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/Analysis/AliasAnalysis.h"
27 #include "llvm/Analysis/CallGraph.h"
28 #include "llvm/Analysis/CaptureTracking.h"
29 #include "llvm/Analysis/MemoryBuiltins.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) {}
46 // runOnSCC - Analyze the SCC, performing the transformation if possible.
47 bool runOnSCC(CallGraphSCC &SCC);
49 // AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
50 bool AddReadAttrs(const CallGraphSCC &SCC);
52 // AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
53 bool AddNoCaptureAttrs(const CallGraphSCC &SCC);
55 // IsFunctionMallocLike - Does this function allocate new memory?
56 bool IsFunctionMallocLike(Function *F,
57 SmallPtrSet<Function*, 8> &) const;
59 // AddNoAliasAttrs - Deduce noalias attributes for the SCC.
60 bool AddNoAliasAttrs(const CallGraphSCC &SCC);
62 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
64 CallGraphSCCPass::getAnalysisUsage(AU);
67 bool PointsToLocalMemory(Value *V);
71 char FunctionAttrs::ID = 0;
72 static RegisterPass<FunctionAttrs>
73 X("functionattrs", "Deduce function attributes");
75 Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); }
78 /// PointsToLocalMemory - Returns whether the given pointer value points to
79 /// memory that is local to the function. Global constants are considered
80 /// local to all functions.
81 bool FunctionAttrs::PointsToLocalMemory(Value *V) {
82 SmallVector<Value*, 16> Worklist;
83 unsigned MaxLookup = 8;
85 Worklist.push_back(V);
88 V = Worklist.pop_back_val()->getUnderlyingObject();
90 // An alloca instruction defines local memory.
91 if (isa<AllocaInst>(V))
94 // A global constant counts as local memory for our purposes.
95 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
96 if (!GV->isConstant())
101 // If both select values point to local memory, then so does the select.
102 if (SelectInst *SI = dyn_cast<SelectInst>(V)) {
103 Worklist.push_back(SI->getTrueValue());
104 Worklist.push_back(SI->getFalseValue());
108 // If all values incoming to a phi node point to local memory, then so does
110 if (PHINode *PN = dyn_cast<PHINode>(V)) {
111 // Don't bother inspecting phi nodes with many operands.
112 if (PN->getNumIncomingValues() > MaxLookup)
114 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
115 Worklist.push_back(PN->getIncomingValue(i));
120 } while (!Worklist.empty() && --MaxLookup);
122 return Worklist.empty();
125 /// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
126 bool FunctionAttrs::AddReadAttrs(const CallGraphSCC &SCC) {
127 SmallPtrSet<Function*, 8> SCCNodes;
129 // Fill SCCNodes with the elements of the SCC. Used for quickly
130 // looking up whether a given CallGraphNode is in this SCC.
131 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
132 SCCNodes.insert((*I)->getFunction());
134 // Check if any of the functions in the SCC read or write memory. If they
135 // write memory then they can't be marked readnone or readonly.
136 bool ReadsMemory = false;
137 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
138 Function *F = (*I)->getFunction();
141 // External node - may write memory. Just give up.
144 if (F->doesNotAccessMemory())
148 // Definitions with weak linkage may be overridden at linktime with
149 // something that writes memory, so treat them like declarations.
150 if (F->isDeclaration() || F->mayBeOverridden()) {
151 if (!F->onlyReadsMemory())
152 // May write memory. Just give up.
159 // Scan the function body for instructions that may read or write memory.
160 for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) {
161 Instruction *I = &*II;
163 // Some instructions can be ignored even if they read or write memory.
164 // Detect these now, skipping to the next instruction if one is found.
165 CallSite CS = CallSite::get(I);
166 if (CS.getInstruction() && CS.getCalledFunction()) {
167 // Ignore calls to functions in the same SCC.
168 if (SCCNodes.count(CS.getCalledFunction()))
170 // Ignore intrinsics that only access local memory.
171 if (unsigned id = CS.getCalledFunction()->getIntrinsicID())
172 if (AliasAnalysis::getModRefBehavior(id) ==
173 AliasAnalysis::AccessesArguments) {
174 // Check that all pointer arguments point to local memory.
175 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
178 if (Arg->getType()->isPointerTy() && !PointsToLocalMemory(Arg))
179 // Writes memory. Just give up.
182 // Only reads and writes local memory.
185 } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
186 // Ignore loads from local memory.
187 if (PointsToLocalMemory(LI->getPointerOperand()))
189 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
190 // Ignore stores to local memory.
191 if (PointsToLocalMemory(SI->getPointerOperand()))
195 // Any remaining instructions need to be taken seriously! Check if they
196 // read or write memory.
197 if (I->mayWriteToMemory())
198 // Writes memory. Just give up.
202 // malloc claims not to write memory! PR3754.
