Differential D101840
[InstCombine] ctlz(x) -> 0 if x is known negative number Authored by xbolva00 on May 4 2021, 8:32 AM.
Details ----------------------------------------
define i32 @src(i32 %x, i1 %zerodef) {
%0:
%assume = icmp slt i32 %x, 0
assume i1 %assume
%c = ctlz i32 %x, %zerodef
ret i32 %c
}
=>
define i32 @tgt(i32 %x, i1 %zerodef) {
%0:
%assume = icmp slt i32 %x, 0
assume i1 %assume
ret i32 0
}
Transformation seems to be correct!Proof: https://alive2.llvm.org/ce/z/rU6o1d
Diff Detail Event Timeline
Comment Actions What's the advantage of doing this in InstSimplfy when InstCombine can already do it via computeKnownBits? Is ctlz common enough for this to matter for the places that use InstSimplify directly? Comment Actions No strong opinion, but everybody should agree. I really dont want to go with “instsimplify -> instcombine -> instsimplify” patches. Comment Actions isKnownNegative is just a wrapper around computeKnownBits. So if we are to do this in InstSimplify we might as well take the whole code that was already flagged in InstCombineCalls that can already do this and more. KnownBits Known = IC.computeKnownBits(Op0, 0, &II);
// Create a mask for bits above (ctlz) or below (cttz) the first known one.
unsigned PossibleZeros = IsTZ ? Known.countMaxTrailingZeros()
: Known.countMaxLeadingZeros();
unsigned DefiniteZeros = IsTZ ? Known.countMinTrailingZeros()
: Known.countMinLeadingZeros();
// If all bits above (ctlz) or below (cttz) the first known one are known
// zero, this value is constant.
// FIXME: This should be in InstSimplify because we're replacing an
// instruction with a constant.
if (PossibleZeros == DefiniteZeros) {
auto *C = ConstantInt::get(Op0->getType(), DefiniteZeros);
return IC.replaceInstUsesWith(II, C);
}Comment Actions
Not sure, if we want to then compute known bits in instcombine just for: if (!Known.One.isNullValue() ||
isKnownNonZero(Op0, IC.getDataLayout(), 0, &IC.getAssumptionCache(), &II,
&IC.getDominatorTree())) {
if (!match(II.getArgOperand(1), m_One()))
return IC.replaceOperand(II, 1, IC.Builder.getTrue());
}For now I think I will move this code to instcombine.
In this case, please feel free to enhance computeKnownBits/computeSignBits. Comment Actions Your test already passes before your change. If the Known bits is negative, PossibleZeros and DefiniteZeros will both be 0 which triggers the PossibleZeros == DefiniteZeros case. | ||||||||||||
Enhance isKnownNegative to know that ashr of negative constant is still negative? Then we could remove the match here.