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222 lines
8.8 KiB
C++
222 lines
8.8 KiB
C++
//===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
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//
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// This file implements the generic AliasAnalysis interface which is used as the
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// common interface used by all clients and implementations of alias analysis.
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//
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// This file also implements the default version of the AliasAnalysis interface
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// that is to be used when no other implementation is specified. This does some
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// simple tests that detect obvious cases: two different global pointers cannot
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// alias, a global cannot alias a malloc, two different mallocs cannot alias,
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// etc.
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//
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// This alias analysis implementation really isn't very good for anything, but
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// it is very fast, and makes a nice clean default implementation. Because it
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// handles lots of little corner cases, other, more complex, alias analysis
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// implementations may choose to rely on this pass to resolve these simple and
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// easy cases.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/BasicAliasAnalysis.h"
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#include "llvm/BasicBlock.h"
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#include "llvm/Support/InstVisitor.h"
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#include "llvm/iMemory.h"
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#include "llvm/iOther.h"
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#include "llvm/Constants.h"
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#include "llvm/GlobalValue.h"
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#include "llvm/DerivedTypes.h"
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// Register the AliasAnalysis interface, providing a nice name to refer to.
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static RegisterAnalysisGroup<AliasAnalysis> X("Alias Analysis");
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// CanModify - Define a little visitor class that is used to check to see if
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// arbitrary chunks of code can modify a specified pointer.
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//
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namespace {
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struct CanModify : public InstVisitor<CanModify, bool> {
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AliasAnalysis &AA;
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const Value *Ptr;
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CanModify(AliasAnalysis *aa, const Value *ptr)
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: AA(*aa), Ptr(ptr) {}
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bool visitInvokeInst(InvokeInst &II) {
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return AA.canInvokeModify(II, Ptr);
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}
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bool visitCallInst(CallInst &CI) {
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return AA.canCallModify(CI, Ptr);
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}
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bool visitStoreInst(StoreInst &SI) {
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return AA.alias(Ptr, SI.getOperand(1));
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}
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// Other instructions do not alias anything.
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bool visitInstruction(Instruction &I) { return false; }
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};
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}
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// AliasAnalysis destructor: DO NOT move this to the header file for
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// AliasAnalysis or else clients of the AliasAnalysis class may not depend on
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// the AliasAnalysis.o file in the current .a file, causing alias analysis
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// support to not be included in the tool correctly!
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//
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AliasAnalysis::~AliasAnalysis() {}
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/// canBasicBlockModify - Return true if it is possible for execution of the
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/// specified basic block to modify the value pointed to by Ptr.
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///
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bool AliasAnalysis::canBasicBlockModify(const BasicBlock &bb,
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const Value *Ptr) {
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CanModify CM(this, Ptr);
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BasicBlock &BB = const_cast<BasicBlock&>(bb);
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for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I)
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if (CM.visit(I)) // Check every instruction in the basic block...
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return true;
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return false;
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}
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/// canInstructionRangeModify - Return true if it is possible for the execution
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/// of the specified instructions to modify the value pointed to by Ptr. The
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/// instructions to consider are all of the instructions in the range of [I1,I2]
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/// INCLUSIVE. I1 and I2 must be in the same basic block.
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///
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bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
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const Instruction &I2,
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const Value *Ptr) {
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assert(I1.getParent() == I2.getParent() &&
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"Instructions not in same basic block!");
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CanModify CM(this, Ptr);
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BasicBlock::iterator I = const_cast<Instruction*>(&I1);
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BasicBlock::iterator E = const_cast<Instruction*>(&I2);
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++E; // Convert from inclusive to exclusive range.
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for (; I != E; ++I)
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if (CM.visit(I)) // Check every instruction in the basic block...
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return true;
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return false;
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}
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//===----------------------------------------------------------------------===//
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// BasicAliasAnalysis Pass Implementation
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//===----------------------------------------------------------------------===//
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//
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// Because of the way .a files work, the implementation of the
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// BasicAliasAnalysis class MUST be in the AliasAnalysis file itself, or else we
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// run the risk of AliasAnalysis being used, but the default implementation not
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// being linked into the tool that uses it. As such, we register and implement
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// the class here.
