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 <ro...@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
