1 //===- FunctionAttrs.cpp - Pass which marks functions readnone or readonly ===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements a simple interprocedural pass which walks the
11 // call-graph, looking for functions which do not access or only read
12 // non-local memory, and marking them readnone/readonly. In addition,
13 // it marks function arguments (of pointer type) 'nocapture' if a call
14 // to the function does not create any copies of the pointer value that
15 // outlive the call. This more or less means that the pointer is only
16 // dereferenced, and not returned from the function or stored in a global.
17 // This pass is implemented as a bottom-up traversal of the call-graph.
19 //===----------------------------------------------------------------------===//
21 #define DEBUG_TYPE "functionattrs"
22 #include "llvm/Transforms/IPO.h"
23 #include "llvm/CallGraphSCCPass.h"
24 #include "llvm/GlobalVariable.h"
25 #include "llvm/IntrinsicInst.h"
26 #include "llvm/LLVMContext.h"
27 #include "llvm/Analysis/AliasAnalysis.h"
28 #include "llvm/Analysis/CallGraph.h"
29 #include "llvm/Analysis/CaptureTracking.h"
30 #include "llvm/ADT/SmallSet.h"
31 #include "llvm/ADT/Statistic.h"
32 #include "llvm/ADT/UniqueVector.h"
33 #include "llvm/Support/InstIterator.h"
36 STATISTIC(NumReadNone, "Number of functions marked readnone");
37 STATISTIC(NumReadOnly, "Number of functions marked readonly");
38 STATISTIC(NumNoCapture, "Number of arguments marked nocapture");
39 STATISTIC(NumNoAlias, "Number of function returns marked noalias");
42 struct FunctionAttrs : public CallGraphSCCPass {
43 static char ID; // Pass identification, replacement for typeid
44 FunctionAttrs() : CallGraphSCCPass(ID), AA(0) {
45 initializeFunctionAttrsPass(*PassRegistry::getPassRegistry());
48 // runOnSCC - Analyze the SCC, performing the transformation if possible.
49 bool runOnSCC(CallGraphSCC &SCC);
51 // AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
52 bool AddReadAttrs(const CallGraphSCC &SCC);
54 // AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
55 bool AddNoCaptureAttrs(const CallGraphSCC &SCC);
57 // IsFunctionMallocLike - Does this function allocate new memory?
58 bool IsFunctionMallocLike(Function *F,
59 SmallPtrSet<Function*, 8> &) const;
61 // AddNoAliasAttrs - Deduce noalias attributes for the SCC.
62 bool AddNoAliasAttrs(const CallGraphSCC &SCC);
64 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
66 AU.addRequired<AliasAnalysis>();
67 CallGraphSCCPass::getAnalysisUsage(AU);
75 char FunctionAttrs::ID = 0;
76 INITIALIZE_PASS_BEGIN(FunctionAttrs, "functionattrs",
77 "Deduce function attributes", false, false)
78 INITIALIZE_AG_DEPENDENCY(CallGraph)
79 INITIALIZE_PASS_END(FunctionAttrs, "functionattrs",
80 "Deduce function attributes", false, false)
82 Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); }
85 /// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
86 bool FunctionAttrs::AddReadAttrs(const CallGraphSCC &SCC) {
87 SmallPtrSet<Function*, 8> SCCNodes;
89 // Fill SCCNodes with the elements of the SCC. Used for quickly
90 // looking up whether a given CallGraphNode is in this SCC.
91 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
92 SCCNodes.insert((*I)->getFunction());
94 // Check if any of the functions in the SCC read or write memory. If they
95 // write memory then they can't be marked readnone or readonly.
96 bool ReadsMemory = false;
97 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
98 Function *F = (*I)->getFunction();
101 // External node - may write memory. Just give up.
104 AliasAnalysis::ModRefBehavior MRB = AA->getModRefBehavior(F);
105 if (MRB == AliasAnalysis::DoesNotAccessMemory)
109 // Definitions with weak linkage may be overridden at linktime with
110 // something that writes memory, so treat them like declarations.
