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functions that don't write can't leak a pointer except through the return value, so a void readonly function is implicitly nocapture. Test these, and add a test that verifies that f1 calling f2 with an otherwise dead pointer gets both of them marked nocapture. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61552 91177308-0d34-0410-b5e6-96231b3b80d8
313 lines
11 KiB
C++
313 lines
11 KiB
C++
//===- FunctionAttrs.cpp - Pass which marks functions readnone or readonly ===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements a simple interprocedural pass which walks the
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// call-graph, looking for functions which do not access or only read
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// non-local memory, and marking them readnone/readonly. It addition,
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// it deduces which function arguments (of pointer type) do not escape,
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// and marks them nocapture. It implements this as a bottom-up traversal
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// of the call-graph.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "functionattrs"
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#include "llvm/Transforms/IPO.h"
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#include "llvm/CallGraphSCCPass.h"
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#include "llvm/GlobalVariable.h"
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#include "llvm/Instructions.h"
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#include "llvm/Analysis/CallGraph.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Support/Compiler.h"
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#include "llvm/Support/InstIterator.h"
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using namespace llvm;
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STATISTIC(NumReadNone, "Number of functions marked readnone");
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STATISTIC(NumReadOnly, "Number of functions marked readonly");
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STATISTIC(NumNoCapture, "Number of arguments marked nocapture");
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namespace {
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struct VISIBILITY_HIDDEN FunctionAttrs : public CallGraphSCCPass {
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static char ID; // Pass identification, replacement for typeid
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FunctionAttrs() : CallGraphSCCPass(&ID) {}
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// runOnSCC - Analyze the SCC, performing the transformation if possible.
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bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
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// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
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bool AddReadAttrs(const std::vector<CallGraphNode *> &SCC);
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// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
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bool AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC);
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// isCaptured - Returns true if this pointer value escapes.
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bool isCaptured(Function &F, Value *V);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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CallGraphSCCPass::getAnalysisUsage(AU);
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}
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bool PointsToLocalMemory(Value *V);
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};
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}
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char FunctionAttrs::ID = 0;
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static RegisterPass<FunctionAttrs>
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X("functionattrs", "Deduce function attributes");
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Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); }
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/// PointsToLocalMemory - Returns whether the given pointer value points to
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/// memory that is local to the function. Global constants are considered
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/// local to all functions.
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bool FunctionAttrs::PointsToLocalMemory(Value *V) {
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V = V->getUnderlyingObject();
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// An alloca instruction defines local memory.
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if (isa<AllocaInst>(V))
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return true;
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// A global constant counts as local memory for our purposes.
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if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
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return GV->isConstant();
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// Could look through phi nodes and selects here, but it doesn't seem
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// to be useful in practice.
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return false;
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}
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/// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
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bool FunctionAttrs::AddReadAttrs(const std::vector<CallGraphNode *> &SCC) {
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SmallPtrSet<CallGraphNode*, 8> SCCNodes;
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CallGraph &CG = getAnalysis<CallGraph>();
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// Fill SCCNodes with the elements of the SCC. Used for quickly
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// looking up whether a given CallGraphNode is in this SCC.
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for (unsigned i = 0, e = SCC.size(); i != e; ++i)
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SCCNodes.insert(SCC[i]);
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// Check if any of the functions in the SCC read or write memory. If they
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// write memory then they can't be marked readnone or readonly.
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bool ReadsMemory = false;
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for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
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Function *F = SCC[i]->getFunction();
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if (F == 0)
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// External node - may write memory. Just give up.
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return false;
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if (F->doesNotAccessMemory())
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// Already perfect!
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continue;
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// Definitions with weak linkage may be overridden at linktime with
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// something that writes memory, so treat them like declarations.
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if (F->isDeclaration() || F->mayBeOverridden()) {
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if (!F->onlyReadsMemory())
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// May write memory. Just give up.
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return false;
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ReadsMemory = true;
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continue;
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}
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// Scan the function body for instructions that may read or write memory.
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for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) {
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Instruction *I = &*II;
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// Some instructions can be ignored even if they read or write memory.
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// Detect these now, skipping to the next instruction if one is found.
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CallSite CS = CallSite::get(I);
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if (CS.getInstruction()) {
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// Ignore calls to functions in the same SCC.
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if (SCCNodes.count(CG[CS.getCalledFunction()]))
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continue;
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} else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
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// Ignore loads from local memory.
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if (PointsToLocalMemory(LI->getPointerOperand()))
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continue;
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} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
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// Ignore stores to local memory.
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if (PointsToLocalMemory(SI->getPointerOperand()))
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continue;
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}
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// Any remaining instructions need to be taken seriously! Check if they
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// read or write memory.
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if (I->mayWriteToMemory())
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// Writes memory. Just give up.
