(), E = F->use_end();
119 CallSite CS = CallSite::get(*UI);
120 if (!CS.getInstruction()) // "Taking the address" of the function
123 // Ensure that this call site is CALLING the function, not passing it as
125 for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end();
127 if (*AI == F) return false; // Passing the function address in!
130 // Check to see which arguments are promotable. If an argument is not
131 // promotable, remove it from the PointerArgs vector.
132 for (unsigned i = 0; i != PointerArgs.size(); ++i)
133 if (!isSafeToPromoteArgument(PointerArgs[i])) {
134 std::swap(PointerArgs[i--], PointerArgs.back());
135 PointerArgs.pop_back();
138 // No promotable pointer arguments.
139 if (PointerArgs.empty()) return false;
141 // Okay, promote all of the arguments are rewrite the callees!
142 Function *NewF = DoPromotion(F, PointerArgs);
144 // Update the call graph to know that the old function is gone.
145 getAnalysis<CallGraph>().changeFunction(F, NewF);
149 /// IsAlwaysValidPointer - Return true if the specified pointer is always legal
151 static bool IsAlwaysValidPointer(Value *V) {
152 if (isa<AllocaInst>(V) || isa<GlobalVariable>(V)) return true;
153 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V))
154 return IsAlwaysValidPointer(GEP->getOperand(0));
155 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
156 if (CE->getOpcode() == Instruction::GetElementPtr)
157 return IsAlwaysValidPointer(CE->getOperand(0));
162 /// AllCalleesPassInValidPointerForArgument - Return true if we can prove that
163 /// all callees pass in a valid pointer for the specified function argument.
164 static bool AllCalleesPassInValidPointerForArgument(Argument *Arg) {
165 Function *Callee = Arg->getParent();
167 unsigned ArgNo = std::distance(Callee->arg_begin(), Function::arg_iterator(Arg));
169 // Look at all call sites of the function. At this pointer we know we only
170 // have direct callees.
171 for (Value::use_iterator UI = Callee->use_begin(), E = Callee->use_end();
173 CallSite CS = CallSite::get(*UI);
174 assert(CS.getInstruction() && "Should only have direct calls!");
176 if (!IsAlwaysValidPointer(CS.getArgument(ArgNo)))
183 /// isSafeToPromoteArgument - As you might guess from the name of this method,
184 /// it checks to see if it is both safe and useful to promote the argument.
185 /// This method limits promotion of aggregates to only promote up to three
186 /// elements of the aggregate in order to avoid exploding the number of
187 /// arguments passed in.
188 bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const {
189 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
191 // We can only promote this argument if all of the uses are loads, or are GEP
192 // instructions (with constant indices) that are subsequently loaded.
193 bool HasLoadInEntryBlock = false;
194 BasicBlock *EntryBlock = Arg->getParent()->begin();
195 std::vector<LoadInst*> Loads;
196 std::vector<std::vector<ConstantInt*> > GEPIndices;
197 for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end();
199 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
200 if (LI->isVolatile()) return false; // Don't hack volatile loads
202 HasLoadInEntryBlock |= LI->getParent() == EntryBlock;
203 } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
204 if (GEP->use_empty()) {
205 // Dead GEP's cause trouble later. Just remove them if we run into
207 getAnalysis<AliasAnalysis>().deleteValue(GEP);
208 GEP->getParent()->getInstList().erase(GEP);
209 return isSafeToPromoteArgument(Arg);
211 // Ensure that all of the indices are constants.
212 std::vector<ConstantInt*> Operands;
213 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i)
214 if (ConstantInt *C = dyn_cast<ConstantInt>(GEP->getOperand(i)))
215 Operands.push_back(C);
217 return false; // Not a constant operand GEP!
219 // Ensure that the only users of the GEP are load instructions.
