https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103733
Bug ID: 103733
Summary: Missed optimization: defaulted op== for trivially
comparable types worse than memcmp
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jengelh at inai dot de
Target Milestone: ---
== Input ==
struct S {
char b[16];
#if __cplusplus >= 202000L
bool operator==(const S &) const = default;
#else
inline bool operator==(const S &o) const { return
__builtin_memcpy(const_cast<char *>(b), o.b, sizeof(b)); }
#endif
};
bool fff(const S &a, const S &b) { return a == b; }
== Observed ==
gcc version 12.0.0 20211214 973f6aedeb6489a9fdc7aeceaed0514348ee1e1a x86_64
$ ./xgcc -B. -c x.cpp -O2 -std=c++20 && objdump -d x.o
0000000000000000 <_Z3fffRK1SS1_>:
0: 31 c0 xor %eax,%eax
2: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
8: 0f b6 14 06 movzbl (%rsi,%rax,1),%edx
c: 38 14 07 cmp %dl,(%rdi,%rax,1)
f: 75 17 jne 28 <_Z3fffRK1SS1_+0x28>
11: 48 83 c0 01 add $0x1,%rax
15: 48 83 f8 10 cmp $0x10,%rax
19: 75 ed jne 8 <_Z3fffRK1SS1_+0x8>
1b: b8 01 00 00 00 mov $0x1,%eax
20: c3 ret
21: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
28: 31 c0 xor %eax,%eax
2a: c3 ret
== Expected ==
Expected the same result as when -std=c++17/memcmp is used.
0000000000000000 <_Z3fffRK1SS1_>:
0: f3 0f 6f 06 movdqu (%rsi),%xmm0
4: b8 01 00 00 00 mov $0x1,%eax
9: 0f 11 07 movups %xmm0,(%rdi)
c: c3 ret
