Re: [ros-dev] [ros-diffs] [tkreuzer] 42353: asm version of DIB_32BPP_ColorFill: - Add frame pointer - Get rid of algin_draw, 32bpp surfaces must be DWORD aligned - Optimize the loop - Add comments

[Timo Kreuzer]

Michael Steil wrote:
>  
>  I wonder, has either of you, Alex or Timo actually *benchmarked* the  
>  code on some sort of native i386 CPU before you argue whether it  
>  should be a stosb or a stosd? If not, writing assembly would be a  
>  clear case of "premature optimization".
>     
I did. on Athlon X2 64, I called the function a bunch ot times, with a
100x100 rect, measuring time with rdtsc  the results were quite random,
but roughly
asm: ~580
gcc 4.2 -march=k8 -fexpensive-optimizations -O3: ~1800
WDK: /GL /Oi /Ot /O2 : ~2600
MSVC 2008 express: /GL /Oi /Ot /O2 ~1800

using a 50x50 rect shifts the advantage slightly in direction of the asm
implementations.

I added volatile to the pointer to prevent the loop to be optimized away.
using memset was a bit slower than a normal loop.
This is what msvc produced with the above settings

_DIB_32BPP_ColorFill:
    push   ebx
    mov   ebx, [eax+8]
    sub    ebx, [eax]
    test    ebx, ebx
    jg      short label1
    xor    al, al
    pop   ebx
    retn

label1:
    mov  ecx, [eax+4]
    push esi
    mov esi, [eax+0Ch]
    sub  esi, ecx
    test  esi, esi
    jg     short label2
    pop  esi
    xor   al, al
    pop  ebx
    retn

label2:
    mov  eax, [edx+4]
    imul  ecx, eax
    add  ecx, [edx]
    cdq
    and  edx, 3
    add  eax, edx
    sar   eax, 2
    add  eax, eax
    push edi
    mov edi, ecx
    add  eax, eax
    jmp  short label3

align 10h
label3:
    mov  ecx, edi
    mov  edx, ebx

label4:
    mov  dword ptr [ecx], 3039h
    add   ecx, 4
    sub   edx, 1
    jnz    short  label4

    dec   esi
    add   edi, eax
    test   esi, esi
    jg     short  label3

    pop  edi
    pop  esi
    mov al, 1
    pop ebx
    retn



I though myself I did something wrong. For me no compiler was able to
generate code as fast as the asm code.
I don't know how Alex managed to get better optimizations, maybe he
knows a secret ninja /Oxxx switch, or maybe express and wdk version both
suck at optimizing or maybe I'm just too supid... ;-)


>   See above: If all you want to optimize is the loop,
> then have C code
>  with asm("rep movsd") in it, or fix the static inline memcpy() to be  
>  more efficient (if it isn't efficient in the first place).
>     
I tried __stosd() which actually resulted in a faster function. with
~610 gcc was aslmost as fast as the asm implementation, msvc actually
won with 590. But that was using not pure portable code. It's the best
solution, it seems, although it will probably still be slower unless we
set our optimization to max.

Btw, I already thought about rewriting our dib code some time ago. Using
inline functions instead of a code generator. The idea is to make it
fully portable, optimizable though inline asm functions where useful and
easier to maintain then the current stuff. It's on my list...

Timo