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

[Krzysztof Parzyszek via llvm-dev]

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.

-Krzysztof

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