205 // If this instruction may read memory, remember that.
206 ReadsMemory |= I->mayReadFromMemory();
210 // Success! Functions in this SCC do not access memory, or only read memory.
211 // Give them the appropriate attribute.
212 bool MadeChange = false;
213 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
214 Function *F = (*I)->getFunction();
216 if (F->doesNotAccessMemory())
220 if (F->onlyReadsMemory() && ReadsMemory)
226 // Clear out any existing attributes.
227 F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone);
229 // Add in the new attribute.
230 F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone);
241 /// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
242 bool FunctionAttrs::AddNoCaptureAttrs(const CallGraphSCC &SCC) {
243 bool Changed = false;
245 // Check each function in turn, determining which pointer arguments are not
247 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
248 Function *F = (*I)->getFunction();
251 // External node - skip it;
254 // Definitions with weak linkage may be overridden at linktime with
255 // something that writes memory, so treat them like declarations.
256 if (F->isDeclaration() || F->mayBeOverridden())
259 for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
260 if (A->getType()->isPointerTy() && !A->hasNoCaptureAttr() &&
261 !PointerMayBeCaptured(A, true, /*StoreCaptures=*/false)) {
262 A->addAttr(Attribute::NoCapture);
271 /// IsFunctionMallocLike - A function is malloc-like if it returns either null
272 /// or a pointer that doesn't alias any other pointer visible to the caller.
273 bool FunctionAttrs::IsFunctionMallocLike(Function *F,
274 SmallPtrSet<Function*, 8> &SCCNodes) const {
275 UniqueVector<Value *> FlowsToReturn;
276 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I)
277 if (ReturnInst *Ret = dyn_cast<ReturnInst>(I->getTerminator()))
278 FlowsToReturn.insert(Ret->getReturnValue());
280 for (unsigned i = 0; i != FlowsToReturn.size(); ++i) {
281 Value *RetVal = FlowsToReturn[i+1]; // UniqueVector[0] is reserved.
283 if (Constant *C = dyn_cast<Constant>(RetVal)) {
284 if (!C->isNullValue() && !isa<UndefValue>(C))
290 if (isa<Argument>(RetVal))
293 if (Instruction *RVI = dyn_cast<Instruction>(RetVal))
294 switch (RVI->getOpcode()) {
295 // Extend the analysis by looking upwards.
296 case Instruction::BitCast:
297 case Instruction::GetElementPtr:
298 FlowsToReturn.insert(RVI->getOperand(0));
300 case Instruction::Select: {
301 SelectInst *SI = cast<SelectInst>(RVI);
302 FlowsToReturn.insert(SI->getTrueValue());
303 FlowsToReturn.insert(SI->getFalseValue());
306 case Instruction::PHI: {
307 PHINode *PN = cast<PHINode>(RVI);
308 for (int i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
309 FlowsToReturn.insert(PN->getIncomingValue(i));
313 // Check whether the pointer came from an allocation.
314 case Instruction::Alloca:
316 case Instruction::Call:
317 case Instruction::Invoke: {
319 if (CS.paramHasAttr(0, Attribute::NoAlias))
321 if (CS.getCalledFunction() &&
322 SCCNodes.count(CS.getCalledFunction()))
326 return false; // Did not come from an allocation.
329 if (PointerMayBeCaptured(RetVal, false, /*StoreCaptures=*/false))
336 /// AddNoAliasAttrs - Deduce noalias attributes for the SCC.
337 bool FunctionAttrs::AddNoAliasAttrs(const CallGraphSCC &SCC) {
338 SmallPtrSet<Function*, 8> SCCNodes;
340 // Fill SCCNodes with the elements of the SCC. Used for quickly
341 // looking up whether a given CallGraphNode is in this SCC.
342 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
343 SCCNodes.insert((*I)->getFunction());
345 // Check each function in turn, determining which functions return noalias
347 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
348 Function *F = (*I)->getFunction();
351 // External node - skip it;
355 if (F->doesNotAlias(0))
358 // Definitions with weak linkage may be overridden at linktime, so
359 // treat them like declarations.
360 if (F->isDeclaration() || F->mayBeOverridden())
363 // We annotate noalias return values, which are only applicable to
365 if (!F->getReturnType()->isPointerTy())
368 if (!IsFunctionMallocLike(F, SCCNodes))
372 bool MadeChange = false;
373 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
374 Function *F = (*I)->getFunction();
375 if (F->doesNotAlias(0) || !F->getReturnType()->isPointerTy())
378 F->setDoesNotAlias(0);
386 bool FunctionAttrs::runOnSCC(CallGraphSCC &SCC) {
387 bool Changed = AddReadAttrs(SCC);
388 Changed |= AddNoCaptureAttrs(SCC);
389 Changed |= AddNoAliasAttrs(SCC);