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"
51 Statistic<> NumArgumentsPromoted("argpromotion",
52 "Number of pointer arguments promoted");
53 Statistic<> NumAggregatesPromoted("argpromotion",
54 "Number of aggregate arguments promoted");
55 Statistic<> NumArgumentsDead("argpromotion",
56 "Number of dead pointer args eliminated");
58 /// ArgPromotion - The 'by reference' to 'by value' argument promotion pass.
60 struct ArgPromotion : public CallGraphSCCPass {
61 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
62 AU.addRequired<AliasAnalysis>();
63 AU.addRequired<TargetData>();
64 CallGraphSCCPass::getAnalysisUsage(AU);
67 virtual bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
69 bool PromoteArguments(CallGraphNode *CGN);
70 bool isSafeToPromoteArgument(Argument *Arg) const;
71 Function *DoPromotion(Function *F, std::vector<Argument*> &ArgsToPromote);
74 RegisterOpt<ArgPromotion> X("argpromotion",
75 "Promote 'by reference' arguments to scalars");
78 ModulePass *llvm::createArgumentPromotionPass() {
79 return new ArgPromotion();
82 bool ArgPromotion::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
83 bool Changed = false, LocalChange;
85 do { // Iterate until we stop promoting from this SCC.
87 // Attempt to promote arguments from all functions in this SCC.
88 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
89 LocalChange |= PromoteArguments(SCC[i]);
90 Changed |= LocalChange; // Remember that we changed something.
91 } while (LocalChange);
96 /// PromoteArguments - This method checks the specified function to see if there
97 /// are any promotable arguments and if it is safe to promote the function (for
98 /// example, all callers are direct). If safe to promote some arguments, it
99 /// calls the DoPromotion method.
101 bool ArgPromotion::PromoteArguments(CallGraphNode *CGN) {
102 Function *F = CGN->getFunction();
104 // Make sure that it is local to this module.
105 if (!F || !F->hasInternalLinkage()) return false;
107 // First check: see if there are any pointer arguments! If not, quick exit.
108 std::vector<Argument*> PointerArgs;
109 for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
110 if (isa<PointerType>(I->getType()))
111 PointerArgs.push_back(I);
112 if (PointerArgs.empty()) return false;
114 // Second check: make sure that all callers are direct callers. We can't
115 // transform functions that have indirect callers.
116 for (Value::use_iterator UI = F->use_begin(), E = F->use_end();
118 CallSite CS = CallSite::get(*UI);
119 if (!CS.getInstruction()) // "Taking the address" of the function
122 // Ensure that this call site is CALLING the function, not passing it as
124 for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end();
126 if (*AI == F) return false; // Passing the function address in!
129 // Check to see which arguments are promotable. If an argument is not
130 // promotable, remove it from the PointerArgs vector.
131 for (unsigned i = 0; i != PointerArgs.size(); ++i)
132 if (!isSafeToPromoteArgument(PointerArgs[i])) {
133 std::swap(PointerArgs[i--], PointerArgs.back());
134 PointerArgs.pop_back();
137 // No promotable pointer arguments.
138 if (PointerArgs.empty()) return false;
140 // Okay, promote all of the arguments are rewrite the callees!
141 Function *NewF = DoPromotion(F, PointerArgs);
143 // Update the call graph to know that the old function is gone.
144 getAnalysis<CallGraph>().changeFunction(F, NewF);
148 /// IsAlwaysValidPointer - Return true if the specified pointer is always legal
150 static bool IsAlwaysValidPointer(Value *V) {
151 if (isa<AllocaInst>(V) || isa<GlobalVariable>(V)) return true;
152 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V))
153 return IsAlwaysValidPointer(GEP->getOperand(0));
154 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
155 if (CE->getOpcode() == Instruction::GetElementPtr)
156 return IsAlwaysValidPointer(CE->getOperand(0));
161 /// AllCalleesPassInValidPointerForArgument - Return true if we can prove that
162 /// all callees pass in a valid pointer for the specified function argument.
163 static bool AllCalleesPassInValidPointerForArgument(Argument *Arg) {
164 Function *Callee = Arg->getParent();
166 unsigned ArgNo = std::distance(Callee->abegin(), Function::aiterator(Arg));
168 // Look at all call sites of the function. At this pointer we know we only
169 // have direct callees.