220 for (Value::use_iterator UI = GEP->use_begin(), E = GEP->use_end();
222 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
223 if (LI->isVolatile()) return false; // Don't hack volatile loads
225 HasLoadInEntryBlock |= LI->getParent() == EntryBlock;
230 // See if there is already a GEP with these indices. If not, check to
231 // make sure that we aren't promoting too many elements. If so, nothing
233 if (std::find(GEPIndices.begin(), GEPIndices.end(), Operands) ==
235 if (GEPIndices.size() == 3) {
236 DEBUG(std::cerr << "argpromotion disable promoting argument '"
237 << Arg->getName() << "' because it would require adding more "
238 << "than 3 arguments to the function.\n");
239 // We limit aggregate promotion to only promoting up to three elements
243 GEPIndices.push_back(Operands);
245 } else if (CallInst* CI = dyn_cast<CallInst>(*UI)) {
246 // Is this a recursive call?
247 if (CI->getCalledFunction() != Arg->getParent())
250 // Find out what position argument we're dealing with.
251 unsigned Position = 0;
252 Function::arg_iterator ArgPos = Arg->getParent()->arg_begin();
253 while (Arg != ArgPos) {
254 assert(ArgPos != Arg->getParent()->arg_end() &&
255 "Arg not in parent's arg list?");
260 // We only know that the call is safe if it's passing the argument in
261 // the same position that it came in at.
262 if (UI.getOperandNo() != Position+1)
265 return false; // Not a load or a GEP.
268 if (Loads.empty()) return true; // No users, this is a dead argument.
270 // If we decide that we want to promote this argument, the value is going to
271 // be unconditionally loaded in all callees. This is only safe to do if the
272 // pointer was going to be unconditionally loaded anyway (i.e. there is a load
273 // of the pointer in the entry block of the function) or if we can prove that
274 // all pointers passed in are always to legal locations (for example, no null
275 // pointers are passed in, no pointers to free'd memory, etc).
276 if (!HasLoadInEntryBlock && !AllCalleesPassInValidPointerForArgument(Arg))
277 return false; // Cannot prove that this is safe!!
279 // Okay, now we know that the argument is only used by load instructions and
280 // it is safe to unconditionally load the pointer. Use alias analysis to
281 // check to see if the pointer is guaranteed to not be modified from entry of
282 // the function to each of the load instructions.
283 Function &F = *Arg->getParent();
285 // Because there could be several/many load instructions, remember which
286 // blocks we know to be transparent to the load.
287 std::set<BasicBlock*> TranspBlocks;
289 TargetData &TD = getAnalysis<TargetData>();
291 for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
292 // Check to see if the load is invalidated from the start of the block to
294 LoadInst *Load = Loads[i];
295 BasicBlock *BB = Load->getParent();
297 const PointerType *LoadTy =
298 cast<PointerType>(Load->getOperand(0)->getType());
299 unsigned LoadSize = (unsigned)TD.getTypeSize(LoadTy->getElementType());
301 if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
302 return false; // Pointer is invalidated!
304 // Now check every path from the entry block to the load for transparency.
305 // To do this, we perform a depth first search on the inverse CFG from the
307 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
308 for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks),
309 E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
310 if (AA.canBasicBlockModify(**I, Arg, LoadSize))
314 // If the path from the entry of the function to each load is free of
315 // instructions that potentially invalidate the load, we can make the
321 /// GEPIdxComparator - Provide a strong ordering for GEP indices. All Value*
322 /// elements are instances of ConstantInt.
324 struct GEPIdxComparator {
325 bool operator()(const std::vector<Value*> &LHS,
326 const std::vector<Value*> &RHS) const {
328 for (; idx < LHS.size() && idx < RHS.size(); ++idx) {
329 if (LHS[idx] != RHS[idx]) {
330 return cast<ConstantInt>(LHS[idx])->getRawValue() <
331 cast<ConstantInt>(RHS[idx])->getRawValue();
335 // Return less than if we ran out of stuff in LHS and we didn't run out of
337 return idx == LHS.size() && idx != RHS.size();
343 /// DoPromotion - This method actually performs the promotion of the specified
344 /// arguments, and returns the new function. At this point, we know that it's
346 Function *ArgPromotion::DoPromotion(Function *F,
347 std::vector<Argument*> &Args2Prom) {
348 std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end());
350 // Start by computing a new prototype for the function, which is the same as
351 // the old function, but has modified arguments.
