[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:
> > > 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?


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  https://reviews.llvm.org/D150100/new/

https://reviews.llvm.org/D150100