170 for (Value::use_iterator UI = Callee->use_begin(), E = Callee->use_end();
172 CallSite CS = CallSite::get(*UI);
173 assert(CS.getInstruction() && "Should only have direct calls!");
175 if (!IsAlwaysValidPointer(CS.getArgument(ArgNo)))
182 /// isSafeToPromoteArgument - As you might guess from the name of this method,
183 /// it checks to see if it is both safe and useful to promote the argument.
184 /// This method limits promotion of aggregates to only promote up to three
185 /// elements of the aggregate in order to avoid exploding the number of
186 /// arguments passed in.
187 bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const {
188 // We can only promote this argument if all of the uses are loads, or are GEP
189 // instructions (with constant indices) that are subsequently loaded.
190 bool HasLoadInEntryBlock = false;
191 BasicBlock *EntryBlock = Arg->getParent()->begin();
192 std::vector<LoadInst*> Loads;
193 std::vector<std::vector<ConstantInt*> > GEPIndices;
194 for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end();
196 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
197 if (LI->isVolatile()) return false; // Don't hack volatile loads
199 HasLoadInEntryBlock |= LI->getParent() == EntryBlock;
200 } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
201 if (GEP->use_empty()) {
202 // Dead GEP's cause trouble later. Just remove them if we run into
204 getAnalysis<AliasAnalysis>().deleteValue(GEP);
205 GEP->getParent()->getInstList().erase(GEP);
206 return isSafeToPromoteArgument(Arg);
208 // Ensure that all of the indices are constants.
209 std::vector<ConstantInt*> Operands;
210 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i)
211 if (ConstantInt *C = dyn_cast<ConstantInt>(GEP->getOperand(i)))
212 Operands.push_back(C);
214 return false; // Not a constant operand GEP!
216 // Ensure that the only users of the GEP are load instructions.
217 for (Value::use_iterator UI = GEP->use_begin(), E = GEP->use_end();
219 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
220 if (LI->isVolatile()) return false; // Don't hack volatile loads
222 HasLoadInEntryBlock |= LI->getParent() == EntryBlock;
227 // See if there is already a GEP with these indices. If not, check to
228 // make sure that we aren't promoting too many elements. If so, nothing
230 if (std::find(GEPIndices.begin(), GEPIndices.end(), Operands) ==
232 if (GEPIndices.size() == 3) {
233 DEBUG(std::cerr << "argpromotion disable promoting argument '"
234 << Arg->getName() << "' because it would require adding more "
235 << "than 3 arguments to the function.\n");
236 // We limit aggregate promotion to only promoting up to three elements
240 GEPIndices.push_back(Operands);
243 return false; // Not a load or a GEP.
246 if (Loads.empty()) return true; // No users, this is a dead argument.
248 // If we decide that we want to promote this argument, the value is going to
249 // be unconditionally loaded in all callees. This is only safe to do if the
250 // pointer was going to be unconditionally loaded anyway (i.e. there is a load
251 // of the pointer in the entry block of the function) or if we can prove that
252 // all pointers passed in are always to legal locations (for example, no null
253 // pointers are passed in, no pointers to free'd memory, etc).
254 if (!HasLoadInEntryBlock && !AllCalleesPassInValidPointerForArgument(Arg))
255 return false; // Cannot prove that this is safe!!
257 // Okay, now we know that the argument is only used by load instructions and
258 // it is safe to unconditionally load the pointer. Use alias analysis to
259 // check to see if the pointer is guaranteed to not be modified from entry of
260 // the function to each of the load instructions.
261 Function &F = *Arg->getParent();
263 // Because there could be several/many load instructions, remember which
264 // blocks we know to be transparent to the load.
265 std::set<BasicBlock*> TranspBlocks;
267 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
268 TargetData &TD = getAnalysis<TargetData>();
270 for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
271 // Check to see if the load is invalidated from the start of the block to
273 LoadInst *Load = Loads[i];
274 BasicBlock *BB = Load->getParent();
276 const PointerType *LoadTy =
277 cast<PointerType>(Load->getOperand(0)->getType());
278 unsigned LoadSize = TD.getTypeSize(LoadTy->getElementType());
280 if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
281 return false; // Pointer is invalidated!
283 // Now check every path from the entry block to the load for transparency.
