Re: [PATCH] Don't call emit_clobber in lower-subreg.cc's resolve_simple_move.

[Jeff Law <jeffreyalaw@gmail.com>]

On 5/8/23 00:43, Richard Biener wrote:
> On Sat, May 6, 2023 at 8:46 PM Jeff Law via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>>
>>
>> On 5/6/23 06:57, Roger Sayle wrote:
>>>
>>> Following up on posts/reviews by Segher and Uros, there's some question
>>> over why the middle-end's lower subreg pass emits a clobber (of a
>>> multi-word register) into the instruction stream before emitting the
>>> sequence of moves of the word-sized parts.  This clobber interferes
>>> with (LRA) register allocation, preventing the multi-word pseudo to
>>> remain in the same hard registers.  This patch eliminates this
>>> (presumably superfluous) clobber and thereby improves register allocation.
>> Those clobbered used to help dataflow analysis know that a multi word
>> register was fully assigned by a subsequent sequence.  I suspect they
>> haven't been terribly useful in quite a while.
> 
> Likely - maybe they still make a difference for some targets though.
> It might be interesting to see whether combining the clobber with the
> first set or making the set a multi-set with a parallel would be any
> better?
Wrapping them inside a PARALLEL might be better, but probably isn't 
worth the effort.  I think all this stuff dates back to the era where we 
had flow.c to provide the register lifetimes used by local-alloc.  We 
also had things like REG_NO_CONFLICT to indicate that the sub-object 
assignments didn't conflict.  In all it was rather hackish.

Jeff