https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102877
Bug ID: 102877
Summary: missed optimization: memcpy produces lots more asm
than otherwise
Product: gcc
Version: 11.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jengelh at inai dot de
Target Milestone: ---
Input (C++)
===========
struct GLOBCNT { unsigned char ab[6]; };
unsigned long long gc_to_num(GLOBCNT gc)
{
unsigned long long value;
auto v = reinterpret_cast<unsigned char *>(&value);
v[0] = 0;
v[1] = 0;
#ifdef WITH_MEMCPY
__builtin_memcpy(v + 2, gc.ab, 6);
#else
v[2] = gc.ab[0]; v[3] = gc.ab[1]; v[4] = gc.ab[2];
v[5] = gc.ab[3]; v[6] = gc.ab[4]; v[7] = gc.ab[5];
#endif
if (__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__)
value = __builtin_bswap64(value);
return value;
}
I hope this is UB-free.
Observed behavior
=================
The use of memcpy/__builtin_memcpy produces a function with 28
instructions/0x5c bytes long.
► g++ -O2 -c t3.cpp -Wall -DWITH_MEMCPY -v
Target: x86_64-suse-linux
gcc version 11.2.1 20210816 [revision 056e324ce46a7924b5cf10f61010cf9dd2ca10e9]
(SUSE Linux)
► objdump -Mintel -d t3.o
0000000000000000 <_Z9gc_to_num7GLOBCNT>:
0: 89 f8 mov eax,edi
2: 89 f9 mov ecx,edi
4: 89 fa mov edx,edi
6: 44 0f b6 c7 movzx r8d,dil
a: c1 e9 10 shr ecx,0x10
d: 0f b6 f4 movzx esi,ah
...
5c: c3 ret
Expected behavior
=================
► g++ -O2 -c t3.cpp -Wall -UWITH_MEMCPY
► objdump -Mintel -d t3.o
0000000000000000 <_Z9gc_to_num7GLOBCNT>:
0: 0f b7 c7 movzx eax,di
3: 48 c1 ef 10 shr rdi,0x10
7: 48 c1 e7 20 shl rdi,0x20
b: 48 c1 e0 10 shl rax,0x10
f: 48 09 f8 or rax,rdi
12: 48 0f c8 bswap rax
15: c3 ret
Other notes
===========
In a twist, clang 13.0.0 produces the short version for memcpy (even shorter
than gcc), and produces a long version for non-memcpy case (even longer than
gcc).
► clang++ -O2 -c t3.cpp -Wall -DWITH_MEMCPY; objdump -Mintel -d t3.o
0000000000000000 <_Z9gc_to_num7GLOBCNT>:
0: 48 89 f8 mov rax,rdi
3: 48 c1 e0 10 shl rax,0x10
7: 48 0f c8 bswap rax
a: c3 ret
► clang++ -O2 -c t3.cpp -Wall -UWITH_MEMCPY; objdump -Mintel -d t3.o
0000000000000000 <_Z9gc_to_num7GLOBCNT>:
0: 48 89 f8 mov rax,rdi
3: 48 b9 ff ff ff ff ff movabs rcx,0xffffffffffff
a: ff 00 00
...
6c: c3 ret
