https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121653
Bug ID: 121653
Summary: Bloaty initialization of two-register structs under
-Os on x86-64
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: pskocik at gmail dot com
Target Milestone: ---
When returning what should be simple two-register structs on x86-64,
gcc outputs really bloaty code under -Os when those registers are split into
multiple fields such as
int a,b,c,d;
OR
int a,b; long C;
OR
int a,b,c;
Here's some examples (which all should've been mov $-1, %eax; xor %edx, %edx;)
and what they compile to:
struct r4{ int a,b;long C; }; struct r4 r4(void){ return (struct r4){-1}; }
struct R4{ int a,b,c,d; }; struct R4 R4(void){ return (struct R4){-1}; }
struct r3{ int a,b,c; }; struct r3 r3(void){ return (struct r3){-1}; }
0000000000000000 <r4>:
0: be ff ff ff ff mov $0xffffffff,%esi
5: 31 c9 xor %ecx,%ecx
7: 31 c0 xor %eax,%eax
9: 48 c1 e6 20 shl $0x20,%rsi
d: 48 21 f1 and %rsi,%rcx
10: be ff ff ff ff mov $0xffffffff,%esi
15: 89 c9 mov %ecx,%ecx
17: 48 89 ca mov %rcx,%rdx
1a: 48 89 c1 mov %rax,%rcx
1d: 48 09 f1 or %rsi,%rcx
20: 48 89 c8 mov %rcx,%rax
23: c3 ret
sz$(r4)=36
0000000000000024 <R4>:
24: be ff ff ff ff mov $0xffffffff,%esi
29: 31 c9 xor %ecx,%ecx
2b: 31 c0 xor %eax,%eax
2d: 48 c1 e6 20 shl $0x20,%rsi
31: 48 21 f1 and %rsi,%rcx
34: be ff ff ff ff mov $0xffffffff,%esi
39: 89 c9 mov %ecx,%ecx
3b: 48 89 ca mov %rcx,%rdx
3e: 48 89 c1 mov %rax,%rcx
41: 48 09 f1 or %rsi,%rcx
44: 48 89 c8 mov %rcx,%rax
47: c3 ret
sz$(R4)=36
0000000000000048 <r3>:
48: 31 c0 xor %eax,%eax
4a: c7 44 24 ec ff ff ff movl $0xffffffff,-0x14(%rsp)
51: ff
52: 48 89 44 24 f0 mov %rax,-0x10(%rsp)
57: 8b 4c 24 f4 mov -0xc(%rsp),%ecx
5b: 48 8b 44 24 ec mov -0x14(%rsp),%rax
60: 48 89 ca mov %rcx,%rdx
63: c3 ret
sz$(r3)=28
-O2 fixes it except for r3 (that one's the least bothersome as padding a
register-returnable struct is no problem). `clang -Os` outputs `mov $-1, %eax;
xor %edx, %edx;` for each as expected.
https://godbolt.org/z/voxb5r9Gv
