https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101708
Bug ID: 101708
Summary: Suboptimal codegen when clearing struct fields
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: cafxx+gcc.gnu.org at strayorange dot com
Target Milestone: ---
GCC trunk (as well as 11.2) currently seem to be performing suboptimal codegen
in reset2 and reset3, that in theory should yield the same (or very similar to
the) result of reset1:
struct example {
unsigned a;
unsigned char b;
unsigned c:1;
unsigned d:2;
unsigned char f;
unsigned char g;
unsigned e;
};
void reset1(struct example *e) {
e->b = 0,
e->d = 0,
e->e = 0,
e->f = 0,
e->g = 0;
}
void reset2(struct example *e) {
*e = (struct example) {
.a = e->a,
.c = e->c
};
}
void reset3(struct example *e) {
struct example tmp = {
.a = e->a,
.c = e->c
};
*e = tmp;
}
compiled with -O2 -Wall yields:
reset1:
and QWORD PTR [rdi+4], 63744
ret
reset2:
movzx eax, BYTE PTR [rdi+5]
mov edx, DWORD PTR [rdi]
mov DWORD PTR [rdi+8], 0
mov QWORD PTR [rdi], 0
and eax, 1
mov DWORD PTR [rdi], edx
mov BYTE PTR [rdi+5], al
ret
reset3:
mov QWORD PTR [rsp-8], 0
mov eax, DWORD PTR [rdi]
mov DWORD PTR [rsp-12], eax
movzx eax, BYTE PTR [rdi+5]
and eax, 1
mov BYTE PTR [rsp-7], al
mov rax, QWORD PTR [rsp-12]
mov QWORD PTR [rdi], rax
mov eax, DWORD PTR [rsp-4]
mov DWORD PTR [rdi+8], eax
ret
Godbolt link: https://godbolt.org/z/asr57TWqq
Bug reported at the request of @gnutools:
https://twitter.com/gnutools/status/1421525698515251207
I tentatively selected rtl-optimization as the component, but I'm not familiar
at all with the internals of GCC: feel free to update as needed.
