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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61873 91177308-0d34-0410-b5e6-96231b3b80d8
348 lines
13 KiB
C++
348 lines
13 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. In addition,
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// it marks function arguments (of pointer type) 'nocapture' if a call
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// to the function does not create any copies of the pointer value that
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// outlive the call. This more or less means that the pointer is only
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// dereferenced, and not returned from the function or stored in a global.
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// This pass is implemented as a bottom-up traversal 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/PointerIntPair.h"
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#include "llvm/ADT/SmallSet.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 - Return true if this pointer value may be captured.
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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 - Return true if this pointer value may be captured.
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bool FunctionAttrs::isCaptured(Function &F, Value *V) {
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typedef PointerIntPair<Use*, 2> UseWithDepth;
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SmallVector<UseWithDepth, 16> Worklist;
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SmallSet<UseWithDepth, 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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UseWithDepth UD(&UI.getUse(), 0);
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Visited.insert(UD);
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Worklist.push_back(UD);
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}
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while (!Worklist.empty()) {
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UseWithDepth UD = Worklist.pop_back_val();
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Use *U = UD.getPointer();
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Instruction *I = cast<Instruction>(U->getUser());
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// The value V may have any type if it comes from tracking a load.
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V = U->get();
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// The depth represents the number of loads that need to be performed to
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// get back the original pointer (or a bitcast etc of it). For example,
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// if the pointer is stored to an alloca, then all uses of the alloca get
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// depth 1: if the alloca is loaded then you get the original pointer back.
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// If a load of the alloca is returned then the pointer has been captured.
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// The depth is needed in order to know which loads dereference the original
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// pointer (these do not capture), and which return a value which needs to
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// be tracked because if it is captured then so is the original pointer.
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unsigned Depth = UD.getInt();
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switch (I->getOpcode()) {
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case Instruction::Call:
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case Instruction::Invoke: {
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CallSite CS = CallSite::get(I);
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// Not captured if the callee is readonly and doesn't return a copy
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// through its return value.
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if (CS.onlyReadsMemory() && I->getType() == Type::VoidTy)
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break;
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// Not captured if only passed via 'nocapture' arguments. Note that
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// calling a function pointer does not in itself cause the pointer to
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// be captured. This is a subtle point considering that (for example)
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// the callee might return its own address. It is analogous to saying
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// that loading a value from a pointer does not cause the pointer to be
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// captured, even though the loaded value might be the pointer itself
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// (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' - captured.
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return true;
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// Only passed via 'nocapture' arguments, or is the called function - not
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// captured.
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break;
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}
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case Instruction::Free:
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// Freeing a pointer does not cause it to be captured.
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break;
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case Instruction::Store:
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if (V == I->getOperand(0)) {
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// Stored the pointer - it may be captured. If it is stored to a local
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// object (alloca) then track that object. Otherwise give up.
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Value *Target = I->getOperand(1)->getUnderlyingObject();
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if (!isa<AllocaInst>(Target))
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// Didn't store to an obviously local object - captured.
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return true;
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if (Depth >= 3)
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// Alloca recursion too deep - give up.
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return true;
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// Analyze all uses of the alloca.
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for (Value::use_iterator UI = Target->use_begin(),
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UE = Target->use_end(); UI != UE; ++UI) {
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UseWithDepth NUD(&UI.getUse(), Depth + 1);
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if (Visited.insert(NUD))
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Worklist.push_back(NUD);
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}
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}
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// Storing to the pointee does not cause the pointer to be captured.
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break;
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case Instruction::BitCast:
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case Instruction::GetElementPtr:
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case Instruction::Load:
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case Instruction::PHI:
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case Instruction::Select:
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// Track any uses of this instruction to see if they are captured.
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// First handle any special cases.
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if (isa<GetElementPtrInst>(I)) {
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// Play safe and do not accept being used as an index.
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if (V != I->getOperand(0))
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return true;
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} else if (isa<SelectInst>(I)) {
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// Play safe and do not accept being used as the condition.
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if (V == I->getOperand(0))
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return true;
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} else if (isa<LoadInst>(I)) {
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// Usually loads can be ignored because they dereference the original
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// pointer. However the loaded value needs to be tracked if loading
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// from an object that the original pointer was stored to.
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if (Depth == 0)
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// Loading the original pointer or a variation of it. This does not
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// cause the pointer to be captured. Note that the loaded value might
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// be the pointer itself (think of self-referential objects), but that
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// is fine as long as it's not this function that stored it there.
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break;
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// Loading a pointer to (a pointer to...) the original pointer or a
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// variation of it. Track uses of the loaded value, noting that one
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// dereference was performed. Note that the loaded value need not be
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// of pointer type. For example, an alloca may have been bitcast to
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// a pointer to another type, which was then loaded.
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--Depth;
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}
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// The original value is not captured via this if the instruction isn't.
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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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UseWithDepth UD(&UI.getUse(), Depth);
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if (Visited.insert(UD))
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Worklist.push_back(UD);
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}
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break;
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default:
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// Something else - be conservative and say it is captured.
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return true;
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}
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}
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// All uses examined - not captured.
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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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// 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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