284 // To do this, we perform a depth first search on the inverse CFG from the
286 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
287 for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks),
288 E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
289 if (AA.canBasicBlockModify(**I, Arg, LoadSize))
293 // If the path from the entry of the function to each load is free of
294 // instructions that potentially invalidate the load, we can make the
300 /// GEPIdxComparator - Provide a strong ordering for GEP indices. All Value*
301 /// elements are instances of ConstantInt.
303 struct GEPIdxComparator {
304 bool operator()(const std::vector<Value*> &LHS,
305 const std::vector<Value*> &RHS) const {
307 for (; idx < LHS.size() && idx < RHS.size(); ++idx) {
308 if (LHS[idx] != RHS[idx]) {
309 return cast<ConstantInt>(LHS[idx])->getRawValue() <
310 cast<ConstantInt>(RHS[idx])->getRawValue();
314 // Return less than if we ran out of stuff in LHS and we didn't run out of
316 return idx == LHS.size() && idx != RHS.size();
322 /// DoPromotion - This method actually performs the promotion of the specified
323 /// arguments, and returns the new function. At this point, we know that it's
325 Function *ArgPromotion::DoPromotion(Function *F,
326 std::vector<Argument*> &Args2Prom) {
327 std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end());
329 // Start by computing a new prototype for the function, which is the same as
330 // the old function, but has modified arguments.
331 const FunctionType *FTy = F->getFunctionType();
332 std::vector<const Type*> Params;
334 typedef std::set<std::vector<Value*>, GEPIdxComparator> ScalarizeTable;
336 // ScalarizedElements - If we are promoting a pointer that has elements
337 // accessed out of it, keep track of which elements are accessed so that we
338 // can add one argument for each.
340 // Arguments that are directly loaded will have a zero element value here, to
341 // handle cases where there are both a direct load and GEP accesses.
343 std::map<Argument*, ScalarizeTable> ScalarizedElements;
345 // OriginalLoads - Keep track of a representative load instruction from the
346 // original function so that we can tell the alias analysis implementation
347 // what the new GEP/Load instructions we are inserting look like.
348 std::map<std::vector<Value*>, LoadInst*> OriginalLoads;
350 for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
351 if (!ArgsToPromote.count(I)) {
352 Params.push_back(I->getType());
353 } else if (I->use_empty()) {
356 // Okay, this is being promoted. Check to see if there are any GEP uses
358 ScalarizeTable &ArgIndices = ScalarizedElements[I];
359 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
361 Instruction *User = cast<Instruction>(*UI);
362 assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User));
363 std::vector<Value*> Indices(User->op_begin()+1, User->op_end());
364 ArgIndices.insert(Indices);
366 if (LoadInst *L = dyn_cast<LoadInst>(User))
369 OrigLoad = cast<LoadInst>(User->use_back());
370 OriginalLoads[Indices] = OrigLoad;
373 // Add a parameter to the function for each element passed in.
374 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
375 E = ArgIndices.end(); SI != E; ++SI)
376 Params.push_back(GetElementPtrInst::getIndexedType(I->getType(), *SI));
378 if (ArgIndices.size() == 1 && ArgIndices.begin()->empty())
379 ++NumArgumentsPromoted;
381 ++NumAggregatesPromoted;
384 const Type *RetTy = FTy->getReturnType();
386 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
387 // have zero fixed arguments.
388 bool ExtraArgHack = false;
389 if (Params.empty() && FTy->isVarArg()) {
391 Params.push_back(Type::IntTy);
393 FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
395 // Create the new function body and insert it into the module...
396 Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
397 F->getParent()->getFunctionList().insert(F, NF);
399 // Get the alias analysis information that we need to update to reflect our
401 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
403 // Loop over all of the callers of the function, transforming the call sites
404 // to pass in the loaded pointers.
406 std::vector<Value*> Args;
407 while (!F->use_empty()) {
408 CallSite CS = CallSite::get(F->use_back());
409 Instruction *Call = CS.getInstruction();
411 // Loop over the operands, inserting GEP and loads in the caller as
413 CallSite::arg_iterator AI = CS.arg_begin();
414 for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I, ++AI)
415 if (!ArgsToPromote.count(I))
416 Args.push_back(*AI); // Unmodified argument
417 else if (!I->use_empty()) {
418 // Non-dead argument: insert GEPs and loads as appropriate.
