[PATCH] D67356: [InstCombine] Simplify @llvm.usub.with.overflow+non-zero check (PR43251)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri marked 2 inline comments as done.
lebedev.ri added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp:2249
+  if (Value *X = foldUnsignedUnderflowCheck(RHS, LHS, /*IsAnd=*/false, Builder))
+    return X;
+
----------------
vsk wrote:
> lebedev.ri wrote:
> > vsk wrote:
> > > Does the 'or of icmps' case arise anywhere?
> > Define arise. We can trivially get one from another via De Morgan laws:
> > `(a && b) ? c : d` <--> `!(a && b) ? d : c` <--> `(!a || !b) ? d : c`,
> > It's *really* bad idea to intentionally not handle such nearby patterns.
> > 
> As in, do unsigned underflow checks tend to contain this specific or-of-icmp pattern, or more generally whether programs in the wild tend to.
> 
> If this isn't the case, then it seems like there's a compile-time cost for optimizing the pattern without any compensating benefit.
I'm not sure what the question is.


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https://reviews.llvm.org/D67356