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llvm-mirror/lib/Transforms/Utils/Evaluator.cpp
David Majnemer 93c48d55ce [ConstnatFolding] Teach the folder how to fold ConstantVector 
A ConstantVector can have ConstantExpr operands and vice versa.
However, the folder had no ability to fold ConstantVectors which, in
some cases, was an optimization barrier.

Instead, rephrase the folder in terms of Constants instead of
ConstantExprs and teach callers how to deal with failure.

llvm-svn: 277099
2016-07-29 03:27:26 +00:00

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//===- Evaluator.cpp - LLVM IR evaluator ----------------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Function evaluator for LLVM IR.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/Evaluator.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/DiagnosticPrinter.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Operator.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#define DEBUG_TYPE "evaluator"
using namespace llvm;
static inline bool
isSimpleEnoughValueToCommit(Constant *C,
SmallPtrSetImpl<Constant *> &SimpleConstants,
const DataLayout &DL);
/// Return true if the specified constant can be handled by the code generator.
/// We don't want to generate something like:
/// void *X = &X/42;
/// because the code generator doesn't have a relocation that can handle that.
///
/// This function should be called if C was not found (but just got inserted)
/// in SimpleConstants to avoid having to rescan the same constants all the
/// time.
static bool
isSimpleEnoughValueToCommitHelper(Constant *C,
SmallPtrSetImpl<Constant *> &SimpleConstants,
const DataLayout &DL) {
// Simple global addresses are supported, do not allow dllimport or
// thread-local globals.
if (auto *GV = dyn_cast<GlobalValue>(C))
return !GV->hasDLLImportStorageClass() && !GV->isThreadLocal();
// Simple integer, undef, constant aggregate zero, etc are all supported.
if (C->getNumOperands() == 0 || isa<BlockAddress>(C))
return true;
// Aggregate values are safe if all their elements are.
if (isa<ConstantAggregate>(C)) {
for (Value *Op : C->operands())
if (!isSimpleEnoughValueToCommit(cast<Constant>(Op), SimpleConstants, DL))
return false;
return true;
}
// We don't know exactly what relocations are allowed in constant expressions,
// so we allow &global+constantoffset, which is safe and uniformly supported
// across targets.
ConstantExpr *CE = cast<ConstantExpr>(C);
switch (CE->getOpcode()) {
case Instruction::BitCast:
// Bitcast is fine if the casted value is fine.
return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants, DL);
case Instruction::IntToPtr:
case Instruction::PtrToInt:
// int <=> ptr is fine if the int type is the same size as the
// pointer type.
if (DL.getTypeSizeInBits(CE->getType()) !=
DL.getTypeSizeInBits(CE->getOperand(0)->getType()))
return false;
return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants, DL);
// GEP is fine if it is simple + constant offset.
case Instruction::GetElementPtr:
for (unsigned i = 1, e = CE->getNumOperands(); i != e; ++i)
if (!isa<ConstantInt>(CE->getOperand(i)))
return false;
return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants, DL);
case Instruction::Add:
// We allow simple+cst.
if (!isa<ConstantInt>(CE->getOperand(1)))
return false;
return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants, DL);
}
return false;
}
static inline bool
isSimpleEnoughValueToCommit(Constant *C,
SmallPtrSetImpl<Constant *> &SimpleConstants,
const DataLayout &DL) {
// If we already checked this constant, we win.
if (!SimpleConstants.insert(C).second)
return true;
// Check the constant.
return isSimpleEnoughValueToCommitHelper(C, SimpleConstants, DL);
}
/// Return true if this constant is simple enough for us to understand. In
/// particular, if it is a cast to anything other than from one pointer type to
/// another pointer type, we punt. We basically just support direct accesses to
/// globals and GEP's of globals. This should be kept up to date with
/// CommitValueTo.
static bool isSimpleEnoughPointerToCommit(Constant *C) {
// Conservatively, avoid aggregate types. This is because we don't
// want to worry about them partially overlapping other stores.
if (!cast<PointerType>(C->getType())->getElementType()->isSingleValueType())
return false;
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C))
// Do not allow weak/*_odr/linkonce linkage or external globals.
return GV->hasUniqueInitializer();
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
// Handle a constantexpr gep.
if (CE->getOpcode() == Instruction::GetElementPtr &&
isa<GlobalVariable>(CE->getOperand(0)) &&
cast<GEPOperator>(CE)->isInBounds()) {
GlobalVariable *GV = cast<GlobalVariable>(CE->getOperand(0));
// Do not allow weak/*_odr/linkonce/dllimport/dllexport linkage or
// external globals.
if (!GV->hasUniqueInitializer())
return false;
// The first index must be zero.