419 ScalarizeTable &ArgIndices = ScalarizedElements[I];
420 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
421 E = ArgIndices.end(); SI != E; ++SI) {
423 LoadInst *OrigLoad = OriginalLoads[*SI];
425 V = new GetElementPtrInst(V, *SI, V->getName()+".idx", Call);
426 AA.copyValue(OrigLoad->getOperand(0), V);
428 Args.push_back(new LoadInst(V, V->getName()+".val", Call));
429 AA.copyValue(OrigLoad, Args.back());
434 Args.push_back(Constant::getNullValue(Type::IntTy));
436 // Push any varargs arguments on the list
437 for (; AI != CS.arg_end(); ++AI)
441 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
442 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
445 New = new CallInst(NF, Args, "", Call);
449 // Update the alias analysis implementation to know that we are replacing
450 // the old call with a new one.
451 AA.replaceWithNewValue(Call, New);
453 if (!Call->use_empty()) {
454 Call->replaceAllUsesWith(New);
455 std::string Name = Call->getName();
460 // Finally, remove the old call from the program, reducing the use-count of
462 Call->getParent()->getInstList().erase(Call);
465 // Since we have now created the new function, splice the body of the old
466 // function right into the new function, leaving the old rotting hulk of the
468 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
470 // Loop over the argument list, transfering uses of the old arguments over to
471 // the new arguments, also transfering over the names as well.
473 for (Function::aiterator I = F->abegin(), E = F->aend(), I2 = NF->abegin();
475 if (!ArgsToPromote.count(I)) {
476 // If this is an unmodified argument, move the name and users over to the
478 I->replaceAllUsesWith(I2);
479 I2->setName(I->getName());
480 AA.replaceWithNewValue(I, I2);
482 } else if (I->use_empty()) {
485 // Otherwise, if we promoted this argument, then all users are load
486 // instructions, and all loads should be using the new argument that we
488 ScalarizeTable &ArgIndices = ScalarizedElements[I];
490 while (!I->use_empty()) {
491 if (LoadInst *LI = dyn_cast<LoadInst>(I->use_back())) {
492 assert(ArgIndices.begin()->empty() &&
493 "Load element should sort to front!");
494 I2->setName(I->getName()+".val");
495 LI->replaceAllUsesWith(I2);
496 AA.replaceWithNewValue(LI, I2);
497 LI->getParent()->getInstList().erase(LI);
498 DEBUG(std::cerr << "*** Promoted load of argument '" << I->getName()
499 << "' in function '" << F->getName() << "'\n");
501 GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->use_back());
502 std::vector<Value*> Operands(GEP->op_begin()+1, GEP->op_end());
505 Function::aiterator TheArg = I2;
506 for (ScalarizeTable::iterator It = ArgIndices.begin();
507 *It != Operands; ++It, ++TheArg) {
508 assert(It != ArgIndices.end() && "GEP not handled??");
511 std::string NewName = I->getName();
512 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
513 if (ConstantInt *CI = dyn_cast<ConstantInt>(Operands[i]))
514 NewName += "."+itostr((int64_t)CI->getRawValue());
517 TheArg->setName(NewName+".val");
519 DEBUG(std::cerr << "*** Promoted agg argument '" << TheArg->getName()
520 << "' of function '" << F->getName() << "'\n");
522 // All of the uses must be load instructions. Replace them all with
523 // the argument specified by ArgNo.
524 while (!GEP->use_empty()) {
525 LoadInst *L = cast<LoadInst>(GEP->use_back());
526 L->replaceAllUsesWith(TheArg);
527 AA.replaceWithNewValue(L, TheArg);
528 L->getParent()->getInstList().erase(L);
531 GEP->getParent()->getInstList().erase(GEP);
535 // Increment I2 past all of the arguments added for this promoted pointer.
536 for (unsigned i = 0, e = ArgIndices.size(); i != e; ++i)
540 // Notify the alias analysis implementation that we inserted a new argument.
542 AA.copyValue(Constant::getNullValue(Type::IntTy), NF->abegin());
545 // Tell the alias analysis that the old function is about to disappear.
546 AA.replaceWithNewValue(F, NF);
548 // Now that the old function is dead, delete it.
549 F->getParent()->getFunctionList().erase(F);