https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119704
Bug ID: 119704
Summary: x86: partially disobeyed strategy rep-based request
for inlined memset
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: mjguzik at gmail dot com
Target Milestone: ---
13.3.0 runs into it, but I also tested on godbolt which claims to have 15.0.1:
gcc
(Compiler-Explorer-Build-gcc-ca4e6e6317ae0ceada8c46ef5db5ece165a6d1c4-binutils-2.42)
15.0.1 20250409 (experimental)
... and got the same result.
I have not verified memcpy, I suspect it might suffer the same problem.
src:
void zero(char *buf)
{
__builtin_memset(buf, 0, SIZE);
}
compiled like so:
cc -O2 -DSIZE=48 -mno-sse
-mmemset-strategy=rep_byte:256:noalign,libcall:-1:noalign -c zero.c
Given rep_byte I expect rep movsb to be emitted. It does happen for some sizes,
but I'm also seeing regular stores or rep movsl.
For sizes 40 bytes and below this still emits regular stores, *not* the
rep-prefixed op, for example:
0000000000000000 <zero>:
0: f3 0f 1e fa endbr64
4: 48 c7 07 00 00 00 00 movq $0x0,(%rdi)
b: 48 c7 47 08 00 00 00 movq $0x0,0x8(%rdi)
12: 00
13: 48 c7 47 10 00 00 00 movq $0x0,0x10(%rdi)
1a: 00
1b: 48 c7 47 18 00 00 00 movq $0x0,0x18(%rdi)
22: 00
23: 48 c7 47 20 00 00 00 movq $0x0,0x20(%rdi)
2a: 00
2b: c3 ret
48 bytes is movsl:
0000000000000000 <zero>:
0: f3 0f 1e fa endbr64
4: b9 0c 00 00 00 mov $0xc,%ecx
9: 31 c0 xor %eax,%eax
b: f3 ab rep stos %eax,%es:(%rdi)
d: c3 ret
64 bytes is movsl:
0000000000000000 <zero>:
0: f3 0f 1e fa endbr64
4: b9 10 00 00 00 mov $0x10,%ecx
9: 31 c0 xor %eax,%eax
b: f3 ab rep stos %eax,%es:(%rdi)
d: c3 ret
65 bytes is movsb:
0000000000000000 <zero>:
0: f3 0f 1e fa endbr64
4: b9 41 00 00 00 mov $0x41,%ecx
9: 31 c0 xor %eax,%eax
b: f3 aa rep stos %al,%es:(%rdi)
d: c3 ret
Given the rep_byte strategy I expect the entire thing to movsb.