ConstantInt *CI = dyn_cast<ConstantInt>(*std::next(CE->op_begin()));
if (!CI || !CI->isZero()) return false;
// The remaining indices must be compile-time known integers within the
// notional bounds of the corresponding static array types.
if (!CE->isGEPWithNoNotionalOverIndexing())
return false;
return ConstantFoldLoadThroughGEPConstantExpr(GV->getInitializer(), CE);
// A constantexpr bitcast from a pointer to another pointer is a no-op,
// and we know how to evaluate it by moving the bitcast from the pointer
// operand to the value operand.
} else if (CE->getOpcode() == Instruction::BitCast &&
isa<GlobalVariable>(CE->getOperand(0))) {
// Do not allow weak/*_odr/linkonce/dllimport/dllexport linkage or
// external globals.
return cast<GlobalVariable>(CE->getOperand(0))->hasUniqueInitializer();
}
}
return false;
}
/// Return the value that would be computed by a load from P after the stores
/// reflected by 'memory' have been performed. If we can't decide, return null.
Constant *Evaluator::ComputeLoadResult(Constant *P) {
// If this memory location has been recently stored, use the stored value: it
// is the most up-to-date.
DenseMap<Constant*, Constant*>::const_iterator I = MutatedMemory.find(P);
if (I != MutatedMemory.end()) return I->second;
// Access it.
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(P)) {
if (GV->hasDefinitiveInitializer())
return GV->getInitializer();
return nullptr;
}
// Handle a constantexpr getelementptr.
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(P))
if (CE->getOpcode() == Instruction::GetElementPtr &&
isa<GlobalVariable>(CE->getOperand(0))) {
GlobalVariable *GV = cast<GlobalVariable>(CE->getOperand(0));
if (GV->hasDefinitiveInitializer())
return ConstantFoldLoadThroughGEPConstantExpr(GV->getInitializer(), CE);
}
return nullptr; // don't know how to evaluate.
}
/// Evaluate all instructions in block BB, returning true if successful, false
/// if we can't evaluate it. NewBB returns the next BB that control flows into,
/// or null upon return.
bool Evaluator::EvaluateBlock(BasicBlock::iterator CurInst,
BasicBlock *&NextBB) {
// This is the main evaluation loop.
while (1) {
Constant *InstResult = nullptr;
DEBUG(dbgs() << "Evaluating Instruction: " << *CurInst << "\n");
if (StoreInst *SI = dyn_cast<StoreInst>(CurInst)) {
if (!SI->isSimple()) {
DEBUG(dbgs() << "Store is not simple! Can not evaluate.\n");
return false; // no volatile/atomic accesses.
}
Constant *Ptr = getVal(SI->getOperand(1));
if (auto *FoldedPtr = ConstantFoldConstant(Ptr, DL, TLI)) {
DEBUG(dbgs() << "Folding constant ptr expression: " << *Ptr);
Ptr = FoldedPtr;
DEBUG(dbgs() << "; To: " << *Ptr << "\n");
}
if (!isSimpleEnoughPointerToCommit(Ptr)) {
// If this is too complex for us to commit, reject it.
DEBUG(dbgs() << "Pointer is too complex for us to evaluate store.");
return false;
}
Constant *Val = getVal(SI->getOperand(0));
// If this might be too difficult for the backend to handle (e.g. the addr
// of one global variable divided by another) then we can't commit it.
if (!isSimpleEnoughValueToCommit(Val, SimpleConstants, DL)) {
DEBUG(dbgs() << "Store value is too complex to evaluate store. " << *Val
<< "\n");
return false;
}
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr)) {
if (CE->getOpcode() == Instruction::BitCast) {
DEBUG(dbgs() << "Attempting to resolve bitcast on constant ptr.\n");
// If we're evaluating a store through a bitcast, then we need
// to pull the bitcast off the pointer type and push it onto the
// stored value.
Ptr = CE->getOperand(0);
Type *NewTy = cast<PointerType>(Ptr->getType())->getElementType();
// In order to push the bitcast onto the stored value, a bitcast
// from NewTy to Val's type must be legal. If it's not, we can try
// introspecting NewTy to find a legal conversion.
while (!Val->getType()->canLosslesslyBitCastTo(NewTy)) {
// If NewTy is a struct, we can convert the pointer to the struct
// into a pointer to its first member.
