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(), Function::arg_iterator(Arg));
166 // Look at all call sites of the function. At this pointer we know we only
167 // have direct callees.
168 for (Value::use_iterator UI = Callee->use_begin(), E = Callee->use_end();
170 CallSite CS = CallSite::get(*UI);
171 assert(CS.getInstruction() && "Should only have direct calls!");
173 if (!IsAlwaysValidPointer(CS.getArgument(ArgNo)))
180 /// isSafeToPromoteArgument - As you might guess from the name of this method,
181 /// it checks to see if it is both safe and useful to promote the argument.
182 /// This method limits promotion of aggregates to only promote up to three
183 /// elements of the aggregate in order to avoid exploding the number of
184 /// arguments passed in.
185 bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const {
186 // We can only promote this argument if all of the uses are loads, or are GEP
187 // instructions (with constant indices) that are subsequently loaded.
188 bool HasLoadInEntryBlock = false;
189 BasicBlock *EntryBlock = Arg->getParent()->begin();
190 std::vector<LoadInst*> Loads;
191 std::vector<std::vector<ConstantInt*> > GEPIndices;
192 for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end();
194 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
195 if (LI->isVolatile()) return false; // Don't hack volatile loads
197 HasLoadInEntryBlock |= LI->getParent() == EntryBlock;
198 } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
199 if (GEP->use_empty()) {
200 // Dead GEP's cause trouble later. Just remove them if we run into
202 getAnalysis<AliasAnalysis>().deleteValue(GEP);
203 GEP->getParent()->getInstList().erase(GEP);
204 return isSafeToPromoteArgument(Arg);
206 // Ensure that all of the indices are constants.
207 std::vector<ConstantInt*> Operands;
208 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i)
209 if (ConstantInt *C = dyn_cast<ConstantInt>(GEP->getOperand(i)))
210 Operands.push_back(C);
212 return false; // Not a constant operand GEP!
214 // Ensure that the only users of the GEP are load instructions.
215 for (Value::use_iterator UI = GEP->use_begin(), E = GEP->use_end();
217 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
218 if (LI->isVolatile()) return false; // Don't hack volatile loads
220 HasLoadInEntryBlock |= LI->getParent() == EntryBlock;
225 // See if there is already a GEP with these indices. If not, check to
226 // make sure that we aren't promoting too many elements. If so, nothing
228 if (std::find(GEPIndices.begin(), GEPIndices.end(), Operands) ==
230 if (GEPIndices.size() == 3) {
231 DOUT << "argpromotion disable promoting argument '"
232 << Arg->getName() << "' because it would require adding more "
233 << "than 3 arguments to the function.\n";
234 // We limit aggregate promotion to only promoting up to three elements
238 GEPIndices.push_back(Operands);
241 return false; // Not a load or a GEP.
244 if (Loads.empty()) return true; // No users, this is a dead argument.
246 // If we decide that we want to promote this argument, the value is going to
247 // be unconditionally loaded in all callees. This is only safe to do if the
248 // pointer was going to be unconditionally loaded anyway (i.e. there is a load
249 // of the pointer in the entry block of the function) or if we can prove that
250 // all pointers passed in are always to legal locations (for example, no null
251 // pointers are passed in, no pointers to free'd memory, etc).
252 if (!HasLoadInEntryBlock && !AllCalleesPassInValidPointerForArgument(Arg))
253 return false; // Cannot prove that this is safe!!
255 // Okay, now we know that the argument is only used by load instructions and
256 // it is safe to unconditionally load the pointer. Use alias analysis to
257 // check to see if the pointer is guaranteed to not be modified from entry of
258 // the function to each of the load instructions.
260 // Because there could be several/many load instructions, remember which
261 // blocks we know to be transparent to the load.
262 std::set<BasicBlock*> TranspBlocks;
264 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
265 TargetData &TD = getAnalysis<TargetData>();
267 for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
268 // Check to see if the load is invalidated from the start of the block to
270 LoadInst *Load = Loads[i];
271 BasicBlock *BB = Load->getParent();
273 const PointerType *LoadTy =
274 cast<PointerType>(Load->getOperand(0)->getType());
275 unsigned LoadSize = (unsigned)TD.getTypeSize(LoadTy->getElementType());
277 if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
278 return false; // Pointer is invalidated!
280 // Now check every path from the entry block to the load for transparency.
281 // To do this, we perform a depth first search on the inverse CFG from the
283 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
284 for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks),
285 E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
286 if (AA.canBasicBlockModify(**I, Arg, LoadSize))
290 // If the path from the entry of the function to each load is free of
291 // instructions that potentially invalidate the load, we can make the
297 /// GEPIdxComparator - Provide a strong ordering for GEP indices. All Value*
298 /// elements are instances of ConstantInt.
300 struct GEPIdxComparator {
301 bool operator()(const std::vector<Value*> &LHS,
302 const std::vector<Value*> &RHS) const {
304 for (; idx < LHS.size() && idx < RHS.size(); ++idx) {
305 if (LHS[idx] != RHS[idx]) {
306 return cast<ConstantInt>(LHS[idx])->getZExtValue() <
307 cast<ConstantInt>(RHS[idx])->getZExtValue();
311 // Return less than if we ran out of stuff in LHS and we didn't run out of
313 return idx == LHS.size() && idx != RHS.size();
319 /// DoPromotion - This method actually performs the promotion of the specified
320 /// arguments, and returns the new function. At this point, we know that it's
322 Function *ArgPromotion::DoPromotion(Function *F,
323 std::vector<Argument*> &Args2Prom) {
324 std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end());
326 // Start by computing a new prototype for the function, which is the same as
327 // the old function, but has modified arguments.
