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);
66 static const int ID; // Pass identifcation, replacement for typeid
67 ArgPromotion() : CallGraphSCCPass((intptr_t)&ID) {}
70 bool PromoteArguments(CallGraphNode *CGN);
71 bool isSafeToPromoteArgument(Argument *Arg) const;
72 Function *DoPromotion(Function *F, std::vector<Argument*> &ArgsToPromote);
75 const int ArgPromotion::ID = 0;
76 RegisterPass<ArgPromotion> X("argpromotion",
77 "Promote 'by reference' arguments to scalars");
80 Pass *llvm::createArgumentPromotionPass() {
81 return new ArgPromotion();
84 bool ArgPromotion::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
85 bool Changed = false, LocalChange;
87 do { // Iterate until we stop promoting from this SCC.
89 // Attempt to promote arguments from all functions in this SCC.
90 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
91 LocalChange |= PromoteArguments(SCC[i]);
92 Changed |= LocalChange; // Remember that we changed something.
93 } while (LocalChange);
98 /// PromoteArguments - This method checks the specified function to see if there
99 /// are any promotable arguments and if it is safe to promote the function (for
100 /// example, all callers are direct). If safe to promote some arguments, it
101 /// calls the DoPromotion method.
103 bool ArgPromotion::PromoteArguments(CallGraphNode *CGN) {
104 Function *F = CGN->getFunction();
106 // Make sure that it is local to this module.
107 if (!F || !F->hasInternalLinkage()) return false;
109 // First check: see if there are any pointer arguments! If not, quick exit.
110 std::vector<Argument*> PointerArgs;
111 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I)
112 if (isa<PointerType>(I->getType()))
113 PointerArgs.push_back(I);
114 if (PointerArgs.empty()) return false;
116 // Second check: make sure that all callers are direct callers. We can't
117 // transform functions that have indirect callers.
118 for (Value::use_iterator UI = F->use_begin(), E = F->use_end();
120 CallSite CS = CallSite::get(*UI);
121 if (!CS.getInstruction()) // "Taking the address" of the function
124 // Ensure that this call site is CALLING the function, not passing it as
126 for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end();
128 if (*AI == F) return false; // Passing the function address in!
131 // Check to see which arguments are promotable. If an argument is not
132 // promotable, remove it from the PointerArgs vector.
133 for (unsigned i = 0; i != PointerArgs.size(); ++i)
134 if (!isSafeToPromoteArgument(PointerArgs[i])) {
135 std::swap(PointerArgs[i--], PointerArgs.back());
136 PointerArgs.pop_back();
139 // No promotable pointer arguments.
140 if (PointerArgs.empty()) return false;
142 // Okay, promote all of the arguments are rewrite the callees!
143 Function *NewF = DoPromotion(F, PointerArgs);
145 // Update the call graph to know that the old function is gone.
146 getAnalysis<CallGraph>().changeFunction(F, NewF);
150 /// IsAlwaysValidPointer - Return true if the specified pointer is always legal
152 static bool IsAlwaysValidPointer(Value *V) {
153 if (isa<AllocaInst>(V) || isa<GlobalVariable>(V)) return true;
154 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V))
155 return IsAlwaysValidPointer(GEP->getOperand(0));
156 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
157 if (CE->getOpcode() == Instruction::GetElementPtr)
158 return IsAlwaysValidPointer(CE->getOperand(0));
163 /// AllCalleesPassInValidPointerForArgument - Return true if we can prove that
164 /// all callees pass in a valid pointer for the specified function argument.
165 static bool AllCalleesPassInValidPointerForArgument(Argument *Arg) {
166 Function *Callee = Arg->getParent();
168 unsigned ArgNo = std::distance(Callee->arg_begin(),
169 Function::arg_iterator(Arg));
171 // Look at all call sites of the function. At this pointer we know we only
172 // have direct callees.
173 for (Value::use_iterator UI = Callee->use_begin(), E = Callee->use_end();
175 CallSite CS = CallSite::get(*UI);
176 assert(CS.getInstruction() && "Should only have direct calls!");
178 if (!IsAlwaysValidPointer(CS.getArgument(ArgNo)))
185 /// isSafeToPromoteArgument - As you might guess from the name of this method,
186 /// it checks to see if it is both safe and useful to promote the argument.
187 /// This method limits promotion of aggregates to only promote up to three
188 /// elements of the aggregate in order to avoid exploding the number of
189 /// arguments passed in.
190 bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const {
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 DOUT << "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);
246 return false; // Not a load or a GEP.
249 if (Loads.empty()) return true; // No users, this is a dead argument.
251 // If we decide that we want to promote this argument, the value is going to
252 // be unconditionally loaded in all callees. This is only safe to do if the
253 // pointer was going to be unconditionally loaded anyway (i.e. there is a load
254 // of the pointer in the entry block of the function) or if we can prove that
255 // all pointers passed in are always to legal locations (for example, no null
256 // pointers are passed in, no pointers to free'd memory, etc).
