//
// 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. It implements
-// this as a bottom-up traversal of the call-graph.
+// 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.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO.h"
#include "llvm/CallGraphSCCPass.h"
#include "llvm/GlobalVariable.h"
-#include "llvm/Instructions.h"
+#include "llvm/IntrinsicInst.h"
+#include "llvm/LLVMContext.h"
+#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CallGraph.h"
-#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
-#include "llvm/Support/Compiler.h"
+#include "llvm/ADT/UniqueVector.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 {
+ struct FunctionAttrs : public CallGraphSCCPass {
static char ID; // Pass identification, replacement for typeid
- FunctionAttrs() : CallGraphSCCPass(&ID) {}
+ FunctionAttrs() : CallGraphSCCPass(ID), AA(0) {
+ initializeFunctionAttrsPass(*PassRegistry::getPassRegistry());
+ }
// runOnSCC - Analyze the SCC, performing the transformation if possible.
- bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
+ bool runOnSCC(CallGraphSCC &SCC);
// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
- bool AddReadAttrs(const std::vector<CallGraphNode *> &SCC);
+ bool AddReadAttrs(const CallGraphSCC &SCC);
// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
- bool AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC);
+ bool AddNoCaptureAttrs(const CallGraphSCC &SCC);
+
+ // IsFunctionMallocLike - Does this function allocate new memory?
+ bool IsFunctionMallocLike(Function *F,
+ SmallPtrSet<Function*, 8> &) const;
- // isCaptured - Returns whether this pointer value is captured.
- bool isCaptured(Function &F, Value *V);
+ // AddNoAliasAttrs - Deduce noalias attributes for the SCC.
+ bool AddNoAliasAttrs(const CallGraphSCC &SCC);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
+ AU.addRequired<AliasAnalysis>();
CallGraphSCCPass::getAnalysisUsage(AU);
}
- bool PointsToLocalMemory(Value *V);
+ private:
+ AliasAnalysis *AA;
};
}
char FunctionAttrs::ID = 0;
-static RegisterPass<FunctionAttrs>
-X("functionattrs", "Deduce function attributes");
+INITIALIZE_PASS_BEGIN(FunctionAttrs, "functionattrs",
+ "Deduce function attributes", false, false)
+INITIALIZE_AG_DEPENDENCY(CallGraph)
+INITIALIZE_PASS_END(FunctionAttrs, "functionattrs",
+ "Deduce function attributes", false, false)
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<CallGraphNode*, 8> SCCNodes;
- CallGraph &CG = getAnalysis<CallGraph>();
+bool FunctionAttrs::AddReadAttrs(const CallGraphSCC &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]);
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
+ SCCNodes.insert((*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();
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
+ Function *F = (*I)->getFunction();
if (F == 0)
// External node - may write memory. Just give up.
return false;
- if (F->doesNotAccessMemory())
+ AliasAnalysis::ModRefBehavior MRB = AA->getModRefBehavior(F);
+ if (MRB == AliasAnalysis::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())
+ if (!AliasAnalysis::onlyReadsMemory(MRB))
// May write memory. Just give up.
return false;
// 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()) {
+ CallSite CS(cast<Value>(I));
+ if (CS) {
// Ignore calls to functions in the same SCC.
- if (SCCNodes.count(CG[CS.getCalledFunction()]))
+ if (CS.getCalledFunction() && SCCNodes.count(CS.getCalledFunction()))
continue;
- } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
- // Ignore loads from local memory.
- if (PointsToLocalMemory(LI->getPointerOperand()))
+ AliasAnalysis::ModRefBehavior MRB = AA->getModRefBehavior(CS);
+ // If the call doesn't access arbitrary memory, we may be able to
+ // figure out something.
+ if (AliasAnalysis::onlyAccessesArgPointees(MRB)) {
+ // If the call does access argument pointees, check each argument.
+ if (AliasAnalysis::doesAccessArgPointees(MRB))
+ // Check whether all pointer arguments point to local memory, and
+ // ignore calls that only access local memory.
+ for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
+ CI != CE; ++CI) {
+ Value *Arg = *CI;
+ if (Arg->getType()->isPointerTy()) {
+ AliasAnalysis::Location Loc(Arg,
+ AliasAnalysis::UnknownSize,
+ I->getMetadata(LLVMContext::MD_tbaa));
+ if (!AA->pointsToConstantMemory(Loc, /*OrLocal=*/true)) {
+ if (MRB & AliasAnalysis::Mod)
+ // Writes non-local memory. Give up.
+ return false;
+ if (MRB & AliasAnalysis::Ref)
+ // Ok, it reads non-local memory.
+ ReadsMemory = true;
+ }
+ }
+ }
continue;
+ }
+ // The call could access any memory. If that includes writes, give up.
+ if (MRB & AliasAnalysis::Mod)
+ return false;
+ // If it reads, note it.
+ if (MRB & AliasAnalysis::Ref)
+ ReadsMemory = true;
+ continue;
+ } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
+ // Ignore non-volatile loads from local memory.
+ if (!LI->isVolatile()) {
+ AliasAnalysis::Location Loc = AA->getLocation(LI);
+ if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
+ continue;
+ }
} else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
- // Ignore stores to local memory.
- if (PointsToLocalMemory(SI->getPointerOperand()))
+ // Ignore non-volatile stores to local memory.
+ if (!SI->isVolatile()) {
+ AliasAnalysis::Location Loc = AA->getLocation(SI);
+ if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
+ continue;
+ }
+ } else if (VAArgInst *VI = dyn_cast<VAArgInst>(I)) {
+ // Ignore vaargs on local memory.
