[PATCH] D118595: [AARCH64][DAGCombine] Add combine for negation of CSEL absolute value pattern.

[Sunho Kim via Phabricator via llvm-commits]

sunho added a comment.

I like your method! It's always reducing negation or keeping the same number of negation unless x and y are both non-negative and it can account for nabs case very well. I was trying to tidy your patch a bit and add some more test cases to address the comments by david-arm and submit it here. However, one thing came to concern me.

When one of x and y are negative maximum of that integer type (e.g. -128 for int8), then folding negation can nullify signed integer overflows that would have happended if not folded. For instance, if you have -csel(-x,-y), then if will be folded to csel(x,y). If x was the negative maximum of that integer type, it would lead to singed integer overflow in the original form (-(-128)), but it will not after folded.

I'm not sure whether I can just consider this case UB and fold it. Could you give me some advice on this?


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