257 if (!HasLoadInEntryBlock && !AllCalleesPassInValidPointerForArgument(Arg))
258 return false; // Cannot prove that this is safe!!
260 // Okay, now we know that the argument is only used by load instructions and
261 // it is safe to unconditionally load the pointer. Use alias analysis to
262 // check to see if the pointer is guaranteed to not be modified from entry of
263 // the function to each of the load instructions.
265 // Because there could be several/many load instructions, remember which
266 // blocks we know to be transparent to the load.
267 std::set<BasicBlock*> TranspBlocks;
269 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
270 TargetData &TD = getAnalysis<TargetData>();
272 for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
273 // Check to see if the load is invalidated from the start of the block to
275 LoadInst *Load = Loads[i];
276 BasicBlock *BB = Load->getParent();
278 const PointerType *LoadTy =
279 cast<PointerType>(Load->getOperand(0)->getType());
280 unsigned LoadSize = (unsigned)TD.getTypeSize(LoadTy->getElementType());
282 if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
283 return false; // Pointer is invalidated!
285 // Now check every path from the entry block to the load for transparency.
286 // To do this, we perform a depth first search on the inverse CFG from the
288 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
289 for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks),
290 E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
291 if (AA.canBasicBlockModify(**I, Arg, LoadSize))
295 // If the path from the entry of the function to each load is free of
296 // instructions that potentially invalidate the load, we can make the
302 /// GEPIdxComparator - Provide a strong ordering for GEP indices. All Value*
303 /// elements are instances of ConstantInt.
305 struct GEPIdxComparator {
306 bool operator()(const std::vector<Value*> &LHS,
307 const std::vector<Value*> &RHS) const {
309 for (; idx < LHS.size() && idx < RHS.size(); ++idx) {
310 if (LHS[idx] != RHS[idx]) {
311 return cast<ConstantInt>(LHS[idx])->getZExtValue() <
312 cast<ConstantInt>(RHS[idx])->getZExtValue();
316 // Return less than if we ran out of stuff in LHS and we didn't run out of
318 return idx == LHS.size() && idx != RHS.size();
324 /// DoPromotion - This method actually performs the promotion of the specified
325 /// arguments, and returns the new function. At this point, we know that it's
327 Function *ArgPromotion::DoPromotion(Function *F,
328 std::vector<Argument*> &Args2Prom) {
329 std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end());
331 // Start by computing a new prototype for the function, which is the same as
332 // the old function, but has modified arguments.
333 const FunctionType *FTy = F->getFunctionType();
334 std::vector<const Type*> Params;
336 typedef std::set<std::vector<Value*>, GEPIdxComparator> ScalarizeTable;
338 // ScalarizedElements - If we are promoting a pointer that has elements
339 // accessed out of it, keep track of which elements are accessed so that we
340 // can add one argument for each.
342 // Arguments that are directly loaded will have a zero element value here, to
343 // handle cases where there are both a direct load and GEP accesses.
345 std::map<Argument*, ScalarizeTable> ScalarizedElements;
347 // OriginalLoads - Keep track of a representative load instruction from the
348 // original function so that we can tell the alias analysis implementation
349 // what the new GEP/Load instructions we are inserting look like.
350 std::map<std::vector<Value*>, LoadInst*> OriginalLoads;
352 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I)
353 if (!ArgsToPromote.count(I)) {
354 Params.push_back(I->getType());
355 } else if (I->use_empty()) {
358 // Okay, this is being promoted. Check to see if there are any GEP uses
360 ScalarizeTable &ArgIndices = ScalarizedElements[I];
361 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
363 Instruction *User = cast<Instruction>(*UI);
364 assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User));
365 std::vector<Value*> Indices(User->op_begin()+1, User->op_end());
366 ArgIndices.insert(Indices);
368 if (LoadInst *L = dyn_cast<LoadInst>(User))
371 OrigLoad = cast<LoadInst>(User->use_back());
372 OriginalLoads[Indices] = OrigLoad;
375 // Add a parameter to the function for each element passed in.
376 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
377 E = ArgIndices.end(); SI != E; ++SI)
378 Params.push_back(GetElementPtrInst::getIndexedType(I->getType(),
382 if (ArgIndices.size() == 1 && ArgIndices.begin()->empty())
383 ++NumArgumentsPromoted;
385 ++NumAggregatesPromoted;
388 const Type *RetTy = FTy->getReturnType();
390 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
391 // have zero fixed arguments.
392 bool ExtraArgHack = false;
393 if (Params.empty() && FTy->isVarArg()) {
395 Params.push_back(Type::Int32Ty);
397 FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
399 // Create the new function body and insert it into the module...
400 Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
401 NF->setCallingConv(F->getCallingConv());
402 F->getParent()->getFunctionList().insert(F, NF);
404 // Get the alias analysis information that we need to update to reflect our
406 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
408 // Loop over all of the callers of the function, transforming the call sites
409 // to pass in the loaded pointers.