352 const FunctionType *FTy = F->getFunctionType();
353 std::vector<const Type*> Params;
355 typedef std::set<std::vector<Value*>, GEPIdxComparator> ScalarizeTable;
357 // ScalarizedElements - If we are promoting a pointer that has elements
358 // accessed out of it, keep track of which elements are accessed so that we
359 // can add one argument for each.
361 // Arguments that are directly loaded will have a zero element value here, to
362 // handle cases where there are both a direct load and GEP accesses.
364 std::map<Argument*, ScalarizeTable> ScalarizedElements;
366 // OriginalLoads - Keep track of a representative load instruction from the
367 // original function so that we can tell the alias analysis implementation
368 // what the new GEP/Load instructions we are inserting look like.
369 std::map<std::vector<Value*>, LoadInst*> OriginalLoads;
371 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I)
372 if (!ArgsToPromote.count(I)) {
373 Params.push_back(I->getType());
374 } else if (I->use_empty()) {
377 // Okay, this is being promoted. Check to see if there are any GEP uses
379 ScalarizeTable &ArgIndices = ScalarizedElements[I];
380 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
382 Instruction *User = cast<Instruction>(*UI);
383 if (!isa<CallInst>(User)) {
384 assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User));
385 std::vector<Value*> Indices(User->op_begin()+1, User->op_end());
386 ArgIndices.insert(Indices);
388 if (LoadInst *L = dyn_cast<LoadInst>(User))
391 OrigLoad = cast<LoadInst>(User->use_back());
392 OriginalLoads[Indices] = OrigLoad;
396 // Add a parameter to the function for each element passed in.
397 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
398 E = ArgIndices.end(); SI != E; ++SI)
399 Params.push_back(GetElementPtrInst::getIndexedType(I->getType(), *SI));
401 if (ArgIndices.size() == 1 && ArgIndices.begin()->empty())
402 ++NumArgumentsPromoted;
404 ++NumAggregatesPromoted;
407 const Type *RetTy = FTy->getReturnType();
409 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
410 // have zero fixed arguments.
411 bool ExtraArgHack = false;
412 if (Params.empty() && FTy->isVarArg()) {
414 Params.push_back(Type::IntTy);
416 FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
418 // Create the new function body and insert it into the module...
419 Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
420 NF->setCallingConv(F->getCallingConv());
421 F->getParent()->getFunctionList().insert(F, NF);
423 // Get the alias analysis information that we need to update to reflect our
425 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
427 // Loop over all of the callers of the function, transforming the call sites
428 // to pass in the loaded pointers.
430 std::vector<Value*> Args;
431 while (!F->use_empty()) {
432 CallSite CS = CallSite::get(F->use_back());
433 Instruction *Call = CS.getInstruction();
435 // Loop over the operands, inserting GEP and loads in the caller as
437 CallSite::arg_iterator AI = CS.arg_begin();
438 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
440 if (!ArgsToPromote.count(I))
441 Args.push_back(*AI); // Unmodified argument
442 else if (!I->use_empty()) {
443 // Non-dead argument: insert GEPs and loads as appropriate.