// FIXME: This could be extended to support arrays as well.
if (StructType *STy = dyn_cast<StructType>(NewTy)) {
NewTy = STy->getTypeAtIndex(0U);
IntegerType *IdxTy = IntegerType::get(NewTy->getContext(), 32);
Constant *IdxZero = ConstantInt::get(IdxTy, 0, false);
Constant * const IdxList[] = {IdxZero, IdxZero};
Ptr = ConstantExpr::getGetElementPtr(nullptr, Ptr, IdxList);
if (auto *FoldedPtr = ConstantFoldConstant(Ptr, DL, TLI))
Ptr = FoldedPtr;
// If we can't improve the situation by introspecting NewTy,
// we have to give up.
} else {
DEBUG(dbgs() << "Failed to bitcast constant ptr, can not "
"evaluate.\n");
return false;
}
}
// If we found compatible types, go ahead and push the bitcast
// onto the stored value.
Val = ConstantExpr::getBitCast(Val, NewTy);
DEBUG(dbgs() << "Evaluated bitcast: " << *Val << "\n");
}
}
MutatedMemory[Ptr] = Val;
} else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(CurInst)) {
InstResult = ConstantExpr::get(BO->getOpcode(),
getVal(BO->getOperand(0)),
getVal(BO->getOperand(1)));
DEBUG(dbgs() << "Found a BinaryOperator! Simplifying: " << *InstResult
<< "\n");
} else if (CmpInst *CI = dyn_cast<CmpInst>(CurInst)) {
InstResult = ConstantExpr::getCompare(CI->getPredicate(),
getVal(CI->getOperand(0)),
getVal(CI->getOperand(1)));
DEBUG(dbgs() << "Found a CmpInst! Simplifying: " << *InstResult
<< "\n");
} else if (CastInst *CI = dyn_cast<CastInst>(CurInst)) {
InstResult = ConstantExpr::getCast(CI->getOpcode(),
getVal(CI->getOperand(0)),
CI->getType());
DEBUG(dbgs() << "Found a Cast! Simplifying: " << *InstResult
<< "\n");
} else if (SelectInst *SI = dyn_cast<SelectInst>(CurInst)) {
InstResult = ConstantExpr::getSelect(getVal(SI->getOperand(0)),
getVal(SI->getOperand(1)),
getVal(SI->getOperand(2)));
DEBUG(dbgs() << "Found a Select! Simplifying: " << *InstResult
<< "\n");
} else if (auto *EVI = dyn_cast<ExtractValueInst>(CurInst)) {
InstResult = ConstantExpr::getExtractValue(
getVal(EVI->getAggregateOperand()), EVI->getIndices());
DEBUG(dbgs() << "Found an ExtractValueInst! Simplifying: " << *InstResult
<< "\n");
} else if (auto *IVI = dyn_cast<InsertValueInst>(CurInst)) {
InstResult = ConstantExpr::getInsertValue(
getVal(IVI->getAggregateOperand()),
getVal(IVI->getInsertedValueOperand()), IVI->getIndices());
DEBUG(dbgs() << "Found an InsertValueInst! Simplifying: " << *InstResult
<< "\n");
} else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(CurInst)) {
Constant *P = getVal(GEP->getOperand(0));
SmallVector<Constant*, 8> GEPOps;
for (User::op_iterator i = GEP->op_begin() + 1, e = GEP->op_end();
i != e; ++i)
GEPOps.push_back(getVal(*i));
InstResult =
ConstantExpr::getGetElementPtr(GEP->getSourceElementType(), P, GEPOps,
cast<GEPOperator>(GEP)->isInBounds());
DEBUG(dbgs() << "Found a GEP! Simplifying: " << *InstResult
<< "\n");
} else if (LoadInst *LI = dyn_cast<LoadInst>(CurInst)) {
if (!LI->isSimple()) {
DEBUG(dbgs() << "Found a Load! Not a simple load, can not evaluate.\n");
return false; // no volatile/atomic accesses.
}
Constant *Ptr = getVal(LI->getOperand(0));
if (auto *FoldedPtr = ConstantFoldConstant(Ptr, DL, TLI)) {
Ptr = FoldedPtr;
DEBUG(dbgs() << "Found a constant pointer expression, constant "
"folding: " << *Ptr << "\n");
}
InstResult = ComputeLoadResult(Ptr);
if (!InstResult) {
DEBUG(dbgs() << "Failed to compute load result. Can not evaluate load."
"\n");
return false; // Could not evaluate load.
}
DEBUG(dbgs() << "Evaluated load: " << *InstResult << "\n");
} else if (AllocaInst *AI = dyn_cast<AllocaInst>(CurInst)) {
if (AI->isArrayAllocation()) {
DEBUG(dbgs() << "Found an array alloca. Can not evaluate.\n");
return false; // Cannot handle array allocs.
