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"
47 #include "llvm/Support/Compiler.h"
51 STATISTIC(NumArgumentsPromoted , "Number of pointer arguments promoted");
52 STATISTIC(NumAggregatesPromoted, "Number of aggregate arguments promoted");
53 STATISTIC(NumArgumentsDead , "Number of dead pointer args eliminated");
56 /// ArgPromotion - The 'by reference' to 'by value' argument promotion pass.
58 struct VISIBILITY_HIDDEN ArgPromotion : public CallGraphSCCPass {
59 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
60 AU.addRequired<AliasAnalysis>();
61 AU.addRequired<TargetData>();
62 CallGraphSCCPass::getAnalysisUsage(AU);
65 virtual bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
67 bool PromoteArguments(CallGraphNode *CGN);
68 bool isSafeToPromoteArgument(Argument *Arg) const;
69 Function *DoPromotion(Function *F, std::vector<Argument*> &ArgsToPromote);
72 RegisterPass<ArgPromotion> X("argpromotion",
73 "Promote 'by reference' arguments to scalars");
76 Pass *llvm::createArgumentPromotionPass() {
77 return new ArgPromotion();
80 bool ArgPromotion::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
81 bool Changed = false, LocalChange;
83 do { // Iterate until we stop promoting from this SCC.
85 // Attempt to promote arguments from all functions in this SCC.
86 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
87 LocalChange |= PromoteArguments(SCC[i]);
88 Changed |= LocalChange; // Remember that we changed something.
89 } while (LocalChange);
94 /// PromoteArguments - This method checks the specified function to see if there
95 /// are any promotable arguments and if it is safe to promote the function (for
96 /// example, all callers are direct). If safe to promote some arguments, it
97 /// calls the DoPromotion method.
99 bool ArgPromotion::PromoteArguments(CallGraphNode *CGN) {
100 Function *F = CGN->getFunction();
102 // Make sure that it is local to this module.
103 if (!F || !F->hasInternalLinkage()) return false;
105 // First check: see if there are any pointer arguments! If not, quick exit.
106 std::vector<Argument*> PointerArgs;
107 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I)
108 if (isa<PointerType>(I->getType()))
109 PointerArgs.push_back(I);
110 if (PointerArgs.empty()) return false;
112 // Second check: make sure that all callers are direct callers. We can't
113 // transform functions that have indirect callers.
114 for (Value::use_iterator UI = F->use_begin(), E = F->use_end();
116 CallSite CS = CallSite::get(*UI);
117 if (!CS.getInstruction()) // "Taking the address" of the function
120 // Ensure that this call site is CALLING the function, not passing it as
122 for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end();
124 if (*AI == F) return false; // Passing the function address in!
127 // Check to see which arguments are promotable. If an argument is not
128 // promotable, remove it from the PointerArgs vector.
129 for (unsigned i = 0; i != PointerArgs.size(); ++i)
130 if (!isSafeToPromoteArgument(PointerArgs[i])) {
131 std::swap(PointerArgs[i--], PointerArgs.back());
132 PointerArgs.pop_back();
135 // No promotable pointer arguments.
136 if (PointerArgs.empty()) return false;
138 // Okay, promote all of the arguments are rewrite the callees!
139 Function *NewF = DoPromotion(F, PointerArgs);
141 // Update the call graph to know that the old function is gone.
142 getAnalysis<CallGraph>().changeFunction(F, NewF);
146 /// IsAlwaysValidPointer - Return true if the specified pointer is always legal
148 static bool IsAlwaysValidPointer(Value *V) {
149 if (isa<AllocaInst>(V) || isa<GlobalVariable>(V)) return true;
150 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V))
151 return IsAlwaysValidPointer(GEP->getOperand(0));
152 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
153 if (CE->getOpcode() == Instruction::GetElementPtr)
154 return IsAlwaysValidPointer(CE->getOperand(0));
159 /// AllCalleesPassInValidPointerForArgument - Return true if we can prove that
160 /// all callees pass in a valid pointer for the specified function argument.
161 static bool AllCalleesPassInValidPointerForArgument(Argument *Arg) {
162 Function *Callee = Arg->getParent();
164 unsigned ArgNo = std::distance(Callee->arg_begin(),
165 Function::arg_iterator(Arg));
167 // Look at all call sites of the function. At this pointer we know we only
168 // have direct callees.
