[PATCH] D154533: [DAG] Improve carry reconstruction in combineCarryDiamond.

[Paweł Bylica via Phabricator via llvm-commits]

chfast added a comment.

In D154533#4480255 <https://reviews.llvm.org/D154533#4480255>, @barannikov88 wrote:

>> In this case, we replace a complex web of instructions by just one add. Reconstructing a carry is worth it. The test cases that were changed bellow are good exemple of that, the carry is passed as an argument to the functions, so it is not possible to know if these are "real" carries or not, and it is nevertheless worth doing the transform.
>
> I was thinking about cases like feeding uaddo by a setcc, but that does not seem to pay off. E.g.:
>
>   declare { i64, i1 } @llvm.usub.with.overflow.i64(i64, i64)
>   
>   define { i64, i1 } @subcarry_fake_carry(i64 %a, i64 %b, i32 %x, i32 %y) {
>       %t1 = call { i64, i1 } @llvm.usub.with.overflow.i64(i64 %a, i64 %b)
>       %partial = extractvalue { i64, i1 } %t1, 0
>       %k1 = extractvalue { i64, i1 } %t1, 1
>   
>       %carryin = icmp eq i32 %x, %y
>       %zcarryin = zext i1 %carryin to i64
>       %s = call { i64, i1 } @llvm.usub.with.overflow.i64(i64 %partial, i64 %zcarryin)
>       %k2 = extractvalue { i64, i1 } %s, 1
>   
>       %carryout = or i1 %k1, %k2
>   
>       %ret = insertvalue { i64, i1 } %s, i1 %carryout, 1
>       ret { i64, i1 } %ret
>   }

It looks this test cases could be added to the repo.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D154533/new/

https://reviews.llvm.org/D154533