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Teach ComputeNumSignBits about signed reminder.

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This optimization a continuation of r231140 that reasoned about signed div.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231433 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Nadav Rotem
2015-03-06 00:23:58 +00:00
parent b0b6966c1d
commit 368d2e9976
2 changed files with 48 additions and 1 deletions
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28
lib/Analysis/ValueTracking.cpp
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@@ -1723,7 +1723,7 @@ unsigned ComputeNumSignBits(Value *V, const DataLayout *TD,
Tmp = TyBits - U->getOperand(0)->getType()->getScalarSizeInBits();
return ComputeNumSignBits(U->getOperand(0), TD, Depth+1, Q) + Tmp;
case Instruction::SDiv:
case Instruction::SDiv: {
const APInt *Denominator;
// sdiv X, C -> adds log(C) sign bits.
if (match(U->getOperand(1), m_APInt(Denominator))) {
@@ -1739,6 +1739,32 @@ unsigned ComputeNumSignBits(Value *V, const DataLayout *TD,
return std::min(TyBits, NumBits + Denominator->logBase2());
}
break;
}
case Instruction::SRem: {
const APInt *Denominator;
// srem X, C -> we know that the result is within 0..C-1 when C is a
// positive constant and the sign bits are at most TypeBits - log2(C).
if (match(U->getOperand(1), m_APInt(Denominator))) {
// Ignore non-positive denominator.
if (!Denominator->isStrictlyPositive())
break;
// Calculate the incoming numerator bits. SRem by a positive constant
// can't lower the number of sign bits.
unsigned NumrBits = ComputeNumSignBits(U->getOperand(0), TD, Depth+1, Q);
// Calculate the leading sign bit constraints by examining the
// denominator. The remainder is in the range 0..C-1, which is
// calculated by the log2(denominator). The sign bits are the bit-width
// minus this value. The result of this subtraction has to be positive.
unsigned ResBits = TyBits - Denominator->logBase2();
return std::max(NumrBits, ResBits);
}
break;
}
case Instruction::AShr: {
Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1, Q);