[Bug middle-end/88097] New: Missing optimization of endian conversion

["bugzilla at poradnik-webmastera dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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: bugzilla@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]