[Bug target/62642] New: x86 rdtsc is moved through barrier

[M8R-ynb11d at mailinator dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62642

            Bug ID: 62642
           Summary: x86 rdtsc is moved through barrier
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: M8R-ynb11d at mailinator dot com

given:

unsigned long long measure(void (*func)(void))
{
    unsigned long long before = __builtin_ia32_rdtsc();
    asm volatile("" ::: "memory");
    func();
    asm volatile("" ::: "memory");
    unsigned long long after = __builtin_ia32_rdtsc();
    return after - before;
}

On x86 linux with -O2, this results in the obviously useless:

measure:
    push    edi
    push    esi
    push    ebx
    call    [DWORD PTR [esp+16]]
    rdtsc
    mov    esi, eax
    mov    edi, edx
    rdtsc
    pop    ebx
    sub    esi, eax
    sbb    edi, edx
    mov    eax, esi
    mov    edx, edi
    pop    esi
    pop    edi
    ret

I can reproduce the problem on 32 bit x86 on Linux and MinGW with 4.8.2 and
4.9.1. (I guess 4.8.0 also exhibits the problem but I don't have that available
for testing, so I set the version to 4.8.2 in the report.)

4.7.x and 4.6.x work correctly, as does 64 bit on both platforms.

If this is not the proper sanctioned way to write this function, I'm all ears
to a better way.  I've tried also adding calls to __builtin_ia32_mfence() which
as I understand it should not be necessary, and it gets even more comical:

        ...
    mfence
    call    [DWORD PTR [esp+16]]
    mfence
    rdtsc
    mov    esi, eax
    mov    edi, edx
    rdtsc
        ...