169 for (Value::use_iterator UI = Callee->use_begin(), E = Callee->use_end();
171 CallSite CS = CallSite::get(*UI);
172 assert(CS.getInstruction() && "Should only have direct calls!");
174 if (!IsAlwaysValidPointer(CS.getArgument(ArgNo)))
181 /// isSafeToPromoteArgument - As you might guess from the name of this method,
182 /// it checks to see if it is both safe and useful to promote the argument.
183 /// This method limits promotion of aggregates to only promote up to three
184 /// elements of the aggregate in order to avoid exploding the number of
185 /// arguments passed in.
186 bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const {
187 // We can only promote this argument if all of the uses are loads, or are GEP
188 // instructions (with constant indices) that are subsequently loaded.
189 bool HasLoadInEntryBlock = false;
190 BasicBlock *EntryBlock = Arg->getParent()->begin();
191 std::vector<LoadInst*> Loads;
192 std::vector<std::vector<ConstantInt*> > GEPIndices;
193 for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end();
195 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
196 if (LI->isVolatile()) return false; // Don't hack volatile loads
198 HasLoadInEntryBlock |= LI->getParent() == EntryBlock;
199 } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
200 if (GEP->use_empty()) {
201 // Dead GEP's cause trouble later. Just remove them if we run into
203 getAnalysis<AliasAnalysis>().deleteValue(GEP);
204 GEP->getParent()->getInstList().erase(GEP);
205 return isSafeToPromoteArgument(Arg);
207 // Ensure that all of the indices are constants.
208 std::vector<ConstantInt*> Operands;
209 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i)
210 if (ConstantInt *C = dyn_cast<ConstantInt>(GEP->getOperand(i)))
211 Operands.push_back(C);
213 return false; // Not a constant operand GEP!
215 // Ensure that the only users of the GEP are load instructions.
216 for (Value::use_iterator UI = GEP->use_begin(), E = GEP->use_end();
218 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
219 if (LI->isVolatile()) return false; // Don't hack volatile loads
221 HasLoadInEntryBlock |= LI->getParent() == EntryBlock;
226 // See if there is already a GEP with these indices. If not, check to
227 // make sure that we aren't promoting too many elements. If so, nothing
229 if (std::find(GEPIndices.begin(), GEPIndices.end(), Operands) ==
231 if (GEPIndices.size() == 3) {
232 DOUT << "argpromotion disable promoting argument '"
233 << Arg->getName() << "' because it would require adding more "
234 << "than 3 arguments to the function.\n";
235 // We limit aggregate promotion to only promoting up to three elements
239 GEPIndices.push_back(Operands);
242 return false; // Not a load or a GEP.
245 if (Loads.empty()) return true; // No users, this is a dead argument.
247 // If we decide that we want to promote this argument, the value is going to
248 // be unconditionally loaded in all callees. This is only safe to do if the
249 // pointer was going to be unconditionally loaded anyway (i.e. there is a load
250 // of the pointer in the entry block of the function) or if we can prove that
251 // all pointers passed in are always to legal locations (for example, no null
252 // pointers are passed in, no pointers to free'd memory, etc).
253 if (!HasLoadInEntryBlock && !AllCalleesPassInValidPointerForArgument(Arg))
254 return false; // Cannot prove that this is safe!!
256 // Okay, now we know that the argument is only used by load instructions and
257 // it is safe to unconditionally load the pointer. Use alias analysis to
258 // check to see if the pointer is guaranteed to not be modified from entry of
259 // the function to each of the load instructions.
261 // Because there could be several/many load instructions, remember which
262 // blocks we know to be transparent to the load.
263 std::set<BasicBlock*> TranspBlocks;
265 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
266 TargetData &TD = getAnalysis<TargetData>();
268 for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
269 // Check to see if the load is invalidated from the start of the block to
271 LoadInst *Load = Loads[i];
272 BasicBlock *BB = Load->getParent();
274 const PointerType *LoadTy =
275 cast<PointerType>(Load->getOperand(0)->getType());
276 unsigned LoadSize = (unsigned)TD.getTypeSize(LoadTy->getElementType());
278 if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
279 return false; // Pointer is invalidated!
