[PATCH] D157755: [InstSimplify] Use knownbits for simplifying `(icmp ugt/ule (or X, Y), X)`; PR64610

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n added inline comments.


================
Comment at: llvm/lib/Analysis/InstructionSimplify.cpp:3135-3140
+        // Get NOT of knownbits by swapping ones/zeros
+        std::swap(RHSKnown.One, RHSKnown.Zero);
+        KnownBits UniqueBits = RHSKnown & YKnown;
+        // We know there is a unique bit so equality conditions cannot be
+        // false.
+        if (UniqueBits.isNonZero())
----------------
goldstein.w.n wrote:
> nikic wrote:
> > goldstein.w.n wrote:
> > > nikic wrote:
> > > > 
> > > Is that right. It's really 'have a common bit not set', it's okay if there are some though
> > Oh, I see, I misunderstood what you're going for here. In that case I don't think your comment in the patch description that this does not add new capability is probably not right. I don't think we'd fold the case where there are common bits elsewhere.
> > 
> > I personally think writing the condition here as `if ((YKnown.One & RHSKnown.Zero) != 0)` would be cleaner.
> I added some tests that have some overlapping bits to make it more clear.
We handle this in other passes. Its new to instsimplify/instcombine.


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https://reviews.llvm.org/D157755