}
Type *Ty = AI->getAllocatedType();
AllocaTmps.push_back(
make_unique<GlobalVariable>(Ty, false, GlobalValue::InternalLinkage,
UndefValue::get(Ty), AI->getName()));
InstResult = AllocaTmps.back().get();
DEBUG(dbgs() << "Found an alloca. Result: " << *InstResult << "\n");
} else if (isa<CallInst>(CurInst) || isa<InvokeInst>(CurInst)) {
CallSite CS(&*CurInst);
// Debug info can safely be ignored here.
if (isa<DbgInfoIntrinsic>(CS.getInstruction())) {
DEBUG(dbgs() << "Ignoring debug info.\n");
++CurInst;
continue;
}
// Cannot handle inline asm.
if (isa<InlineAsm>(CS.getCalledValue())) {
DEBUG(dbgs() << "Found inline asm, can not evaluate.\n");
return false;
}
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CS.getInstruction())) {
if (MemSetInst *MSI = dyn_cast<MemSetInst>(II)) {
if (MSI->isVolatile()) {
DEBUG(dbgs() << "Can not optimize a volatile memset " <<
"intrinsic.\n");
return false;
}
Constant *Ptr = getVal(MSI->getDest());
Constant *Val = getVal(MSI->getValue());
Constant *DestVal = ComputeLoadResult(getVal(Ptr));
if (Val->isNullValue() && DestVal && DestVal->isNullValue()) {
// This memset is a no-op.
DEBUG(dbgs() << "Ignoring no-op memset.\n");
++CurInst;
continue;
}
}
if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
II->getIntrinsicID() == Intrinsic::lifetime_end) {
DEBUG(dbgs() << "Ignoring lifetime intrinsic.\n");
++CurInst;
continue;
}
if (II->getIntrinsicID() == Intrinsic::invariant_start) {
// We don't insert an entry into Values, as it doesn't have a
// meaningful return value.
if (!II->use_empty()) {
DEBUG(dbgs() << "Found unused invariant_start. Can't evaluate.\n");
return false;
}
ConstantInt *Size = cast<ConstantInt>(II->getArgOperand(0));
Value *PtrArg = getVal(II->getArgOperand(1));
Value *Ptr = PtrArg->stripPointerCasts();
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr)) {
Type *ElemTy = GV->getValueType();
if (!Size->isAllOnesValue() &&
Size->getValue().getLimitedValue() >=
DL.getTypeStoreSize(ElemTy)) {
Invariants.insert(GV);
DEBUG(dbgs() << "Found a global var that is an invariant: " << *GV
<< "\n");
} else {
DEBUG(dbgs() << "Found a global var, but can not treat it as an "
"invariant.\n");
}
}
// Continue even if we do nothing.
++CurInst;
continue;
} else if (II->getIntrinsicID() == Intrinsic::assume) {
DEBUG(dbgs() << "Skipping assume intrinsic.\n");
++CurInst;
continue;
}
DEBUG(dbgs() << "Unknown intrinsic. Can not evaluate.\n");
return false;
}
// Resolve function pointers.
Function *Callee = dyn_cast<Function>(getVal(CS.getCalledValue()));
if (!Callee || Callee->isInterposable()) {
DEBUG(dbgs() << "Can not resolve function pointer.\n");
return false; // Cannot resolve.
}
SmallVector<Constant*, 8> Formals;
for (User::op_iterator i = CS.arg_begin(), e = CS.arg_end(); i != e; ++i)
Formals.push_back(getVal(*i));
if (Callee->isDeclaration()) {
// If this is a function we can constant fold, do it.
if (Constant *C = ConstantFoldCall(Callee, Formals, TLI)) {
InstResult = C;
DEBUG(dbgs() << "Constant folded function call. Result: " <<
*InstResult << "\n");
} else {
DEBUG(dbgs() << "Can not constant fold function call.\n");
return false;
}
} else {
if (Callee->getFunctionType()->isVarArg()) {
DEBUG(dbgs() << "Can not constant fold vararg function call.\n");
return false;
}
Constant *RetVal = nullptr;
// Execute the call, if successful, use the return value.
