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[ValueTracking] simplify logic in ComputeNumSignBits (NFCI) 

This was noted in http://reviews.llvm.org/D21610 . The previous code
predated the use of APInt ( http://reviews.llvm.org/rL47654 ), so it
had to account for the fixed width of uint64_t.

Now that we're using the variable width APInt, we can remove some
complexity.

llvm-svn: 273584
This commit is contained in:
Sanjay Patel 2016-06-23 17:41:59 +00:00
parent ef3358d579
commit e053621071
1 changed files with 9 additions and 16 deletions
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25
llvm/lib/Analysis/ValueTracking.cpp
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@ -2153,25 +2153,18 @@ unsigned ComputeNumSignBits(Value *V, unsigned Depth, const Query &Q) {
return VecSignBits;
APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
APInt Mask;
computeKnownBits(V, KnownZero, KnownOne, Depth, Q);
if (KnownZero.isNegative()) { // sign bit is 0
Mask = KnownZero;
} else if (KnownOne.isNegative()) { // sign bit is 1;
Mask = KnownOne;
} else {
// Nothing known.
return FirstAnswer;
}
// If we know that the sign bit is either zero or one, determine the number of
// identical bits in the top of the input value.
if (KnownZero.isNegative())
return std::max(FirstAnswer, KnownZero.countLeadingOnes());
// 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::max(FirstAnswer, std::min(TyBits, Mask.countLeadingZeros()));
if (KnownOne.isNegative())
return std::max(FirstAnswer, KnownOne.countLeadingOnes());
// computeKnownBits gave us no extra information about the top bits.
return FirstAnswer;
}
/// This function computes the integer multiple of Base that equals V.