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/Instructions.h"
26 #include "llvm/Analysis/CallGraph.h"
27 #include "llvm/ADT/PointerIntPair.h"
28 #include "llvm/ADT/SmallSet.h"
29 #include "llvm/ADT/Statistic.h"
30 #include "llvm/Support/Compiler.h"
31 #include "llvm/Support/InstIterator.h"
34 STATISTIC(NumReadNone, "Number of functions marked readnone");
35 STATISTIC(NumReadOnly, "Number of functions marked readonly");
36 STATISTIC(NumNoCapture, "Number of arguments marked nocapture");
39 struct VISIBILITY_HIDDEN FunctionAttrs : public CallGraphSCCPass {
40 static char ID; // Pass identification, replacement for typeid
41 FunctionAttrs() : CallGraphSCCPass(&ID) {}
43 // runOnSCC - Analyze the SCC, performing the transformation if possible.
44 bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
46 // AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
47 bool AddReadAttrs(const std::vector<CallGraphNode *> &SCC);
49 // AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
50 bool AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC);
52 // isCaptured - Return true if this pointer value may be captured.
53 bool isCaptured(Function &F, Value *V);
55 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
57 CallGraphSCCPass::getAnalysisUsage(AU);
60 bool PointsToLocalMemory(Value *V);
64 char FunctionAttrs::ID = 0;
65 static RegisterPass<FunctionAttrs>
66 X("functionattrs", "Deduce function attributes");
68 Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); }
71 /// PointsToLocalMemory - Returns whether the given pointer value points to
72 /// memory that is local to the function. Global constants are considered
73 /// local to all functions.
74 bool FunctionAttrs::PointsToLocalMemory(Value *V) {
75 V = V->getUnderlyingObject();
76 // An alloca instruction defines local memory.
77 if (isa<AllocaInst>(V))
79 // A global constant counts as local memory for our purposes.
80 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
81 return GV->isConstant();
82 // Could look through phi nodes and selects here, but it doesn't seem
83 // to be useful in practice.
87 /// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
88 bool FunctionAttrs::AddReadAttrs(const std::vector<CallGraphNode *> &SCC) {
89 SmallPtrSet<CallGraphNode*, 8> SCCNodes;
90 CallGraph &CG = getAnalysis<CallGraph>();
92 // Fill SCCNodes with the elements of the SCC. Used for quickly
93 // looking up whether a given CallGraphNode is in this SCC.
94 for (unsigned i = 0, e = SCC.size(); i != e; ++i)
95 SCCNodes.insert(SCC[i]);
97 // Check if any of the functions in the SCC read or write memory. If they
98 // write memory then they can't be marked readnone or readonly.
99 bool ReadsMemory = false;
100 for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
101 Function *F = SCC[i]->getFunction();
104 // External node - may write memory. Just give up.
107 if (F->doesNotAccessMemory())
111 // Definitions with weak linkage may be overridden at linktime with
112 // something that writes memory, so treat them like declarations.
113 if (F->isDeclaration() || F->mayBeOverridden()) {
114 if (!F->onlyReadsMemory())
115 // May write memory. Just give up.
122 // Scan the function body for instructions that may read or write memory.
123 for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) {
124 Instruction *I = &*II;
126 // Some instructions can be ignored even if they read or write memory.
127 // Detect these now, skipping to the next instruction if one is found.
128 CallSite CS = CallSite::get(I);
129 if (CS.getInstruction()) {
130 // Ignore calls to functions in the same SCC.
131 if (SCCNodes.count(CG[CS.getCalledFunction()]))
133 } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
134 // Ignore loads from local memory.
135 if (PointsToLocalMemory(LI->getPointerOperand()))
137 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
138 // Ignore stores to local memory.
139 if (PointsToLocalMemory(SI->getPointerOperand()))
143 // Any remaining instructions need to be taken seriously! Check if they
144 // read or write memory.
145 if (I->mayWriteToMemory())
146 // Writes memory. Just give up.
148 // If this instruction may read memory, remember that.
149 ReadsMemory |= I->mayReadFromMemory();
153 // Success! Functions in this SCC do not access memory, or only read memory.
154 // Give them the appropriate attribute.
155 bool MadeChange = false;
156 for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
157 Function *F = SCC[i]->getFunction();
159 if (F->doesNotAccessMemory())
163 if (F->onlyReadsMemory() && ReadsMemory)
169 // Clear out any existing attributes.
170 F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone);
172 // Add in the new attribute.
173 F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone);
184 /// isCaptured - Return true if this pointer value may be captured.
185 bool FunctionAttrs::isCaptured(Function &F, Value *V) {
186 SmallVector<Use*, 16> Worklist;
187 SmallSet<Use*, 16> Visited;
189 for (Value::use_iterator UI = V->use_begin(), UE = V->use_end(); UI != UE;
191 Use *U = &UI.getUse();
193 Worklist.push_back(U);
196 while (!Worklist.empty()) {
197 Use *U = Worklist.pop_back_val();
198 Instruction *I = cast<Instruction>(U->getUser());
201 switch (I->getOpcode()) {
202 case Instruction::Call:
203 case Instruction::Invoke: {
204 CallSite CS = CallSite::get(I);
205 // Not captured if the callee is readonly and doesn't return a copy
206 // through its return value.
207 if (CS.onlyReadsMemory() && I->getType() == Type::VoidTy)
210 // Not captured if only passed via 'nocapture' arguments. Note that
211 // calling a function pointer does not in itself cause the pointer to
212 // be captured. This is a subtle point considering that (for example)
213 // the callee might return its own address. It is analogous to saying
214 // that loading a value from a pointer does not cause the pointer to be
215 // captured, even though the loaded value might be the pointer itself
216 // (think of self-referential objects).
217 CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
218 for (CallSite::arg_iterator A = B; A != E; ++A)
219 if (A->get() == V && !CS.paramHasAttr(A - B + 1, Attribute::NoCapture))
220 // The parameter is not marked 'nocapture' - captured.
222 // Only passed via 'nocapture' arguments, or is the called function - not
226 case Instruction::Free:
227 // Freeing a pointer does not cause it to be captured.
229 case Instruction::Load:
230 // Loading from a pointer does not cause it to be captured.
232 case Instruction::Store:
233 if (V == I->getOperand(0))
234 // Stored the pointer - it may be captured.
236 // Storing to the pointee does not cause the pointer to be captured.
238 case Instruction::BitCast:
239 case Instruction::GetElementPtr:
240 case Instruction::PHI:
241 case Instruction::Select:
242 // The original value is not captured via this if the new value isn't.
243 for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
245 Use *U = &UI.getUse();
246 if (Visited.insert(U))
247 Worklist.push_back(U);
251 // Something else - be conservative and say it is captured.
256 // All uses examined - not captured.
260 /// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
261 bool FunctionAttrs::AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC) {
262 bool Changed = false;
264 // Check each function in turn, determining which pointer arguments are not
266 for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
267 Function *F = SCC[i]->getFunction();
270 // External node - skip it;
273 // Definitions with weak linkage may be overridden at linktime with
274 // something that writes memory, so treat them like declarations.
275 if (F->isDeclaration() || F->mayBeOverridden())
278 for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
279 if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr() &&
280 !isCaptured(*F, A)) {
281 A->addAttr(Attribute::NoCapture);
290 bool FunctionAttrs::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
291 bool Changed = AddReadAttrs(SCC);
292 Changed |= AddNoCaptureAttrs(SCC);