[PATCH] D150100: [KnownBits] Improve implementation of `KnownBits::abs`
Noah Goldstein via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Sat May 13 12:52:49 PDT 2023
goldstein.w.n added inline comments.
================
Comment at: llvm/lib/Support/KnownBits.cpp:423
+ KnownBits KnownNeg = computeForAddSub(
+ /*Add*/ false, /*NSW*/ false, Zero, *this);
+
----------------
nikic wrote:
> goldstein.w.n wrote:
> > nikic wrote:
> > > goldstein.w.n wrote:
> > > > nikic wrote:
> > > > > You can pass IntMinIsPoison to the NSW argument.
> > > > > You can pass IntMinIsPoison to the NSW argument.
> > > >
> > > > Can we? The `0-Negative_X` is positive unless `Negative_X == INT_MIN` so if `IntMinIsPoison` we can't set `NSW`. Also would do slightly better and just do `KnownAbs.isNonNegative()` because it uses the `IntMinIsPoison` condition (and others) to try and set output sign.
> > > >
> > > > But either way, since `KnownAbs` already has logic for setting output sign and we combine our result from negation with `KnownAbs` its kind of a non-issue.
> > > I don't understand. The only value for which `0-X` wraps is INT_MIN, which is exactly what the IntMinIsPoison flag controls. The entire purpose of that flag is to allow an `nsw` assumption on the negation.
> > Oh I mistakenly though something like `sub nsw i8 0, 129` would be `poison` but guess not.
> > Either way though, we 'join' with the original `abs` value to get the signbit so `nsw` is unneeded no?
> But can we drop that join if NSW is passed? Just making this `if (isNegative(X)) return neg(X);` would make this logic a lot cleaner.
> But can we drop that join if NSW is passed? Just making this `if (isNegative(X)) return neg(X);` would make this logic a lot cleaner.
I doesn't seem to cover all cases:
```
KnownBits Zero(getBitWidth());
Zero.setAllZero();
KnownBits KnownNeg = computeForAddSub(
/*Add*/ false, KnownAbs.isNegative(), Zero, *this);
// Preserve signbit from `KnownAbs` as it has additional logic for figuring
// it out that we don't want to duplicate here.
KnownBits KnownAbsNoSign = KnownAbs;
KnownBits KnownNegNoSign = KnownNeg;
KnownAbsNoSign.One.clearSignBit();
KnownAbsNoSign.Zero.clearSignBit();
KnownNegNoSign.One.clearSignBit();
KnownNegNoSign.Zero.clearSignBit();
assert(KnownAbsNoSign.One.isSubsetOf(KnownNegNoSign.One));
assert(KnownAbsNoSign.Zero.isSubsetOf(KnownNegNoSign.Zero));
assert(KnownAbs.One.isSubsetOf(KnownNeg.One));
assert(KnownAbs.Zero.isSubsetOf(KnownNeg.Zero));
KnownNeg.One.clearSignBit();
KnownNeg.Zero.clearSignBit();
KnownAbs.One |= KnownNeg.One;
KnownAbs.Zero |= KnownNeg.Zero;
```
On the exhaustive tests fails:
```
KnownBits.cpp:440: llvm::KnownBits llvm::KnownBits::abs(bool) const: Assertion `KnownAbs.Zero.isSubsetOf(KnownNeg.Zero)' failed.
```
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D150100/new/
https://reviews.llvm.org/D150100
More information about the llvm-commits
mailing list