https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108724
Bug ID: 108724
Summary: [11 regression] Poor codegen when summing two arrays
without AVX or SSE
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: gbs at canishe dot com
Target Milestone: ---
This program:
void foo(int *a, const int *__restrict b, const int *__restrict c)
{
for (int i = 0; i < 16; i++) {
a[i] = b[i] + c[i];
}
}
When compiled for x86 by GCC 11.1+ with -O3 -mno-avx -mno-sse, produces:
foo:
movq %rdx, %rax
subq $8, %rsp
movl (%rsi), %edx
movq %rsi, %rcx
addl (%rax), %edx
movl 4(%rax), %esi
movq $0, (%rsp)
movl %edx, (%rsp)
movq (%rsp), %rdx
addl 4(%rcx), %esi
movq %rdx, -8(%rsp)
movl %esi, -4(%rsp)
movq -8(%rsp), %rdx
movq %rdx, (%rdi)
movl 8(%rax), %edx
addl 8(%rcx), %edx
movq $0, -16(%rsp)
movl %edx, -16(%rsp)
movq -16(%rsp), %rdx
movl 12(%rcx), %esi
addl 12(%rax), %esi
movq %rdx, -24(%rsp)
movl %esi, -20(%rsp)
movq -24(%rsp), %rdx
movq %rdx, 8(%rdi)
[snip more of the same]
movl 48(%rcx), %edx
movq $0, -96(%rsp)
addl 48(%rax), %edx
movl %edx, -96(%rsp)
movq -96(%rsp), %rdx
movl 52(%rcx), %esi
addl 52(%rax), %esi
movq %rdx, -104(%rsp)
movl %esi, -100(%rsp)
movq -104(%rsp), %rdx
movq %rdx, 48(%rdi)
movl 56(%rcx), %edx
movq $0, -112(%rsp)
addl 56(%rax), %edx
movl %edx, -112(%rsp)
movq -112(%rsp), %rdx
movl 60(%rcx), %ecx
addl 60(%rax), %ecx
movq %rdx, -120(%rsp)
movl %ecx, -116(%rsp)
movq -120(%rsp), %rdx
movq %rdx, 56(%rdi)
addq $8, %rsp
ret
(Godbolt link: https://godbolt.org/z/qq9dbP8ed)
This is bizarre - it's storing intermediate results on the stack, instead of
keeping them in registers or writing them directly to *a, which is bound to be
slow. (GCC 10.4, and Clang, produce more or less what I would expect, using
only the provided arrays and a register.) I haven't done any benchmarking
myself, but Jonathan Wakely's results (on list:
https://gcc.gnu.org/pipermail/gcc-help/2023-February/142181.html) seem to bear
this out.
>From a bisect, this behavior seems to have been introduced by commit
33c0f246f799b7403171e97f31276a8feddd05c9 (tree-optimization/97626 - handle SCCs
properly in SLP stmt analysis) from Oct 2020, and persists into GCC trunk.
