Re: [PATCH 0/7] ira/lra: Support subreg coalesce

[Jeff Law <jeffreyalaw@gmail.com>]

On 11/10/23 03:39, Richard Sandiford wrote:
> Lehua Ding <lehua.ding@rivai.ai> writes:
>> On 2023/11/10 18:16, Richard Sandiford wrote:
>>> Lehua Ding <lehua.ding@rivai.ai> writes:
>>>> Hi Richard,
>>>>
>>>> On 2023/11/8 17:40, Richard Sandiford wrote:
>>>>> Tracking subreg liveness will sometimes expose dead code that
>>>>> wasn't obvious without it.  PR89606 has an example of this.
>>>>> There the dead code was introduced by init-regs, and there's a
>>>>> debate about (a) whether init-regs should still be run and (b) if it
>>>>> should still be run, whether it should use subreg liveness tracking too.
>>>>>
>>>>> But I think such dead code is possible even without init-regs.
>>>>> So for the purpose of this series, I think the init-regs behaviour
>>>>> in that PR creates a helpful example.
>>>>
>>>> Yes, I think the init-regs should be enhanced to reduce unnecessary
>>>> initialization. My previous internal patchs did this in a separate
>>>> patch. Maybe I should split the live_subreg problem out of the second
>>>> patch and not couple it with these patches. That way it can be reviewed
>>>> separately.
>>>
>>> But my point was that this kind of dead code is possible even without
>>> init-regs.  So I think we should have something that removes the dead
>>> code.  And we can try it on that PR (without changing init-regs).
>>
>> Got it, so we should add a fast remove dead code job after init-regs pass.
> 
> I'm just not sure how fast it would be, given that it needs the subreg
> liveness info.  Could it be done during RA itself, during one of the existing
> instruction walks?  E.g. if IRA sees a dead instruction, it could remove it
> rather than recording conflict information for it.
> 
Yea, it's a real concern.  I haven't done the analysis yet, but I have a 
  sense that Joern's ext-dce work which Jivan and I are working on 
(which does sub-object liveness tracking) is having a compile-time 
impact as well.

Jeff