Re: [RFC] RISC-V: elide sign extend when expanding cmp_and_jump

[Jeff Law <jeffreyalaw@gmail.com>]

On 10/25/23 10:25, Vineet Gupta wrote:
> Hey Robin,
> 
> On 10/25/23 00:12, Robin Dapp wrote:
>> Hi Vineet,
>>
>> I was thinking of two things while skimming the code:
>>
>>   - Couldn't we do this in the expanders directly?  Or is the
>>     subreg-promoted info gone until we reach that?
> 
> Following is the call stack involved:
> 
>    expand_gimple_cond
>      do_compare_and_jump
>         emit_cmp_and_jump_insns
>             gen_cbranchqi4
>                 riscv_expand_conditional_branch
>                     riscv_emit_int_compare
>                        riscv_extend_comparands
> 
> 
> Last function is what introduces the extraneous sign extends, w/o taking 
> subreg-promoted into consideration and what my patch attempts to address.
> 
>>   - Should some common-code part be more suited to handle that?
>>     We already elide redundant sign-zero extensions for other
>>     reasons.  Maybe we could add subreg promoted handling there?
> 
> Not in the context of this specific issue.
Robin's point (IIUC) is that if we put this logic into a zero/sign 
extend expander, then it'll get used for *any* attempt to zero/sign 
extend that goes through the target expander.

It doesn't work for your case because we use gen_rtx_{ZERO,SIGN}_EXTEND 
directly.   But if those were adjusted to use the expander, then Robin's 
idea would be applicable to this case too.

Jeff