llvm-6502/lib/Transforms/IPO/FunctionAttrs.cpp
2009-09-18 22:35:49 +00:00

351 lines
12 KiB
C++

//===- FunctionAttrs.cpp - Pass which marks functions readnone or readonly ===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a simple interprocedural pass which walks the
// call-graph, looking for functions which do not access or only read
// non-local memory, and marking them readnone/readonly. In addition,
// it marks function arguments (of pointer type) 'nocapture' if a call
// to the function does not create any copies of the pointer value that
// outlive the call. This more or less means that the pointer is only
// dereferenced, and not returned from the function or stored in a global.
// This pass is implemented as a bottom-up traversal of the call-graph.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "functionattrs"
#include "llvm/Transforms/IPO.h"
#include "llvm/CallGraphSCCPass.h"
#include "llvm/GlobalVariable.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/MallocHelper.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/UniqueVector.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/InstIterator.h"
using namespace llvm;
STATISTIC(NumReadNone, "Number of functions marked readnone");
STATISTIC(NumReadOnly, "Number of functions marked readonly");
STATISTIC(NumNoCapture, "Number of arguments marked nocapture");
STATISTIC(NumNoAlias, "Number of function returns marked noalias");
namespace {
struct VISIBILITY_HIDDEN FunctionAttrs : public CallGraphSCCPass {
static char ID; // Pass identification, replacement for typeid
FunctionAttrs() : CallGraphSCCPass(&ID) {}
// runOnSCC - Analyze the SCC, performing the transformation if possible.
bool runOnSCC(std::vector<CallGraphNode *> &SCC);
// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
bool AddReadAttrs(const std::vector<CallGraphNode *> &SCC);
// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
bool AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC);
// IsFunctionMallocLike - Does this function allocate new memory?
bool IsFunctionMallocLike(Function *F,
SmallPtrSet<Function*, 8> &) const;
// AddNoAliasAttrs - Deduce noalias attributes for the SCC.
bool AddNoAliasAttrs(const std::vector<CallGraphNode *> &SCC);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
CallGraphSCCPass::getAnalysisUsage(AU);
}
bool PointsToLocalMemory(Value *V);
};
}
char FunctionAttrs::ID = 0;
static RegisterPass<FunctionAttrs>
X("functionattrs", "Deduce function attributes");
Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); }
/// PointsToLocalMemory - Returns whether the given pointer value points to
/// memory that is local to the function. Global constants are considered
/// local to all functions.
bool FunctionAttrs::PointsToLocalMemory(Value *V) {
V = V->getUnderlyingObject();
// An alloca instruction defines local memory.
if (isa<AllocaInst>(V))
return true;
// A global constant counts as local memory for our purposes.
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
return GV->isConstant();
// Could look through phi nodes and selects here, but it doesn't seem
// to be useful in practice.
return false;
}
/// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
bool FunctionAttrs::AddReadAttrs(const std::vector<CallGraphNode *> &SCC) {
SmallPtrSet<Function*, 8> SCCNodes;
// Fill SCCNodes with the elements of the SCC. Used for quickly
// looking up whether a given CallGraphNode is in this SCC.
for (unsigned i = 0, e = SCC.size(); i != e; ++i)
SCCNodes.insert(SCC[i]->getFunction());
// Check if any of the functions in the SCC read or write memory. If they
// write memory then they can't be marked readnone or readonly.
bool ReadsMemory = false;
for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
Function *F = SCC[i]->getFunction();
if (F == 0)
// External node - may write memory. Just give up.
return false;
if (F->doesNotAccessMemory())
// Already perfect!
continue;
// Definitions with weak linkage may be overridden at linktime with
// something that writes memory, so treat them like declarations.
if (F->isDeclaration() || F->mayBeOverridden()) {
if (!F->onlyReadsMemory())
// May write memory. Just give up.
return false;
ReadsMemory = true;
continue;
}
// Scan the function body for instructions that may read or write memory.
for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) {
Instruction *I = &*II;
// Some instructions can be ignored even if they read or write memory.
// Detect these now, skipping to the next instruction if one is found.
CallSite CS = CallSite::get(I);
if (CS.getInstruction() && CS.getCalledFunction()) {
// Ignore calls to functions in the same SCC.
if (SCCNodes.count(CS.getCalledFunction()))
continue;
} else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
// Ignore loads from local memory.
if (PointsToLocalMemory(LI->getPointerOperand()))
continue;
} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
// Ignore stores to local memory.
if (PointsToLocalMemory(SI->getPointerOperand()))
continue;
}
// Any remaining instructions need to be taken seriously! Check if they
// read or write memory.
if (I->mayWriteToMemory())
// Writes memory. Just give up.
return false;
if (isa<MallocInst>(I) || isMalloc(I))
// malloc claims not to write memory! PR3754.
return false;
// If this instruction may read memory, remember that.
ReadsMemory |= I->mayReadFromMemory();
}
}
// Success! Functions in this SCC do not access memory, or only read memory.
