https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88097
Bug ID: 88097
Summary: Missing optimization of endian conversion
Product: gcc
Version: 8.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: bugzi...@poradnik-webmastera.com
Target Milestone: ---
I have found some old code network code which looked like this:
[code]
#include <stdint.h>
#include <arpa/inet.h>
struct Test
{
uint16_t Word1;
uint16_t Word2;
};
uint32_t test(Test* ip)
{
return ((ntohs(ip->Word1) << 16) | ntohs(ip->Word2));
}
[/code]
gcc 8.2 compiles it in following way (with -O3):
[asm]
test(Test*):
movzx eax, WORD PTR [rdi]
movzx edx, WORD PTR [rdi+2]
rorw $8, ax
rorw $8, dx
sal eax, 16
movzx edx, dx
or eax, edx
ret
[/asm]
clang 7.0.0 recognizes that both 16-bit fields are next to each other, so
32-bit byte swap can be used:
[asm]
test(Test*): # @test(Test*)
mov eax, dword ptr [rdi]
bswap eax
ret
[/asm]
And this is with -mmovbe added:
[asm]
test(Test*): # @test(Test*)
movbe eax, dword ptr [rdi]
ret
[/asm]