328 const FunctionType *FTy = F->getFunctionType();
329 std::vector<const Type*> Params;
331 typedef std::set<std::vector<Value*>, GEPIdxComparator> ScalarizeTable;
333 // ScalarizedElements - If we are promoting a pointer that has elements
334 // accessed out of it, keep track of which elements are accessed so that we
335 // can add one argument for each.
337 // Arguments that are directly loaded will have a zero element value here, to
338 // handle cases where there are both a direct load and GEP accesses.
340 std::map<Argument*, ScalarizeTable> ScalarizedElements;
342 // OriginalLoads - Keep track of a representative load instruction from the
343 // original function so that we can tell the alias analysis implementation
344 // what the new GEP/Load instructions we are inserting look like.
345 std::map<std::vector<Value*>, LoadInst*> OriginalLoads;
347 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I)
348 if (!ArgsToPromote.count(I)) {
349 Params.push_back(I->getType());
350 } else if (I->use_empty()) {
353 // Okay, this is being promoted. Check to see if there are any GEP uses
355 ScalarizeTable &ArgIndices = ScalarizedElements[I];
356 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
358 Instruction *User = cast<Instruction>(*UI);
359 assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User));
360 std::vector<Value*> Indices(User->op_begin()+1, User->op_end());
361 ArgIndices.insert(Indices);
363 if (LoadInst *L = dyn_cast<LoadInst>(User))
366 OrigLoad = cast<LoadInst>(User->use_back());
367 OriginalLoads[Indices] = OrigLoad;
370 // Add a parameter to the function for each element passed in.
371 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
372 E = ArgIndices.end(); SI != E; ++SI)
373 Params.push_back(GetElementPtrInst::getIndexedType(I->getType(), *SI));
375 if (ArgIndices.size() == 1 && ArgIndices.begin()->empty())
376 ++NumArgumentsPromoted;
378 ++NumAggregatesPromoted;
381 const Type *RetTy = FTy->getReturnType();
383 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
384 // have zero fixed arguments.
385 bool ExtraArgHack = false;
386 if (Params.empty() && FTy->isVarArg()) {
388 Params.push_back(Type::Int32Ty);
390 FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
392 // Create the new function body and insert it into the module...
393 Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
394 NF->setCallingConv(F->getCallingConv());
395 F->getParent()->getFunctionList().insert(F, NF);
397 // Get the alias analysis information that we need to update to reflect our
399 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
401 // Loop over all of the callers of the function, transforming the call sites
402 // to pass in the loaded pointers.
404 std::vector<Value*> Args;
405 while (!F->use_empty()) {
406 CallSite CS = CallSite::get(F->use_back());
407 Instruction *Call = CS.getInstruction();
409 // Loop over the operands, inserting GEP and loads in the caller as
411 CallSite::arg_iterator AI = CS.arg_begin();
412 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
414 if (!ArgsToPromote.count(I))
415 Args.push_back(*AI); // Unmodified argument
416 else if (!I->use_empty()) {
417 // Non-dead argument: insert GEPs and loads as appropriate.
418 ScalarizeTable &ArgIndices = ScalarizedElements[I];
419 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
420 E = ArgIndices.end(); SI != E; ++SI) {
422 LoadInst *OrigLoad = OriginalLoads[*SI];
424 V = new GetElementPtrInst(V, *SI, V->getName()+".idx", Call);
425 AA.copyValue(OrigLoad->getOperand(0), V);
427 Args.push_back(new LoadInst(V, V->getName()+".val", Call));
428 AA.copyValue(OrigLoad, Args.back());
433 Args.push_back(Constant::getNullValue(Type::Int32Ty));
435 // Push any varargs arguments on the list
436 for (; AI != CS.arg_end(); ++AI)
440 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
441 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
443 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
445 New = new CallInst(NF, Args, "", Call);
446 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
447 if (cast<CallInst>(Call)->isTailCall())
448 cast<CallInst>(New)->setTailCall();
452 // Update the alias analysis implementation to know that we are replacing
453 // the old call with a new one.
454 AA.replaceWithNewValue(Call, New);
456 if (!Call->use_empty()) {
457 Call->replaceAllUsesWith(New);
461 // Finally, remove the old call from the program, reducing the use-count of
463 Call->getParent()->getInstList().erase(Call);
466 // Since we have now created the new function, splice the body of the old
467 // function right into the new function, leaving the old rotting hulk of the
469 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
471 // Loop over the argument list, transfering uses of the old arguments over to
472 // the new arguments, also transfering over the names as well.
474 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(), I2 = NF->arg_begin();
476 if (!ArgsToPromote.count(I)) {
477 // If this is an unmodified argument, move the name and users over to the
479 I->replaceAllUsesWith(I2);
481 AA.replaceWithNewValue(I, I2);
483 } else if (I->use_empty()) {
486 // Otherwise, if we promoted this argument, then all users are load
487 // instructions, and all loads should be using the new argument that we
489 ScalarizeTable &ArgIndices = ScalarizedElements[I];
491 while (!I->use_empty()) {
492 if (LoadInst *LI = dyn_cast<LoadInst>(I->use_back())) {
493 assert(ArgIndices.begin()->empty() &&
494 "Load element should sort to front!");
495 I2->setName(I->getName()+".val");
496 LI->replaceAllUsesWith(I2);
497 AA.replaceWithNewValue(LI, I2);
498 LI->getParent()->getInstList().erase(LI);
499 DOUT << "*** Promoted load of argument '" << I->getName()
500 << "' in function '" << F->getName() << "'\n";
502 GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->use_back());
503 std::vector<Value*> Operands(GEP->op_begin()+1, GEP->op_end());
505 Function::arg_iterator 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->getZExtValue());
517 TheArg->setName(NewName+".val");
519 DOUT << "*** 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::Int32Ty), NF->arg_begin());
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);