111 if (F->isDeclaration() || F->mayBeOverridden()) {
112 if (!AliasAnalysis::onlyReadsMemory(MRB))
113 // May write memory. Just give up.
120 // Scan the function body for instructions that may read or write memory.
121 for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) {
122 Instruction *I = &*II;
124 // Some instructions can be ignored even if they read or write memory.
125 // Detect these now, skipping to the next instruction if one is found.
126 CallSite CS(cast<Value>(I));
128 // Ignore calls to functions in the same SCC.
129 if (CS.getCalledFunction() && SCCNodes.count(CS.getCalledFunction()))
131 switch (AA->getModRefBehavior(CS)) {
132 case AliasAnalysis::DoesNotAccessMemory:
133 // Ignore calls that don't access memory.
135 case AliasAnalysis::OnlyReadsMemory:
136 // Handle calls that only read from memory.
139 case AliasAnalysis::AccessesArguments:
140 // Check whether all pointer arguments point to local memory, and
141 // ignore calls that only access local memory.
142 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
145 if (Arg->getType()->isPointerTy()) {
146 AliasAnalysis::Location Loc(Arg,
147 AliasAnalysis::UnknownSize,
148 I->getMetadata(LLVMContext::MD_tbaa));
149 if (!AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
150 // Writes memory. Just give up.
154 // Only reads and writes local memory.
156 case AliasAnalysis::AccessesArgumentsReadonly:
157 // Check whether all pointer arguments point to local memory, and
158 // ignore calls that only access local memory.
159 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
162 if (Arg->getType()->isPointerTy()) {
163 AliasAnalysis::Location Loc(Arg,
164 AliasAnalysis::UnknownSize,
165 I->getMetadata(LLVMContext::MD_tbaa));
166 if (!AA->pointsToConstantMemory(Loc, /*OrLocal=*/true)) {
167 // Reads non-local memory.
173 // Only reads memory.
176 // Otherwise, be conservative.
179 } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
180 // Ignore non-volatile loads from local memory.
181 if (!LI->isVolatile()) {
182 AliasAnalysis::Location Loc(LI->getPointerOperand(),
183 AA->getTypeStoreSize(LI->getType()),
184 LI->getMetadata(LLVMContext::MD_tbaa));
185 if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
188 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
189 // Ignore non-volatile stores to local memory.
190 if (!SI->isVolatile()) {
191 const Type *StoredType = SI->getValueOperand()->getType();
192 AliasAnalysis::Location Loc(SI->getPointerOperand(),
193 AA->getTypeStoreSize(StoredType),
194 SI->getMetadata(LLVMContext::MD_tbaa));
195 if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
198 } else if (VAArgInst *VI = dyn_cast<VAArgInst>(I)) {
199 // Ignore vaargs on local memory.
200 AliasAnalysis::Location Loc(VI->getPointerOperand(),
201 AliasAnalysis::UnknownSize,
202 VI->getMetadata(LLVMContext::MD_tbaa));
203 if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
207 // Any remaining instructions need to be taken seriously! Check if they
208 // read or write memory.
209 if (I->mayWriteToMemory())
210 // Writes memory. Just give up.
213 // If this instruction may read memory, remember that.
214 ReadsMemory |= I->mayReadFromMemory();
218 // Success! Functions in this SCC do not access memory, or only read memory.
219 // Give them the appropriate attribute.
220 bool MadeChange = false;
221 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
222 Function *F = (*I)->getFunction();
224 if (F->doesNotAccessMemory())
228 if (F->onlyReadsMemory() && ReadsMemory)
234 // Clear out any existing attributes.
235 F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone);
237 // Add in the new attribute.
238 F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone);
249 /// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
250 bool FunctionAttrs::AddNoCaptureAttrs(const CallGraphSCC &SCC) {
251 bool Changed = false;
253 // Check each function in turn, determining which pointer arguments are not
255 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
256 Function *F = (*I)->getFunction();
259 // External node - skip it;
262 // Definitions with weak linkage may be overridden at linktime with
263 // something that writes memory, so treat them like declarations.
