Author: Ricardo Gibert
Date: 18:54:59 01/22/02
Go up one level in this thread
On January 22, 2002 at 21:21:57, Ricardo Gibert wrote:
>On January 22, 2002 at 21:15:17, Ricardo Gibert wrote:
>
>>On January 21, 2002 at 16:16:10, Rafael Andrist wrote:
>>
>>>Well, I just rewrote the following function in assembler to get better speed (no
>>>conditional jumps, less memory access) but the speedup was only minimal. A
>>>possible problem of the asm code is, that the instructions doesn't pair well,
>>>but it should be still considerably faster. Has anyone an idea what the problem
>>>with the code below is? Should I perhaps throw this function out and use a
>>>look-up-table?
>>>
>>>INLINE int Diag045Rot(const int iSqNr)
>>>{
>>>#if defined (Use_Asm)
>>>// 0 <= iSqNr <= 63
>>>__asm
>>>{
>>> mov eax, iFeldNr;
>>> mov ah, al;
>>> and al, 007h; //x (iFeldNr%8) --> al
>>> shr ah, 3; //y (iFeldNr/8) --> ah
>>> sub al, ah; //x-y --> al
>>> mov ah, al; // --> ah
>>> and ah, 080h; //ah &= 0x80 (isolate sign bit)
>>> add ah, 080h; //ah += 0x80 (setting the carry bit)
>>> adc ah, 0; //ah += carry bit
>>> shl ah, 3; //ah <<= 3;
>>> add al, ah; //al += 8*(x-y < 0)
>>> xor ah, ah;
>>>}
>>>#else
>>> int x, y;
>>> x = iSqNr%8;
>>> y = iSqNr/8;
>>> return x-y + 8*(x-y < 0);
>>
>>Isn't this is the same as "return abs(x-y);"? If so, maybe the compiler will do
>>a better job of optimizing with it.
>
>Oops! No it's not, but how about return (x-y+8)%8 ?
Come to think of it, even better is "return (iSqNr-iSqNr/8)%8". I decided to
test this and it works fine. You'll have to benchmark it to see if it produces
faster code.
>
>>
>>Also, if you are using msvc6, it might help the compiler to insert an
>>"__assume((0 <= iSqNr) && (iSqNr <= 63));". I don't use msvc6 myself, so I can't
>>tell you if this really helps here.
>>
>>>#endif
>>>}
>>>
>>>
>>>Thanks in Advance
>>>Rafael B. Andrist
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