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Add a ComputeNumSignBits function for use by instcombine, based on the

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code in SelectionDAG.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51279 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2008-05-19 22:14:15 +00:00
parent c215b3ef5d
commit 45b4e48b18
1 changed files with 149 additions and 5 deletions
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154
lib/Transforms/Scalar/InstructionCombining.cpp
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@ -378,8 +378,9 @@ namespace {
Value *EvaluateInDifferentType(Value *V, const Type *Ty, bool isSigned);
void ComputeMaskedBits(Value *V, const APInt &Mask, APInt& KnownZero,
APInt& KnownOne, unsigned Depth = 0);
APInt& KnownOne, unsigned Depth = 0) const;
bool MaskedValueIsZero(Value *V, const APInt& Mask, unsigned Depth = 0);
unsigned ComputeNumSignBits(Value *Op, unsigned Depth = 0) const;
bool CanEvaluateInDifferentType(Value *V, const IntegerType *Ty,
unsigned CastOpc,
int &NumCastsRemoved);
@ -596,10 +597,10 @@ static User *dyn_castGetElementPtr(Value *V) {
/// getOpcode - If this is an Instruction or a ConstantExpr, return the
/// opcode value. Otherwise return UserOp1.
static unsigned getOpcode(User *U) {
if (Instruction *I = dyn_cast<Instruction>(U))
static unsigned getOpcode(Value *V) {
if (Instruction *I = dyn_cast<Instruction>(V))
return I->getOpcode();
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U))
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
return CE->getOpcode();
// Use UserOp1 to mean there's no opcode.
return Instruction::UserOp1;
@ -666,7 +667,7 @@ static bool MultiplyOverflows(ConstantInt *C1, ConstantInt *C2, bool sign) {
/// this won't lose us code quality.
void InstCombiner::ComputeMaskedBits(Value *V, const APInt &Mask,
APInt& KnownZero, APInt& KnownOne,
unsigned Depth) {
unsigned Depth) const {
assert(V && "No Value?");
assert(Depth <= 6 && "Limit Search Depth");
uint32_t BitWidth = Mask.getBitWidth();
@ -678,6 +679,7 @@ void InstCombiner::ComputeMaskedBits(Value *V, const APInt &Mask,
KnownZero.getBitWidth() == BitWidth &&
KnownOne.getBitWidth() == BitWidth &&
"V, Mask, KnownOne and KnownZero should have same BitWidth");
if (ConstantInt *CI = dyn_cast<ConstantInt>(V)) {
// We know all of the bits for a constant!
KnownOne = CI->getValue() & Mask;
@ -2059,6 +2061,148 @@ Value *InstCombiner::SimplifyDemandedVectorElts(Value *V, uint64_t DemandedElts,
return MadeChange ? I : 0;
}
/// ComputeNumSignBits - Return the number of times the sign bit of the
/// register is replicated into the other bits. We know that at least 1 bit
/// is always equal to the sign bit (itself), but other cases can give us
/// information. For example, immediately after an "ashr X, 2", we know that
/// the top 3 bits are all equal to each other, so we return 3.
///
unsigned InstCombiner::ComputeNumSignBits(Value *V, unsigned Depth) const{
const IntegerType *Ty = cast<IntegerType>(V->getType());
unsigned TyBits = Ty->getBitWidth();
unsigned Tmp, Tmp2;
if (Depth == 6)
return 1; // Limit search depth.
User *U = dyn_cast<User>(V);
switch (getOpcode(V)) {
default: break;
case Instruction::SExt:
Tmp = TyBits-cast<IntegerType>(U->getOperand(0)->getType())->getBitWidth();
return ComputeNumSignBits(U->getOperand(0), Depth+1) + Tmp;
case Instruction::AShr:
Tmp = ComputeNumSignBits(U->getOperand(0), Depth+1);
// SRA X, C -> adds C sign bits.
if (ConstantInt *C = dyn_cast<ConstantInt>(U->getOperand(1))) {
Tmp += C->getZExtValue();
if (Tmp > TyBits) Tmp = TyBits;
}
return Tmp;
case Instruction::Shl:
if (ConstantInt *C = dyn_cast<ConstantInt>(U->getOperand(1))) {
// shl destroys sign bits.
Tmp = ComputeNumSignBits(U->getOperand(0), Depth+1);
if (C->getZExtValue() >= TyBits || // Bad shift.
