Re: Testing memory performance

[Sad Clouds <cryintothebluesky%gmail.com@localhost>]

Looked at disassembly of memcpy() and NetBSD version looks way more
complicated. I don't know anything about x86 assembly, but maybe the
clue is somewhere here:

NetBSD (gcc 5.5.0):

Dump of assembler code for function memcpy:
=> 0x00007f7e5940b980 <+0>:     mov    %rdx,%rcx
   0x00007f7e5940b983 <+3>:     mov    %rdi,%rax
   0x00007f7e5940b986 <+6>:     mov    %rdi,%r11
   0x00007f7e5940b989 <+9>:     shr    $0x3,%rcx
   0x00007f7e5940b98d <+13>:    je     0x7f7e5940b9cc <memcpy+76>
   0x00007f7e5940b98f <+15>:    lea    -0x8(%rdi,%rdx,1),%r9
   0x00007f7e5940b994 <+20>:    mov    -0x8(%rsi,%rdx,1),%r10
   0x00007f7e5940b999 <+25>:    and    $0x7,%r11
   0x00007f7e5940b99d <+29>:    jne    0x7f7e5940b9a6 <memcpy+38>
   0x00007f7e5940b99f <+31>:    rep movsq %ds:(%rsi),%es:(%rdi)
   0x00007f7e5940b9a2 <+34>:    mov    %r10,(%r9)
   0x00007f7e5940b9a5 <+37>:    retq   
   0x00007f7e5940b9a6 <+38>:    lea    -0x9(%r11,%rdx,1),%rcx
   0x00007f7e5940b9ab <+43>:    neg    %r11
   0x00007f7e5940b9ae <+46>:    mov    (%rsi),%rdx
   0x00007f7e5940b9b1 <+49>:    mov    %rdi,%r8
   0x00007f7e5940b9b4 <+52>:    lea    0x8(%rsi,%r11,1),%rsi
   0x00007f7e5940b9b9 <+57>:    lea    0x8(%rdi,%r11,1),%rdi
   0x00007f7e5940b9be <+62>:    shr    $0x3,%rcx
   0x00007f7e5940b9c2 <+66>:    rep movsq %ds:(%rsi),%es:(%rdi)
   0x00007f7e5940b9c5 <+69>:    mov    %rdx,(%r8)
   0x00007f7e5940b9c8 <+72>:    mov    %r10,(%r9)
   0x00007f7e5940b9cb <+75>:    retq   
   0x00007f7e5940b9cc <+76>:    mov    %rdx,%rcx
   0x00007f7e5940b9cf <+79>:    rep movsb %ds:(%rsi),%es:(%rdi)
   0x00007f7e5940b9d1 <+81>:    retq   
End of assembler dump.



Linux (gcc 6.3.0):

Dump of assembler code for function memcpy:
=> 0x00007ffff78a0e90 <+0>:   mov    %rdi,%rax
   0x00007ffff78a0e93 <+3>:   cmp    $0x10,%rdx
   0x00007ffff78a0e97 <+7>:   jb     0x7ffff78a0f77
   0x00007ffff78a0e9d <+13>:  cmp    $0x20,%rdx
   0x00007ffff78a0ea1 <+17>:  ja     0x7ffff78a0fc6
   0x00007ffff78a0ea7 <+23>:  movups (%rsi),%xmm0
   0x00007ffff78a0eaa <+26>:  movups -0x10(%rsi,%rdx,1),%xmm1
   0x00007ffff78a0eaf <+31>:  movups %xmm0,(%rdi)
   0x00007ffff78a0eb2 <+34>:  movups %xmm1,-0x10(%rdi,%rdx,1)
   0x00007ffff78a0eb7 <+39>:  retq   
End of assembler dump.