264 if (F->isDeclaration() || F->mayBeOverridden())
267 for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
268 if (A->getType()->isPointerTy() && !A->hasNoCaptureAttr() &&
269 !PointerMayBeCaptured(A, true, /*StoreCaptures=*/false)) {
270 A->addAttr(Attribute::NoCapture);
279 /// IsFunctionMallocLike - A function is malloc-like if it returns either null
280 /// or a pointer that doesn't alias any other pointer visible to the caller.
281 bool FunctionAttrs::IsFunctionMallocLike(Function *F,
282 SmallPtrSet<Function*, 8> &SCCNodes) const {
283 UniqueVector<Value *> FlowsToReturn;
284 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I)
285 if (ReturnInst *Ret = dyn_cast<ReturnInst>(I->getTerminator()))
286 FlowsToReturn.insert(Ret->getReturnValue());
288 for (unsigned i = 0; i != FlowsToReturn.size(); ++i) {
289 Value *RetVal = FlowsToReturn[i+1]; // UniqueVector[0] is reserved.
291 if (Constant *C = dyn_cast<Constant>(RetVal)) {
292 if (!C->isNullValue() && !isa<UndefValue>(C))
298 if (isa<Argument>(RetVal))
301 if (Instruction *RVI = dyn_cast<Instruction>(RetVal))
302 switch (RVI->getOpcode()) {
303 // Extend the analysis by looking upwards.
304 case Instruction::BitCast:
305 case Instruction::GetElementPtr:
306 FlowsToReturn.insert(RVI->getOperand(0));
308 case Instruction::Select: {
309 SelectInst *SI = cast<SelectInst>(RVI);
310 FlowsToReturn.insert(SI->getTrueValue());
311 FlowsToReturn.insert(SI->getFalseValue());
314 case Instruction::PHI: {
315 PHINode *PN = cast<PHINode>(RVI);
316 for (int i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
317 FlowsToReturn.insert(PN->getIncomingValue(i));
321 // Check whether the pointer came from an allocation.
322 case Instruction::Alloca:
324 case Instruction::Call:
325 case Instruction::Invoke: {
327 if (CS.paramHasAttr(0, Attribute::NoAlias))
329 if (CS.getCalledFunction() &&
330 SCCNodes.count(CS.getCalledFunction()))
334 return false; // Did not come from an allocation.
337 if (PointerMayBeCaptured(RetVal, false, /*StoreCaptures=*/false))
344 /// AddNoAliasAttrs - Deduce noalias attributes for the SCC.
345 bool FunctionAttrs::AddNoAliasAttrs(const CallGraphSCC &SCC) {
346 SmallPtrSet<Function*, 8> SCCNodes;
348 // Fill SCCNodes with the elements of the SCC. Used for quickly
349 // looking up whether a given CallGraphNode is in this SCC.
350 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
351 SCCNodes.insert((*I)->getFunction());
353 // Check each function in turn, determining which functions return noalias
355 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
356 Function *F = (*I)->getFunction();
359 // External node - skip it;
363 if (F->doesNotAlias(0))
366 // Definitions with weak linkage may be overridden at linktime, so
367 // treat them like declarations.
368 if (F->isDeclaration() || F->mayBeOverridden())
371 // We annotate noalias return values, which are only applicable to
373 if (!F->getReturnType()->isPointerTy())
376 if (!IsFunctionMallocLike(F, SCCNodes))
380 bool MadeChange = false;
381 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
382 Function *F = (*I)->getFunction();
383 if (F->doesNotAlias(0) || !F->getReturnType()->isPointerTy())
386 F->setDoesNotAlias(0);
394 bool FunctionAttrs::runOnSCC(CallGraphSCC &SCC) {
395 AA = &getAnalysis<AliasAnalysis>();
397 bool Changed = AddReadAttrs(SCC);
398 Changed |= AddNoCaptureAttrs(SCC);
399 Changed |= AddNoAliasAttrs(SCC);