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

[Jeff Law]

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.


> 
> A concrete example of the observed improvement is PR target/43644.
> For the test case:
> __int128 foo(__int128 x, __int128 y) { return x+y; }
> 
> on x86_64-pc-linux-gnu, gcc -O2 currently generates:
> 
> foo:    movq    %rsi, %rax
>          movq    %rdi, %r8
>          movq    %rax, %rdi
>          movq    %rdx, %rax
>          movq    %rcx, %rdx
>          addq    %r8, %rax
>          adcq    %rdi, %rdx
>          ret
> 
> with this patch, we now generate the much improved:
> 
> foo:    movq    %rdx, %rax
>          movq    %rcx, %rdx
>          addq    %rdi, %rax
>          adcq    %rsi, %rdx
>          ret
> 
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32} with
> no new failures.  OK for mainline?
> 
> 
> 2023-05-06  Roger Sayle  <roger@nextmovesoftware.com>
> 
> gcc/ChangeLog
>          PR target/43644
>          * lower-subreg.cc (resolve_simple_move): Don't emit a clobber
>          immediately before moving a multi-word register by parts.
> 
> gcc/testsuite/ChangeLog
>          PR target/43644
>          * gcc.target/i386/pr43644.c: New test case.
OK for the trunk.  I won't be at all surprised to see fallout in the 
various target tests.  We can fault in fixes as needed.  More 
importantly I think we want as much soak time for this change as we can 
in case there are unexpected consequences.

jeff