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 RegisterPass<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::arg_iterator I = F->arg_begin(), E = F->arg_end(); 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->arg_begin(), Function::arg_iterator(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 DOUT << "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.
262 // Because there could be several/many load instructions, remember which
263 // blocks we know to be transparent to the load.
264 std::set<BasicBlock*> TranspBlocks;
266 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
267 TargetData &TD = getAnalysis<TargetData>();
269 for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
270 // Check to see if the load is invalidated from the start of the block to
272 LoadInst *Load = Loads[i];
273 BasicBlock *BB = Load->getParent();
275 const PointerType *LoadTy =
276 cast<PointerType>(Load->getOperand(0)->getType());
277 unsigned LoadSize = (unsigned)TD.getTypeSize(LoadTy->getElementType());
279 if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
280 return false; // Pointer is invalidated!
282 // Now check every path from the entry block to the load for transparency.
283 // To do this, we perform a depth first search on the inverse CFG from the
285 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
286 for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks),
287 E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
288 if (AA.canBasicBlockModify(**I, Arg, LoadSize))
292 // If the path from the entry of the function to each load is free of
293 // instructions that potentially invalidate the load, we can make the
299 /// GEPIdxComparator - Provide a strong ordering for GEP indices. All Value*
300 /// elements are instances of ConstantInt.
302 struct GEPIdxComparator {
303 bool operator()(const std::vector<Value*> &LHS,
304 const std::vector<Value*> &RHS) const {
306 for (; idx < LHS.size() && idx < RHS.size(); ++idx) {
307 if (LHS[idx] != RHS[idx]) {
308 return cast<ConstantInt>(LHS[idx])->getZExtValue() <
309 cast<ConstantInt>(RHS[idx])->getZExtValue();
313 // Return less than if we ran out of stuff in LHS and we didn't run out of
315 return idx == LHS.size() && idx != RHS.size();
321 /// DoPromotion - This method actually performs the promotion of the specified
322 /// arguments, and returns the new function. At this point, we know that it's
324 Function *ArgPromotion::DoPromotion(Function *F,
325 std::vector<Argument*> &Args2Prom) {
326 std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end());
328 // Start by computing a new prototype for the function, which is the same as
329 // the old function, but has modified arguments.
330 const FunctionType *FTy = F->getFunctionType();
331 std::vector<const Type*> Params;
333 typedef std::set<std::vector<Value*>, GEPIdxComparator> ScalarizeTable;
335 // ScalarizedElements - If we are promoting a pointer that has elements
336 // accessed out of it, keep track of which elements are accessed so that we
337 // can add one argument for each.
339 // Arguments that are directly loaded will have a zero element value here, to
340 // handle cases where there are both a direct load and GEP accesses.
342 std::map<Argument*, ScalarizeTable> ScalarizedElements;
344 // OriginalLoads - Keep track of a representative load instruction from the
345 // original function so that we can tell the alias analysis implementation
346 // what the new GEP/Load instructions we are inserting look like.
347 std::map<std::vector<Value*>, LoadInst*> OriginalLoads;
349 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I)
350 if (!ArgsToPromote.count(I)) {
351 Params.push_back(I->getType());
352 } else if (I->use_empty()) {
355 // Okay, this is being promoted. Check to see if there are any GEP uses
357 ScalarizeTable &ArgIndices = ScalarizedElements[I];
358 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
360 Instruction *User = cast<Instruction>(*UI);
361 assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User));
362 std::vector<Value*> Indices(User->op_begin()+1, User->op_end());
363 ArgIndices.insert(Indices);
365 if (LoadInst *L = dyn_cast<LoadInst>(User))
368 OrigLoad = cast<LoadInst>(User->use_back());
369 OriginalLoads[Indices] = OrigLoad;
372 // Add a parameter to the function for each element passed in.
373 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
374 E = ArgIndices.end(); SI != E; ++SI)
375 Params.push_back(GetElementPtrInst::getIndexedType(I->getType(), *SI));
377 if (ArgIndices.size() == 1 && ArgIndices.begin()->empty())
378 ++NumArgumentsPromoted;
380 ++NumAggregatesPromoted;
383 const Type *RetTy = FTy->getReturnType();
385 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
386 // have zero fixed arguments.
387 bool ExtraArgHack = false;
388 if (Params.empty() && FTy->isVarArg()) {
390 Params.push_back(Type::IntTy);
392 FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
394 // Create the new function body and insert it into the module...
395 Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
396 NF->setCallingConv(F->getCallingConv());
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::arg_iterator I = F->arg_begin(), E = F->arg_end();
416 if (!ArgsToPromote.count(I))
417 Args.push_back(*AI); // Unmodified argument
418 else if (!I->use_empty()) {
419 // Non-dead argument: insert GEPs and loads as appropriate.
420 ScalarizeTable &ArgIndices = ScalarizedElements[I];
421 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
422 E = ArgIndices.end(); SI != E; ++SI) {
424 LoadInst *OrigLoad = OriginalLoads[*SI];
426 V = new GetElementPtrInst(V, *SI, V->getName()+".idx", Call);
427 AA.copyValue(OrigLoad->getOperand(0), V);
429 Args.push_back(new LoadInst(V, V->getName()+".val", Call));
430 AA.copyValue(OrigLoad, Args.back());
435 Args.push_back(Constant::getNullValue(Type::IntTy));
437 // Push any varargs arguments on the list
438 for (; AI != CS.arg_end(); ++AI)
442 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
443 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
445 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
447 New = new CallInst(NF, Args, "", Call);
448 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
449 if (cast<CallInst>(Call)->isTailCall())
450 cast<CallInst>(New)->setTailCall();
454 // Update the alias analysis implementation to know that we are replacing
455 // the old call with a new one.
456 AA.replaceWithNewValue(Call, New);
458 if (!Call->use_empty()) {
459 Call->replaceAllUsesWith(New);
460 std::string Name = Call->getName();
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(), I2 = NF->arg_begin();
480 if (!ArgsToPromote.count(I)) {
481 // If this is an unmodified argument, move the name and users over to the
483 I->replaceAllUsesWith(I2);
484 I2->setName(I->getName());
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 += "."+itostr((int64_t)CI->getZExtValue());
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::IntTy), 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);