[PATCH] D133919: [InstCombine] Fold ((x?1:4)&(y?1:4))==0 to x^y

[Sanjay Patel via Phabricator via llvm-commits]

spatel added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp:6478
+    ) && match(Op1, m_Zero())) {
+      if ((*C & *D) == 0)
+        return BinaryOperator::CreateXor(A, B);
----------------
marcauberer wrote:
> @spatel do you know how to check for shared bits between C and D the best. The current solution does not work.
> Besides that, by feeding C and D with m_APInt to the match function, the vector test case does not work properly. We might need to fall back to m_Value, do we?
Before making enhancements to generalize the fold, we have to make sure the transform is sound. 

The Alive2 proof says that the select values must be non-zero, but that is not checked. We need more tests to verify that the matching is complete.

Once you're confident that the transform is correct for the basic pattern, look at how to make it handle other patterns:
1. Why are there one-use restrictions? Hint: if we're only creating 1 instruction, then we can never end up with more instructions than we started with.
2. Almost any fold that checks for "X == 0" alone can be generalized to handle "X != 0".
3. We don't need to capture all of the constants if we're looking for equivalent values; use m_Specific or m_Deferred for that. See m_ImmConstant() for a more general match than m_APInt().


Repository:
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  https://reviews.llvm.org/D133919/new/

https://reviews.llvm.org/D133919