[llvm-dev] Deprecating ADDC/ADDE/SUBC/SUBE

[Friedman, Eli via llvm-dev]

On 5/30/2018 11:28 AM, Krzysztof Parzyszek wrote:
> On 5/30/2018 1:16 PM, Friedman, Eli wrote:
>> On 5/30/2018 10:29 AM, Krzysztof Parzyszek via llvm-dev wrote:
>>> For targets where ADDCARRY and SUBCARRY are legal, would it make 
>>> sense to expand ADDC/UADDO/ADDE/etc. into ADDCARRY (and same for sub)?
>>
>> SelectionDAG will never generate ADDC/ADDE on targets where they 
>> aren't legal.  Targets which custom-lower ADDCARRY generally also 
>> custom-lower UADDO; not sure what sort of expansion you're thinking of.
>
> If ADDC/ADDE cannot ever appear, then that's ok. The expansion of a 
> long ADD will generate UADDO for the first addition. UADDO is 
> unnecessary since ADDCARRY already includes UADDO's functionality, so 
> if target sets UADDO to Expand, it could be replaced with ADDCARRY. 
> Targets can handle both manually, but why should they have to?
>
> I was actually working on using ADDCARRY on Hexagon and I find the 
> UADDO generation a little annnoying.
>

If the expansion of UADDO would be useful, patch welcome, I guess. (It 
isn't useful on architectures like ARM; we have to special-case UADDO 
anyway to generate "adds" instead of "adcs".)

-Eli

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