// Give them the appropriate attribute.
bool MadeChange = false;
for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
Function *F = SCC[i]->getFunction();
if (F->doesNotAccessMemory())
// Already perfect!
continue;
if (F->onlyReadsMemory() && ReadsMemory)
// No change.
continue;
MadeChange = true;
// Clear out any existing attributes.
F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone);
// Add in the new attribute.
F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone);
if (ReadsMemory)
++NumReadOnly;
else
++NumReadNone;
}
return MadeChange;
}
/// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
bool FunctionAttrs::AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC) {
bool Changed = false;
// Check each function in turn, determining which pointer arguments are not
// captured.
for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
Function *F = SCC[i]->getFunction();
if (F == 0)
// External node - skip it;
continue;
// Definitions with weak linkage may be overridden at linktime with
// something that writes memory, so treat them like declarations.
if (F->isDeclaration() || F->mayBeOverridden())
continue;
for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr() &&
!PointerMayBeCaptured(A, true)) {
A->addAttr(Attribute::NoCapture);
++NumNoCapture;
Changed = true;
}
}
return Changed;
}
/// IsFunctionMallocLike - A function is malloc-like if it returns either null
/// or a pointer that doesn't alias any other pointer visible to the caller.
bool FunctionAttrs::IsFunctionMallocLike(Function *F,
SmallPtrSet<Function*, 8> &SCCNodes) const {
UniqueVector<Value *> FlowsToReturn;
for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I)
if (ReturnInst *Ret = dyn_cast<ReturnInst>(I->getTerminator()))
FlowsToReturn.insert(Ret->getReturnValue());
for (unsigned i = 0; i != FlowsToReturn.size(); ++i) {
Value *RetVal = FlowsToReturn[i+1]; // UniqueVector[0] is reserved.
if (Constant *C = dyn_cast<Constant>(RetVal)) {
if (!C->isNullValue() && !isa<UndefValue>(C))
return false;
continue;
}
if (isa<Argument>(RetVal))
return false;
if (Instruction *RVI = dyn_cast<Instruction>(RetVal))
switch (RVI->getOpcode()) {
// Extend the analysis by looking upwards.
case Instruction::BitCast:
if (isMalloc(RVI))
break;
// fall through
case Instruction::GetElementPtr:
FlowsToReturn.insert(RVI->getOperand(0));
continue;
case Instruction::Select: {
SelectInst *SI = cast<SelectInst>(RVI);
FlowsToReturn.insert(SI->getTrueValue());
FlowsToReturn.insert(SI->getFalseValue());
} continue;
case Instruction::PHI: {
PHINode *PN = cast<PHINode>(RVI);
for (int i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
FlowsToReturn.insert(PN->getIncomingValue(i));
} continue;
// Check whether the pointer came from an allocation.
case Instruction::Alloca:
case Instruction::Malloc:
break;
case Instruction::Call:
if (isMalloc(RVI))
break;
case Instruction::Invoke: {
CallSite CS(RVI);
if (CS.paramHasAttr(0, Attribute::NoAlias))
break;
if (CS.getCalledFunction() &&
SCCNodes.count(CS.getCalledFunction()))
break;
} // fall-through
default:
return false; // Did not come from an allocation.
}
if (PointerMayBeCaptured(RetVal, false))
return false;
}
return true;
}
/// AddNoAliasAttrs - Deduce noalias attributes for the SCC.
bool FunctionAttrs::AddNoAliasAttrs(const std::vector<CallGraphNode *> &SCC) {
SmallPtrSet<Function*, 8> SCCNodes;
// Fill SCCNodes with the elements of the SCC. Used for quickly
// looking up whether a given CallGraphNode is in this SCC.
for (unsigned i = 0, e = SCC.size(); i != e; ++i)
SCCNodes.insert(SCC[i]->getFunction());
// Check each function in turn, determining which functions return noalias
// pointers.
for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
Function *F = SCC[i]->getFunction();
if (F == 0)
// External node - skip it;
return false;
// Already noalias.
if (F->doesNotAlias(0))
continue;
// Definitions with weak linkage may be overridden at linktime, so
// treat them like declarations.
if (F->isDeclaration() || F->mayBeOverridden())
return false;
// We annotate noalias return values, which are only applicable to
// pointer types.
if (!isa<PointerType>(F->getReturnType()))
continue;
if (!IsFunctionMallocLike(F, SCCNodes))
return false;
}
bool MadeChange = false;
for (unsigned i = 0, e = SCC.size(); i != e; ++i) {
Function *F = SCC[i]->getFunction();
if (F->doesNotAlias(0) || !isa<PointerType>(F->getReturnType()))
continue;
F->setDoesNotAlias(0);
++NumNoAlias;
MadeChange = true;
}
return MadeChange;
}
bool FunctionAttrs::runOnSCC(std::vector<CallGraphNode *> &SCC) {
bool Changed = AddReadAttrs(SCC);
Changed |= AddNoCaptureAttrs(SCC);
Changed |= AddNoAliasAttrs(SCC);
return Changed;
}