C->getZExtValue() >= Tmp) break; // Shifted all sign bits out.
return Tmp - C->getZExtValue();
}
break;
case Instruction::And:
case Instruction::Or:
case Instruction::Xor: // NOT is handled here.
// Logical binary ops preserve the number of sign bits.
Tmp = ComputeNumSignBits(U->getOperand(0), Depth+1);
if (Tmp == 1) return 1; // Early out.
Tmp2 = ComputeNumSignBits(U->getOperand(1), Depth+1);
return std::min(Tmp, Tmp2);
case Instruction::Select:
Tmp = ComputeNumSignBits(U->getOperand(0), Depth+1);
if (Tmp == 1) return 1; // Early out.
Tmp2 = ComputeNumSignBits(U->getOperand(1), Depth+1);
return std::min(Tmp, Tmp2);
case Instruction::Add:
// Add can have at most one carry bit. Thus we know that the output
// is, at worst, one more bit than the inputs.
Tmp = ComputeNumSignBits(U->getOperand(0), Depth+1);
if (Tmp == 1) return 1; // Early out.
// Special case decrementing a value (ADD X, -1):
if (ConstantInt *CRHS = dyn_cast<ConstantInt>(U->getOperand(0)))
if (CRHS->isAllOnesValue()) {
APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
APInt Mask = APInt::getAllOnesValue(TyBits);
ComputeMaskedBits(U->getOperand(0), Mask, KnownZero, KnownOne, Depth+1);
// If the input is known to be 0 or 1, the output is 0/-1, which is all
// sign bits set.
if ((KnownZero | APInt(TyBits, 1)) == Mask)
return TyBits;
// If we are subtracting one from a positive number, there is no carry
// out of the result.
if (KnownZero.isNegative())
return Tmp;
}
Tmp2 = ComputeNumSignBits(U->getOperand(1), Depth+1);
if (Tmp2 == 1) return 1;
return std::min(Tmp, Tmp2)-1;
break;
case Instruction::Sub:
Tmp2 = ComputeNumSignBits(U->getOperand(1), Depth+1);
if (Tmp2 == 1) return 1;
// Handle NEG.
if (ConstantInt *CLHS = dyn_cast<ConstantInt>(U->getOperand(0)))
if (CLHS->isNullValue()) {
APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
APInt Mask = APInt::getAllOnesValue(TyBits);
ComputeMaskedBits(U->getOperand(1), Mask, KnownZero, KnownOne, Depth+1);
// If the input is known to be 0 or 1, the output is 0/-1, which is all
// sign bits set.
if ((KnownZero | APInt(TyBits, 1)) == Mask)
return TyBits;
// If the input is known to be positive (the sign bit is known clear),
// the output of the NEG has the same number of sign bits as the input.
if (KnownZero.isNegative())
return Tmp2;
// Otherwise, we treat this like a SUB.
}
// Sub can have at most one carry bit. Thus we know that the output
// is, at worst, one more bit than the inputs.
Tmp = ComputeNumSignBits(U->getOperand(0), Depth+1);
if (Tmp == 1) return 1; // Early out.
return std::min(Tmp, Tmp2)-1;
break;
case Instruction::Trunc:
// FIXME: it's tricky to do anything useful for this, but it is an important
// case for targets like X86.
break;
}
// Finally, if we can prove that the top bits of the result are 0's or 1's,
// use this information.
APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
APInt Mask = APInt::getAllOnesValue(TyBits);
ComputeMaskedBits(V, Mask, KnownZero, KnownOne, Depth);
if (KnownZero.isNegative()) { // sign bit is 0
Mask = KnownZero;
} else if (KnownOne.isNegative()) { // sign bit is 1;
Mask = KnownOne;
} else {
// Nothing known.
return 1;
}
// Okay, we know that the sign bit in Mask is set. Use CLZ to determine
// the number of identical bits in the top of the input value.
Mask = ~Mask;
Mask <<= Mask.getBitWidth()-TyBits;
// Return # leading zeros. We use 'min' here in case Val was zero before
// shifting. We don't want to return '64' as for an i32 "0".
return std::min(TyBits, Mask.countLeadingZeros());
}
/// AssociativeOpt - Perform an optimization on an associative operator. This
/// function is designed to check a chain of associative operators for a
/// potential to apply a certain optimization. Since the optimization may be