[PATCH] D113783: [InstCombine] Fold (A^B)|~A-->~(A&B)

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a reviewer: rampitec.
spatel added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp:2676
   // A | (~A ^ B) -> A | ~B
+  // ~A | (A ^ B) -> ~(A & B)
   // (A & B) | (A ^ B)
----------------
This surprised me, but this seems to work. Note that 'not' is also an 'xor' operation.
But canonicalization rules will always put the 'not' operand on the right (operand 1). So the swap above here then guarantees that the 'not' is operand 0.


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Comment at: llvm/test/Transforms/InstCombine/or-xor.ll:62
 
+; ~X | (X ^ Y) --> ~(X & Y)
+
----------------
We need another test where X and Y are commuted.
We also need at least 3 tests with extra uses: (1) extra use of the 'not', (2) extra use of the regular 'xor', (3) extra uses of both of those.
The transform should be fine as long there is only one use for either of the operands because we are eliminating an instruction in this sequence.
When creating more tests, it's good to vary the data types a bit so we have better coverage. For example, have one test use vector types and another use a weird type like 'i65'.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D113783/new/

https://reviews.llvm.org/D113783