[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:
> 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.


================
Comment at: llvm/lib/Support/KnownBits.cpp:426
+    // NB: KnownAbs is not always a subset of KnownNeg so we want to get input
+    // from both.
+    KnownAbs.One |= KnownNeg.One;
----------------
nikic wrote:
> Under which circumstance is KnownAbs better? Seems fishy.
> Under which circumstance is KnownAbs better? Seems fishy.

The output sign at the very least. But when testing (even when I was getting the right sign from the `computeForAddSub` `assert(KnownAbs.Zero.isSubsetOf(KnownNeg.Zero) && KnownAbs.isSubsetOf(KnownNeg.One))` was failing on the exhaustive tests.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D150100/new/

https://reviews.llvm.org/D150100