[PATCH] D70079: [SelectionDAG] Combine U{ADD,SUB}O diamonds into {ADD,SUB}CARRY

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D70079#1745241 <https://reviews.llvm.org/D70079#1745241>, @davezarzycki wrote:

> 1. Why is UADDO/USUBO an intrinsic but ADDCARRY/SUBCARRY is not? This is unfortunate given that clang and perhaps other languages have addcarry/subcarry intrinsics.


I don't know the full history, but my guess is that the *.with.overflow intrinsics were assumed to be good enough to hold the patterns together through IR optimization, and the backend could relatively easily map those to DAG nodes. If that's not correct, then we could make a case for adding to, extending, or replacing the current set of intrinsics.

That does lead to questions I was already wondering about:

1. Can we do this transform in IR (instcombine)?
2. There are tests with intrinsics and tests with "raw" IR - are we missing IR transforms that should canonicalize to 1 form or the other?


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