444 ScalarizeTable &ArgIndices = ScalarizedElements[I];
445 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
446 E = ArgIndices.end(); SI != E; ++SI) {
448 LoadInst *OrigLoad = OriginalLoads[*SI];
450 V = new GetElementPtrInst(V, *SI, V->getName()+".idx", Call);
451 AA.copyValue(OrigLoad->getOperand(0), V);
453 Args.push_back(new LoadInst(V, V->getName()+".val", Call));
454 AA.copyValue(OrigLoad, Args.back());
459 Args.push_back(Constant::getNullValue(Type::IntTy));
461 // Push any varargs arguments on the list
462 for (; AI != CS.arg_end(); ++AI)
466 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
467 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
469 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
471 New = new CallInst(NF, Args, "", Call);
472 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
473 if (cast<CallInst>(Call)->isTailCall())
474 cast<CallInst>(New)->setTailCall();
478 // Update the alias analysis implementation to know that we are replacing
479 // the old call with a new one.
480 AA.replaceWithNewValue(Call, New);
482 if (!Call->use_empty()) {
483 Call->replaceAllUsesWith(New);
484 std::string Name = Call->getName();
489 // Finally, remove the old call from the program, reducing the use-count of
491 Call->getParent()->getInstList().erase(Call);
494 // Since we have now created the new function, splice the body of the old
495 // function right into the new function, leaving the old rotting hulk of the
497 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
499 // Loop over the argument list, transfering uses of the old arguments over to
500 // the new arguments, also transfering over the names as well.
502 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(), I2 = NF->arg_begin();
504 if (!ArgsToPromote.count(I)) {
505 // If this is an unmodified argument, move the name and users over to the
507 I->replaceAllUsesWith(I2);
508 I2->setName(I->getName());
509 AA.replaceWithNewValue(I, I2);
511 } else if (I->use_empty()) {
514 // Otherwise, if we promoted this argument, then all users are load
515 // instructions, and all loads should be using the new argument that we
517 ScalarizeTable &ArgIndices = ScalarizedElements[I];
519 while (!I->use_empty()) {
520 if (LoadInst *LI = dyn_cast<LoadInst>(I->use_back())) {
521 assert(ArgIndices.begin()->empty() &&
522 "Load element should sort to front!");
523 I2->setName(I->getName()+".val");
524 LI->replaceAllUsesWith(I2);
525 AA.replaceWithNewValue(LI, I2);
526 LI->getParent()->getInstList().erase(LI);
527 DEBUG(std::cerr << "*** Promoted load of argument '" << I->getName()
528 << "' in function '" << F->getName() << "'\n");
530 GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->use_back());
531 std::vector<Value*> Operands(GEP->op_begin()+1, GEP->op_end());
534 Function::arg_iterator TheArg = I2;
535 for (ScalarizeTable::iterator It = ArgIndices.begin();
536 *It != Operands; ++It, ++TheArg) {
537 assert(It != ArgIndices.end() && "GEP not handled??");
540 std::string NewName = I->getName();
541 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
542 if (ConstantInt *CI = dyn_cast<ConstantInt>(Operands[i]))
543 NewName += "."+itostr((int64_t)CI->getRawValue());
546 TheArg->setName(NewName+".val");
548 DEBUG(std::cerr << "*** Promoted agg argument '" << TheArg->getName()
549 << "' of function '" << F->getName() << "'\n");
551 // All of the uses must be load instructions. Replace them all with
552 // the argument specified by ArgNo.
553 while (!GEP->use_empty()) {
554 LoadInst *L = cast<LoadInst>(GEP->use_back());
555 L->replaceAllUsesWith(TheArg);
556 AA.replaceWithNewValue(L, TheArg);
557 L->getParent()->getInstList().erase(L);
560 GEP->getParent()->getInstList().erase(GEP);
564 // Increment I2 past all of the arguments added for this promoted pointer.
565 for (unsigned i = 0, e = ArgIndices.size(); i != e; ++i)
569 // Notify the alias analysis implementation that we inserted a new argument.
571 AA.copyValue(Constant::getNullValue(Type::IntTy), NF->arg_begin());
574 // Tell the alias analysis that the old function is about to disappear.
575 AA.replaceWithNewValue(F, NF);
577 // Now that the old function is dead, delete it.
578 F->getParent()->getFunctionList().erase(F);