[llvm-dev] Semantics of Generic MIR overflowing operations?

[Cassie Jones via llvm-dev]

Hi,

I was looking at the overflowing operations because I was trying to deal with the legalization and selection of wide adds, and I wasn’t sure on the finer points of some of the semantics.
I’m specifically looking at the addition/subtraction operations:

    G_UADDO, G_SADDO, G_USUBO, G_SSUBO
    G_UADDE, G_SADDE, G_USUBE, G_SSUBE

Is it correct that the S* operations set the carry-out value when they have a signed overflow, and the U* operations when they have an unsigned overflow?
Particularly, is there any difference between them if the carry-out isn’t used by anything?
I would expect that a decomposed wide add would have a sequence like the following:

    G_UADDO
    G_UADDE
    ...

Would that just need to end with a G_SADDE when implementing an operation like Rust's i128::checked_add()?

Finally, Is there a reason that G_SADDE and G_SSUBE aren’t legal in the AArch64 backend? (Particularly while G_SADDO and G_SSUBO are.)
They seem like they have the potential to produce bad code if used in the middle of a sequence like that, but I don’t know if that’s a reason to make them illegal so I was wondering if there was something else I was missing.

Thanks,
— Cassie

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