+ AliasAnalysis::Location Loc = AA->getLocation(VI);
+ if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
continue;
}
if (I->mayWriteToMemory())
// Writes memory. Just give up.
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();
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
+ Function *F = (*I)->getFunction();
if (F->doesNotAccessMemory())
// Already perfect!
F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone);
if (ReadsMemory)
- NumReadOnly++;
+ ++NumReadOnly;
else
- NumReadNone++;
+ ++NumReadNone;
}
return MadeChange;
}
-/// isCaptured - Returns whether this pointer value is captured.
-bool FunctionAttrs::isCaptured(Function &F, Value *V) {
- SmallVector<Use*, 16> Worklist;
- SmallPtrSet<Use*, 16> Visited;
-
- for (Value::use_iterator UI = V->use_begin(), UE = V->use_end(); UI != UE;
- ++UI) {
- Use *U = &UI.getUse();
- Visited.insert(U);
- Worklist.push_back(U);
- }
+/// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
+bool FunctionAttrs::AddNoCaptureAttrs(const CallGraphSCC &SCC) {
+ bool Changed = false;
- while (!Worklist.empty()) {
- Use *U = Worklist.pop_back_val();
- Instruction *I = cast<Instruction>(U->getUser());
- V = U->get();
+ // Check each function in turn, determining which pointer arguments are not
+ // captured.
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
+ Function *F = (*I)->getFunction();
- if (isa<LoadInst>(I)) {
- // Loading a pointer does not cause it to escape.
+ if (F == 0)
+ // External node - skip it;
continue;
- }
- if (isa<StoreInst>(I)) {
- if (V == I->getOperand(0))
- // Stored the pointer - escapes. TODO: improve this.
- return true;
- // Storing to the pointee does not cause the pointer to escape.
+ // 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;
- }
- CallSite CS = CallSite::get(I);
- if (CS.getInstruction()) {
- // Does not escape if only passed via 'nocapture' arguments. Note
- // that calling a function pointer does not in itself cause that
- // function pointer to escape. This is a subtle point considering
- // that (for example) the callee might return its own address. It
- // is analogous to saying that loading a value from a pointer does
- // not cause the pointer to escape, even though the loaded value
- // might be the pointer itself (think of self-referential objects).
- CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
- for (CallSite::arg_iterator A = B; A != E; ++A)
- if (A->get() == V && !CS.paramHasAttr(A-B+1, Attribute::NoCapture))
- // The parameter is not marked 'nocapture' - escapes.
- return true;
- // Only passed via 'nocapture' arguments, or is the called function.
- // Does not escape.
+ for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
+ if (A->getType()->isPointerTy() && !A->hasNoCaptureAttr() &&
+ !PointerMayBeCaptured(A, true, /*StoreCaptures=*/false)) {
+ 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<BitCastInst>(I) || isa<GetElementPtrInst>(I) ||
- isa<PHINode>(I) || isa<SelectInst>(I)) {
- // Type conversion, calculating an offset, or merging values.
- // The original value does not escape via this if the new value doesn't.
- // Note that in the case of a select instruction it is important that
- // the value not be used as the condition, since otherwise one bit of
- // information might escape. It cannot be the condition because it has
- // the wrong type.
- for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
- UI != UE; ++UI) {
- Use *U = &UI.getUse();
- if (Visited.insert(U))
- Worklist.push_back(U);
+ 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:
+ 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:
+ break;
+ case Instruction::Call:
+ 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.
}
- continue;
- }
- // Something else - be conservative and say it escapes.
- return true;
+ if (PointerMayBeCaptured(RetVal, false, /*StoreCaptures=*/false))
+ return false;
}
- return false;
+ return true;
}
-/// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
-bool FunctionAttrs::AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC) {
- bool Changed = false;
+/// AddNoAliasAttrs - Deduce noalias attributes for the SCC.
+bool FunctionAttrs::AddNoAliasAttrs(const CallGraphSCC &SCC) {
+ SmallPtrSet<Function*, 8> SCCNodes;
- // 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();
+ // Fill SCCNodes with the elements of the SCC. Used for quickly
+ // looking up whether a given CallGraphNode is in this SCC.
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
+ SCCNodes.insert((*I)->getFunction());
+
+ // Check each function in turn, determining which functions return noalias
+ // pointers.
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
+ Function *F = (*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 with
- // something that writes memory, so treat them like declarations.
+ // 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 (!F->getReturnType()->isPointerTy())
continue;
- for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
- if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr() &&
- !isCaptured(*F, A)) {
- A->addAttr(Attribute::NoCapture);
- NumNoCapture++;
- Changed = true;
- }
+ if (!IsFunctionMallocLike(F, SCCNodes))
+ return false;
}
- return Changed;
+ bool MadeChange = false;
+ for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
+ Function *F = (*I)->getFunction();
+ if (F->doesNotAlias(0) || !F->getReturnType()->isPointerTy())
+ continue;
+
+ F->setDoesNotAlias(0);
+ ++NumNoAlias;
+ MadeChange = true;
+ }
+
+ return MadeChange;
}
-bool FunctionAttrs::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
+bool FunctionAttrs::runOnSCC(CallGraphSCC &SCC) {
+ AA = &getAnalysis<AliasAnalysis>();
+
bool Changed = AddReadAttrs(SCC);
Changed |= AddNoCaptureAttrs(SCC);
+ Changed |= AddNoAliasAttrs(SCC);
return Changed;
}
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