[PATCH] D150100: [KnownBits] Improve implementation of `KnownBits::abs`

[Noah Goldstein via Phabricator via llvm-commits]

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