411 std::vector<Value*> Args;
412 while (!F->use_empty()) {
413 CallSite CS = CallSite::get(F->use_back());
414 Instruction *Call = CS.getInstruction();
416 // Loop over the operands, inserting GEP and loads in the caller as
418 CallSite::arg_iterator AI = CS.arg_begin();
419 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
421 if (!ArgsToPromote.count(I))
422 Args.push_back(*AI); // Unmodified argument
423 else if (!I->use_empty()) {
424 // Non-dead argument: insert GEPs and loads as appropriate.
425 ScalarizeTable &ArgIndices = ScalarizedElements[I];
426 for (ScalarizeTable::iterator SI = ArgIndices.begin(),
427 E = ArgIndices.end(); SI != E; ++SI) {
429 LoadInst *OrigLoad = OriginalLoads[*SI];
431 V = new GetElementPtrInst(V, &(*SI)[0], SI->size(),
432 V->getName()+".idx", Call);
433 AA.copyValue(OrigLoad->getOperand(0), V);
435 Args.push_back(new LoadInst(V, V->getName()+".val", Call));
436 AA.copyValue(OrigLoad, Args.back());
441 Args.push_back(Constant::getNullValue(Type::Int32Ty));
443 // Push any varargs arguments on the list
444 for (; AI != CS.arg_end(); ++AI)
448 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
449 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
450 &Args[0], Args.size(), "", Call);
451 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
453 New = new CallInst(NF, &Args[0], Args.size(), "", Call);
454 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
455 if (cast<CallInst>(Call)->isTailCall())
456 cast<CallInst>(New)->setTailCall();
460 // Update the alias analysis implementation to know that we are replacing
461 // the old call with a new one.
462 AA.replaceWithNewValue(Call, New);
464 if (!Call->use_empty()) {
465 Call->replaceAllUsesWith(New);
469 // Finally, remove the old call from the program, reducing the use-count of
471 Call->getParent()->getInstList().erase(Call);
474 // Since we have now created the new function, splice the body of the old
475 // function right into the new function, leaving the old rotting hulk of the
477 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
479 // Loop over the argument list, transfering uses of the old arguments over to
480 // the new arguments, also transfering over the names as well.
482 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
483 I2 = NF->arg_begin(); I != E; ++I)
484 if (!ArgsToPromote.count(I)) {
485 // If this is an unmodified argument, move the name and users over to the
487 I->replaceAllUsesWith(I2);
489 AA.replaceWithNewValue(I, I2);
491 } else if (I->use_empty()) {
494 // Otherwise, if we promoted this argument, then all users are load
495 // instructions, and all loads should be using the new argument that we
497 ScalarizeTable &ArgIndices = ScalarizedElements[I];
499 while (!I->use_empty()) {
500 if (LoadInst *LI = dyn_cast<LoadInst>(I->use_back())) {
501 assert(ArgIndices.begin()->empty() &&
502 "Load element should sort to front!");
503 I2->setName(I->getName()+".val");
504 LI->replaceAllUsesWith(I2);
505 AA.replaceWithNewValue(LI, I2);
506 LI->getParent()->getInstList().erase(LI);
507 DOUT << "*** Promoted load of argument '" << I->getName()
508 << "' in function '" << F->getName() << "'\n";
510 GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->use_back());
511 std::vector<Value*> Operands(GEP->op_begin()+1, GEP->op_end());
513 Function::arg_iterator TheArg = I2;
514 for (ScalarizeTable::iterator It = ArgIndices.begin();
515 *It != Operands; ++It, ++TheArg) {
516 assert(It != ArgIndices.end() && "GEP not handled??");
519 std::string NewName = I->getName();
520 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
521 if (ConstantInt *CI = dyn_cast<ConstantInt>(Operands[i]))
522 NewName += "." + CI->getValue().toString(10);
525 TheArg->setName(NewName+".val");
527 DOUT << "*** Promoted agg argument '" << TheArg->getName()
528 << "' of function '" << F->getName() << "'\n";
530 // All of the uses must be load instructions. Replace them all with
531 // the argument specified by ArgNo.
532 while (!GEP->use_empty()) {
533 LoadInst *L = cast<LoadInst>(GEP->use_back());
534 L->replaceAllUsesWith(TheArg);
535 AA.replaceWithNewValue(L, TheArg);
536 L->getParent()->getInstList().erase(L);
539 GEP->getParent()->getInstList().erase(GEP);
543 // Increment I2 past all of the arguments added for this promoted pointer.
544 for (unsigned i = 0, e = ArgIndices.size(); i != e; ++i)
548 // Notify the alias analysis implementation that we inserted a new argument.
550 AA.copyValue(Constant::getNullValue(Type::Int32Ty), NF->arg_begin());
553 // Tell the alias analysis that the old function is about to disappear.
554 AA.replaceWithNewValue(F, NF);
556 // Now that the old function is dead, delete it.
557 F->getParent()->getFunctionList().erase(F);