ValueStack.emplace_back();
if (!EvaluateFunction(Callee, RetVal, Formals)) {
DEBUG(dbgs() << "Failed to evaluate function.\n");
return false;
}
ValueStack.pop_back();
InstResult = RetVal;
if (InstResult) {
DEBUG(dbgs() << "Successfully evaluated function. Result: "
<< *InstResult << "\n\n");
} else {
DEBUG(dbgs() << "Successfully evaluated function. Result: 0\n\n");
}
}
} else if (isa<TerminatorInst>(CurInst)) {
DEBUG(dbgs() << "Found a terminator instruction.\n");
if (BranchInst *BI = dyn_cast<BranchInst>(CurInst)) {
if (BI->isUnconditional()) {
NextBB = BI->getSuccessor(0);
} else {
ConstantInt *Cond =
dyn_cast<ConstantInt>(getVal(BI->getCondition()));
if (!Cond) return false; // Cannot determine.
NextBB = BI->getSuccessor(!Cond->getZExtValue());
}
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(CurInst)) {
ConstantInt *Val =
dyn_cast<ConstantInt>(getVal(SI->getCondition()));
if (!Val) return false; // Cannot determine.
NextBB = SI->findCaseValue(Val).getCaseSuccessor();
} else if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(CurInst)) {
Value *Val = getVal(IBI->getAddress())->stripPointerCasts();
if (BlockAddress *BA = dyn_cast<BlockAddress>(Val))
NextBB = BA->getBasicBlock();
else
return false; // Cannot determine.
} else if (isa<ReturnInst>(CurInst)) {
NextBB = nullptr;
} else {
// invoke, unwind, resume, unreachable.
DEBUG(dbgs() << "Can not handle terminator.");
return false; // Cannot handle this terminator.
}
// We succeeded at evaluating this block!
DEBUG(dbgs() << "Successfully evaluated block.\n");
return true;
} else {
// Did not know how to evaluate this!
DEBUG(dbgs() << "Failed to evaluate block due to unhandled instruction."
"\n");
return false;
}
if (!CurInst->use_empty()) {
if (auto *FoldedInstResult = ConstantFoldConstant(InstResult, DL, TLI))
InstResult = FoldedInstResult;
setVal(&*CurInst, InstResult);
}
// If we just processed an invoke, we finished evaluating the block.
if (InvokeInst *II = dyn_cast<InvokeInst>(CurInst)) {
NextBB = II->getNormalDest();
DEBUG(dbgs() << "Found an invoke instruction. Finished Block.\n\n");
return true;
}
// Advance program counter.
++CurInst;
}
}
/// Evaluate a call to function F, returning true if successful, false if we
/// can't evaluate it. ActualArgs contains the formal arguments for the
/// function.
bool Evaluator::EvaluateFunction(Function *F, Constant *&RetVal,
const SmallVectorImpl<Constant*> &ActualArgs) {
// Check to see if this function is already executing (recursion). If so,
// bail out. TODO: we might want to accept limited recursion.
if (std::find(CallStack.begin(), CallStack.end(), F) != CallStack.end())
return false;
CallStack.push_back(F);
// Initialize arguments to the incoming values specified.
unsigned ArgNo = 0;
for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end(); AI != E;
++AI, ++ArgNo)
setVal(&*AI, ActualArgs[ArgNo]);
// ExecutedBlocks - We only handle non-looping, non-recursive code. As such,
// we can only evaluate any one basic block at most once. This set keeps
// track of what we have executed so we can detect recursive cases etc.
SmallPtrSet<BasicBlock*, 32> ExecutedBlocks;
// CurBB - The current basic block we're evaluating.
BasicBlock *CurBB = &F->front();
BasicBlock::iterator CurInst = CurBB->begin();
while (1) {
BasicBlock *NextBB = nullptr; // Initialized to avoid compiler warnings.
DEBUG(dbgs() << "Trying to evaluate BB: " << *CurBB << "\n");
if (!EvaluateBlock(CurInst, NextBB))
return false;
if (!NextBB) {
// Successfully running until there's no next block means that we found
// the return. Fill it the return value and pop the call stack.
ReturnInst *RI = cast<ReturnInst>(CurBB->getTerminator());
if (RI->getNumOperands())
RetVal = getVal(RI->getOperand(0));
CallStack.pop_back();
return true;
}
// Okay, we succeeded in evaluating this control flow. See if we have
// executed the new block before. If so, we have a looping function,
// which we cannot evaluate in reasonable time.
if (!ExecutedBlocks.insert(NextBB).second)
return false; // looped!
// Okay, we have never been in this block before. Check to see if there
// are any PHI nodes. If so, evaluate them with information about where
// we came from.
PHINode *PN = nullptr;
for (CurInst = NextBB->begin();
(PN = dyn_cast<PHINode>(CurInst)); ++CurInst)
setVal(PN, getVal(PN->getIncomingValueForBlock(CurBB)));
// Advance to the next block.
CurBB = NextBB;
}
}
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