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return false;
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// If this instruction may read memory, remember that.
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ReadsMemory |= I->mayReadFromMemory();
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}
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}
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// Success! Functions in this SCC do not access memory, or only read memory.
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// Give them the appropriate attribute.
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bool MadeChange = false;
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for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
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Function *F = SCC[i]->getFunction();
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if (F->doesNotAccessMemory())
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// Already perfect!
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continue;
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if (F->onlyReadsMemory() && ReadsMemory)
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// No change.
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continue;
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MadeChange = true;
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// Clear out any existing attributes.
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F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone);
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// Add in the new attribute.
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F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone);
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if (ReadsMemory)
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NumReadOnly++;
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else
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NumReadNone++;
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}
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return MadeChange;
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}
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/// isCaptured - Returns whether this pointer value is captured.
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bool FunctionAttrs::isCaptured(Function &F, Value *V) {
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SmallVector<Use*, 16> Worklist;
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SmallPtrSet<Use*, 16> Visited;
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for (Value::use_iterator UI = V->use_begin(), UE = V->use_end(); UI != UE;
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++UI) {
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Use *U = &UI.getUse();
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Visited.insert(U);
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Worklist.push_back(U);
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}
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while (!Worklist.empty()) {
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Use *U = Worklist.pop_back_val();
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Instruction *I = cast<Instruction>(U->getUser());
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V = U->get();
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if (isa<LoadInst>(I)) {
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// Loading a pointer does not cause it to escape.
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continue;
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}
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if (isa<StoreInst>(I)) {
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if (V == I->getOperand(0))
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// Stored the pointer - escapes. TODO: improve this.
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return true;
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// Storing to the pointee does not cause the pointer to escape.
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continue;
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}
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if (isa<FreeInst>(I)) {
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// Freeing a pointer does not cause it to escape.
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continue;
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}
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CallSite CS = CallSite::get(I);
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if (CS.getInstruction()) {
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// Does not escape if the callee is readonly and doesn't return a
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// copy through its own return value.
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if (CS.onlyReadsMemory() && I->getType() == Type::VoidTy)
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continue;
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// Does not escape if passed via 'nocapture' arguments. Note that
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// calling a function pointer does not in itself cause that
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// function pointer to escape. This is a subtle point considering
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// that (for example) the callee might return its own address. It
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// is analogous to saying that loading a value from a pointer does
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// not cause the pointer to escape, even though the loaded value
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// might be the pointer itself (think of self-referential objects).
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CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
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for (CallSite::arg_iterator A = B; A != E; ++A)
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if (A->get() == V && !CS.paramHasAttr(A-B+1, Attribute::NoCapture))
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// The parameter is not marked 'nocapture' - escapes.
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return true;
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// Only passed via 'nocapture' arguments, or is the called function.
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// Does not escape.
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continue;
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}
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if (isa<BitCastInst>(I) || isa<GetElementPtrInst>(I) ||
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isa<PHINode>(I) || isa<SelectInst>(I)) {
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// Type conversion, calculating an offset, or merging values.
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// The original value does not escape via this if the new value doesn't.
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// Note that in the case of a select instruction it is important that
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// the value not be used as the condition, since otherwise one bit of
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// information might escape. It cannot be the condition because it has
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// the wrong type.
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for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
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UI != UE; ++UI) {
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Use *U = &UI.getUse();
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if (Visited.insert(U))
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Worklist.push_back(U);
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}
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continue;
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}
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// Something else - be conservative and say it escapes.
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return true;
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}
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return false;
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}
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/// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
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bool FunctionAttrs::AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC) {
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bool Changed = false;
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// Check each function in turn, determining which pointer arguments are not
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// captured.
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for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
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Function *F = SCC[i]->getFunction();
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if (F == 0)
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// External node - skip it;
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continue;
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// If the function is readonly and doesn't return any value, we
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// know that the pointer value can't escape. Mark all of its pointer
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// arguments nocapture.
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if (F->onlyReadsMemory() && F->getReturnType() == Type::VoidTy) {
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for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end();
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A != E; ++A)
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if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr()) {
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A->addAttr(Attribute::NoCapture);
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++NumNoCapture;
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Changed = true;
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}
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continue;
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}
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// Definitions with weak linkage may be overridden at linktime with
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// something that writes memory, so treat them like declarations.
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if (F->isDeclaration() || F->mayBeOverridden())
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continue;
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for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
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if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr() &&
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!isCaptured(*F, A)) {
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A->addAttr(Attribute::NoCapture);
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++NumNoCapture;
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Changed = true;
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}
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}
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return Changed;
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}
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bool FunctionAttrs::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
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bool Changed = AddReadAttrs(SCC);
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Changed |= AddNoCaptureAttrs(SCC);
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return Changed;
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}
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