memcpy vs slice copy

[Sergey Gromov]

Sun, 15 Mar 2009 13:17:50 +0000 (UTC), Moritz Warning wrote:

> On Sat, 14 Mar 2009 23:50:58 -0400, bearophile wrote:
> 
>> While doing some string processing I've seen some unusual timings
>> compared to the C code, so I have written this to see the situation
>> better. When USE_MEMCPY is false this little benchmark runs about 3+
>> times slower:
> 
> I did a little benchmark:
> 
> ldc -release -O5
> true: 0.51
> false: 0.63
> 
> dmd -release -O
> true: 4.47
> false: 3.58
> 
> I don't see a very big difference between slice copying and memcpy (but 
> between compilers).
> 
> Btw.: http://www.digitalmars.com/pnews/read.php?
> server=news.digitalmars.com&group=digitalmars.D.bugs&artnum=14933

The original benchmark swapped insanely on my 1GB laptop so I've cut the
number of iterations in half, to 50_000_000.  Compiled with -O -release
-inline.  Results:

slice: 2.31
memcpy: 	0.73

That's 3 times difference.  Disassembly:

slice:
L31:            mov     ECX,EDX
                mov     EAX,6
                lea     ESI,010h[ESP]
                mov     ECX,EAX
                mov     EDI,EDX
                rep
                movsb
                add     EDX,6
                add     EBX,6
                cmp     EBX,011E1A300h
                jb      L31

memcpy:
L35:            push    6
                lea     ECX,014h[ESP]
                push    ECX
                push    EBX
                call    near ptr _memcpy
                add     EBX,6
                add     ESI,6
                add     ESP,0Ch
                cmp     ESI,011E1A300h
                jb      L35

Seems like rep movsb is /way/ sub-optimal for copying data.