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//
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namespace {
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// Register this pass...
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RegisterOpt<BasicAliasAnalysis>
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X("basicaa", "Basic Alias Analysis (default AA impl)");
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// Declare that we implement the AliasAnalysis interface
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RegisterAnalysisGroup<AliasAnalysis, BasicAliasAnalysis, true> Y;
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} // End of anonymous namespace
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// hasUniqueAddress - Return true if the
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static inline bool hasUniqueAddress(const Value *V) {
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return isa<GlobalValue>(V) || isa<MallocInst>(V) || isa<AllocaInst>(V);
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}
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static const Value *getUnderlyingObject(const Value *V) {
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if (!isa<PointerType>(V->getType())) return 0;
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// If we are at some type of object... return it.
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if (hasUniqueAddress(V)) return V;
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// Traverse through different addressing mechanisms...
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if (const Instruction *I = dyn_cast<Instruction>(V)) {
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if (isa<CastInst>(I) || isa<GetElementPtrInst>(I))
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return getUnderlyingObject(I->getOperand(0));
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}
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return 0;
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}
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// alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
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// as array references. Note that this function is heavily tail recursive.
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// Hopefully we have a smart C++ compiler. :)
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//
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AliasAnalysis::Result BasicAliasAnalysis::alias(const Value *V1,
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const Value *V2) {
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// Strip off constant pointer refs if they exist
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if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V1))
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V1 = CPR->getValue();
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if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V2))
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V2 = CPR->getValue();
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// Are we checking for alias of the same value?
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if (V1 == V2) return MustAlias;
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if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) &&
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V1->getType() != Type::LongTy && V2->getType() != Type::LongTy)
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return NoAlias; // Scalars cannot alias each other
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// Strip off cast instructions...
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if (const Instruction *I = dyn_cast<CastInst>(V1))
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return alias(I->getOperand(0), V2);
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if (const Instruction *I = dyn_cast<CastInst>(V2))
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return alias(I->getOperand(0), V1);
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// If we have two gep instructions with identical indices, return an alias
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// result equal to the alias result of the original pointer...
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//
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if (const GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(V1))
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if (const GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(V2))
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if (GEP1->getNumOperands() == GEP2->getNumOperands() &&
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GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType()) {
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if (std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
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return alias(GEP1->getOperand(0), GEP2->getOperand(0));
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// If all of the indexes to the getelementptr are constant, but
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// different (well we already know they are different), then we know
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// that there cannot be an alias here if the two base pointers DO alias.
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//
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bool AllConstant = true;
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for (unsigned i = 1, e = GEP1->getNumOperands(); i != e; ++i)
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if (!isa<Constant>(GEP1->getOperand(i)) ||
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!isa<Constant>(GEP2->getOperand(i))) {
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AllConstant = false;
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break;
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}
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// If we are all constant, then look at where the the base pointers
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// alias. If they are known not to alias, then we are dealing with two
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// different arrays or something, so no alias is possible. If they are
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// known to be the same object, then we cannot alias because we are
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// indexing into a different part of the object. As usual, MayAlias
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// doesn't tell us anything.
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//
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if (AllConstant &&
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alias(GEP1->getOperand(0), GEP2->getOperand(1)) != MayAlias)
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return NoAlias;
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}
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// Figure out what objects these things are pointing to if we can...
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const Value *O1 = getUnderlyingObject(V1);
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const Value *O2 = getUnderlyingObject(V2);
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// Pointing at a discernable object?
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if (O1 && O2) {
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// If they are two different objects, we know that we have no alias...
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if (O1 != O2) return NoAlias;
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// If they are the same object, they we can look at the indexes. If they
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// index off of the object is the same for both pointers, they must alias.
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// If they are provably different, they must not alias. Otherwise, we can't
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// tell anything.
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} else if (O1 && isa<ConstantPointerNull>(V2)) {
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return NoAlias; // Unique values don't alias null
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} else if (O2 && isa<ConstantPointerNull>(V1)) {
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return NoAlias; // Unique values don't alias null
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}
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return MayAlias;
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}
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