281 // Now check every path from the entry block to the load for transparency.
282 // To do this, we perform a depth first search on the inverse CFG from the
284 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
285 for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks),
286 E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
287 if (AA.canBasicBlockModify(**I, Arg, LoadSize))
291 // If the path from the entry of the function to each load is free of
292 // instructions that potentially invalidate the load, we can make the
298 /// GEPIdxComparator - Provide a strong ordering for GEP indices. All Value*
299 /// elements are instances of ConstantInt.
301 struct GEPIdxComparator {
302 bool operator()(const std::vector<Value*> &LHS,
303 const std::vector<Value*> &RHS) const {
305 for (; idx < LHS.size() && idx < RHS.size(); ++idx) {
306 if (LHS[idx] != RHS[idx]) {
307 return cast<ConstantInt>(LHS[idx])->getZExtValue() <
308 cast<ConstantInt>(RHS[idx])->getZExtValue();
312 // Return less than if we ran out of stuff in LHS and we didn't run out of
314 return idx == LHS.size() && idx != RHS.size();
320 /// DoPromotion - This method actually performs the promotion of the specified
321 /// arguments, and returns the new function. At this point, we know that it's
323 Function *ArgPromotion::DoPromotion(Function *F,
324 std::vector<Argument*> &Args2Prom) {
325 std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end());
327 // Start by computing a new prototype for the function, which is the same as
328 // the old function, but has modified arguments.
329 const FunctionType *FTy = F->getFunctionType();
330 std::vector<const Type*> Params;
332 typedef std::set<std::vector<Value*>, GEPIdxComparator> ScalarizeTable;
334 // ScalarizedElements - If we are promoting a pointer that has elements
335 // accessed out of it, keep track of which elements are accessed so that we
336 // can add one argument for each.
338 // Arguments that are directly loaded will have a zero element value here, to
339 // handle cases where there are both a direct load and GEP accesses.
341 std::map<Argument*, ScalarizeTable> ScalarizedElements;
343 // OriginalLoads - Keep track of a representative load instruction from the
344 // original function so that we can tell the alias analysis implementation
345 // what the new GEP/Load instructions we are inserting look like.
346 std::map<std::vector<Value*>, LoadInst*> OriginalLoads;
348 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I)
349 if (!ArgsToPromote.count(I)) {
350 Params.push_back(I->getType());
351 } else if (I->use_empty()) {
354 // Okay, this is being promoted. Check to see if there are any GEP uses
356 ScalarizeTable &ArgIndices = ScalarizedElements[I];
357 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
359 Instruction *User = cast<Instruction>(*UI);
360 assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User));
361 std::vector<Value*> Indices(User->op_begin()+1, User->op_end());
362 ArgIndices.insert(Indices);
364 if (LoadInst *L = dyn_cast<LoadInst>(User))
367 OrigLoad = cast<LoadInst>(User->use_back());
368 OriginalLoads[Indices] = OrigLoad;
371 // Add a parameter to the function for each element passed in.
372 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
373 E = ArgIndices.end(); SI != E; ++SI)
374 Params.push_back(GetElementPtrInst::getIndexedType(I->getType(),
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::Int32Ty);
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 NF->setCallingConv(F->getCallingConv());
398 F->getParent()->getFunctionList().insert(F, NF);
400 // Get the alias analysis information that we need to update to reflect our
402 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
404 // Loop over all of the callers of the function, transforming the call sites
405 // to pass in the loaded pointers.
407 std::vector<Value*> Args;
408 while (!F->use_empty()) {
409 CallSite CS = CallSite::get(F->use_back());
410 Instruction *Call = CS.getInstruction();
412 // Loop over the operands, inserting GEP and loads in the caller as
414 CallSite::arg_iterator AI = CS.arg_begin();
415 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
417 if (!ArgsToPromote.count(I))
418 Args.push_back(*AI); // Unmodified argument
419 else if (!I->use_empty()) {
420 // Non-dead argument: insert GEPs and loads as appropriate.
