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 typedef PointerIntPair<Use*, 2> UseWithDepth;
187 SmallVector<UseWithDepth, 16> Worklist;
188 SmallSet<UseWithDepth, 16> Visited;
190 for (Value::use_iterator UI = V->use_begin(), UE = V->use_end(); UI != UE;
192 UseWithDepth UD(&UI.getUse(), 0);
194 Worklist.push_back(UD);
197 while (!Worklist.empty()) {
198 UseWithDepth UD = Worklist.pop_back_val();
199 Use *U = UD.getPointer();
200 Instruction *I = cast<Instruction>(U->getUser());
201 // The value V may have any type if it comes from tracking a load.
203 // The depth represents the number of loads that need to be performed to
204 // get back the original pointer (or a bitcast etc of it). For example,
205 // if the pointer is stored to an alloca, then all uses of the alloca get
206 // depth 1: if the alloca is loaded then you get the original pointer back.
207 // If a load of the alloca is returned then the pointer has been captured.
208 // The depth is needed in order to know which loads dereference the original
209 // pointer (these do not capture), and which return a value which needs to
210 // be tracked because if it is captured then so is the original pointer.
211 unsigned Depth = UD.getInt();
213 if (isa<StoreInst>(I)) {
214 if (V == I->getOperand(0)) {
215 // Stored the pointer - it may be captured. If it is stored to a local
216 // object (alloca) then track that object. Otherwise give up.
217 Value *Target = I->getOperand(1)->getUnderlyingObject();
218 if (!isa<AllocaInst>(Target))
219 // Didn't store to an obviously local object - captured.
222 // Alloca recursion too deep - give up.
224 // Analyze all uses of the alloca.
225 for (Value::use_iterator UI = Target->use_begin(),
226 UE = Target->use_end(); UI != UE; ++UI) {
227 UseWithDepth NUD(&UI.getUse(), Depth + 1);
228 if (Visited.insert(NUD))
229 Worklist.push_back(NUD);
232 // Storing to the pointee does not cause the pointer to be captured.
233 } else if (isa<FreeInst>(I)) {
234 // Freeing a pointer does not cause it to be captured.
235 } else if (isa<CallInst>(I) || isa<InvokeInst>(I)) {
236 CallSite CS = CallSite::get(I);
237 // Not captured if the callee is readonly and doesn't return a copy
238 // through its return value.
239 if (CS.onlyReadsMemory() && I->getType() == Type::VoidTy)
242 // Not captured if only passed via 'nocapture' arguments. Note that
243 // calling a function pointer does not in itself cause the pointer to
244 // be captured. This is a subtle point considering that (for example)
245 // the callee might return its own address. It is analogous to saying
246 // that loading a value from a pointer does not cause the pointer to be
247 // captured, even though the loaded value might be the pointer itself
248 // (think of self-referential objects).
249 CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
250 for (CallSite::arg_iterator A = B; A != E; ++A)
251 if (A->get() == V && !CS.paramHasAttr(A - B + 1, Attribute::NoCapture))
252 // The parameter is not marked 'nocapture' - captured.
254 // Only passed via 'nocapture' arguments, or is the called function - not
256 } else if (isa<BitCastInst>(I) || isa<LoadInst>(I) || isa<PHINode>(I) ||
257 // Play safe and exclude GEP indices.
258 (isa<GetElementPtrInst>(I) && V == I->getOperand(0)) ||
259 // Play safe and exclude the select condition.
260 (isa<SelectInst>(I) && V != I->getOperand(0))) {
262 // Usually loads can be ignored because they dereference the original
263 // pointer. However the loaded value needs to be tracked if loading
264 // from an object that the original pointer was stored to.
265 if (isa<LoadInst>(I)) {
267 // Loading the original pointer or a variation of it. This does not
268 // cause the pointer to be captured. Note that the loaded value might
269 // be the pointer itself (think of self-referential objects), but that
270 // is fine as long as it's not this function that stored it there.
272 // Loading a pointer to (a pointer to...) the original pointer or a
273 // variation of it. Track uses of the loaded value, noting that one
274 // dereference was performed. Note that the loaded value need not be
275 // of pointer type. For example, an alloca may have been bitcast to
276 // a pointer to another type, which was then loaded.
280 // The original value is not captured via this if the instruction isn't.
281 for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
283 UseWithDepth UD(&UI.getUse(), Depth);
284 if (Visited.insert(UD))
285 Worklist.push_back(UD);
288 // Something else - be conservative and say it is captured.
293 // All uses examined - not captured.
297 /// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
298 bool FunctionAttrs::AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC) {
299 bool Changed = false;
301 // Check each function in turn, determining which pointer arguments are not
303 for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
304 Function *F = SCC[i]->getFunction();
307 // External node - skip it;
310 // Definitions with weak linkage may be overridden at linktime with
311 // something that writes memory, so treat them like declarations.
312 if (F->isDeclaration() || F->mayBeOverridden())
315 for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
316 if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr() &&
317 !isCaptured(*F, A)) {
318 A->addAttr(Attribute::NoCapture);
327 bool FunctionAttrs::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
328 bool Changed = AddReadAttrs(SCC);
329 Changed |= AddNoCaptureAttrs(SCC);