421 ScalarizeTable &ArgIndices = ScalarizedElements[I];
422 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
423 E = ArgIndices.end(); SI != E; ++SI) {
425 LoadInst *OrigLoad = OriginalLoads[*SI];
427 V = new GetElementPtrInst(V, &(*SI)[0], SI->size(),
428 V->getName()+".idx", Call);
429 AA.copyValue(OrigLoad->getOperand(0), V);
431 Args.push_back(new LoadInst(V, V->getName()+".val", Call));
432 AA.copyValue(OrigLoad, Args.back());
437 Args.push_back(Constant::getNullValue(Type::Int32Ty));
439 // Push any varargs arguments on the list
440 for (; AI != CS.arg_end(); ++AI)
444 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
445 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
446 &Args[0], Args.size(), "", Call);
447 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
449 New = new CallInst(NF, &Args[0], Args.size(), "", Call);
450 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
451 if (cast<CallInst>(Call)->isTailCall())
452 cast<CallInst>(New)->setTailCall();
456 // Update the alias analysis implementation to know that we are replacing
457 // the old call with a new one.
458 AA.replaceWithNewValue(Call, New);
460 if (!Call->use_empty()) {
461 Call->replaceAllUsesWith(New);
465 // Finally, remove the old call from the program, reducing the use-count of
467 Call->getParent()->getInstList().erase(Call);
470 // Since we have now created the new function, splice the body of the old
471 // function right into the new function, leaving the old rotting hulk of the
473 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
475 // Loop over the argument list, transfering uses of the old arguments over to
476 // the new arguments, also transfering over the names as well.
478 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
479 I2 = NF->arg_begin(); I != E; ++I)
480 if (!ArgsToPromote.count(I)) {
481 // If this is an unmodified argument, move the name and users over to the
483 I->replaceAllUsesWith(I2);
485 AA.replaceWithNewValue(I, I2);
487 } else if (I->use_empty()) {
490 // Otherwise, if we promoted this argument, then all users are load
491 // instructions, and all loads should be using the new argument that we
493 ScalarizeTable &ArgIndices = ScalarizedElements[I];
495 while (!I->use_empty()) {
496 if (LoadInst *LI = dyn_cast<LoadInst>(I->use_back())) {
497 assert(ArgIndices.begin()->empty() &&
498 "Load element should sort to front!");
499 I2->setName(I->getName()+".val");
500 LI->replaceAllUsesWith(I2);
501 AA.replaceWithNewValue(LI, I2);
502 LI->getParent()->getInstList().erase(LI);
503 DOUT << "*** Promoted load of argument '" << I->getName()
504 << "' in function '" << F->getName() << "'\n";
506 GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->use_back());
507 std::vector<Value*> Operands(GEP->op_begin()+1, GEP->op_end());
509 Function::arg_iterator TheArg = I2;
510 for (ScalarizeTable::iterator It = ArgIndices.begin();
511 *It != Operands; ++It, ++TheArg) {
512 assert(It != ArgIndices.end() && "GEP not handled??");
515 std::string NewName = I->getName();
516 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
517 if (ConstantInt *CI = dyn_cast<ConstantInt>(Operands[i]))
518 NewName += "." + CI->getValue().toString(10);
521 TheArg->setName(NewName+".val");
523 DOUT << "*** Promoted agg argument '" << TheArg->getName()
524 << "' of function '" << F->getName() << "'\n";
526 // All of the uses must be load instructions. Replace them all with
527 // the argument specified by ArgNo.
528 while (!GEP->use_empty()) {
529 LoadInst *L = cast<LoadInst>(GEP->use_back());
530 L->replaceAllUsesWith(TheArg);
531 AA.replaceWithNewValue(L, TheArg);
532 L->getParent()->getInstList().erase(L);
535 GEP->getParent()->getInstList().erase(GEP);
539 // Increment I2 past all of the arguments added for this promoted pointer.
540 for (unsigned i = 0, e = ArgIndices.size(); i != e; ++i)
544 // Notify the alias analysis implementation that we inserted a new argument.
546 AA.copyValue(Constant::getNullValue(Type::Int32Ty), NF->arg_begin());
549 // Tell the alias analysis that the old function is about to disappear.
550 AA.replaceWithNewValue(F, NF);
552 // Now that the old function is dead, delete it.
553 F->getParent()->getFunctionList().erase(F);