Author: Ed Schröder
Date: 13:06:19 01/22/02
Go up one level in this thread
On January 21, 2002 at 18:38:44, Tim Foden 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;
>>}
>
>maybe you could try something like this, which is shorter, and looks to be quite
>pairable. On the down side it uses 3 registers.
>
>Disclaimer: It compiles, but I haven't tested it to see if it produces the
>correct answer :)
>
>__asm
>{
> mov eax, square
> mov ebx, eax
I think that:
mov eax, square
mov ebx, square
is faster, because it can be paired but it might be processor dependent.
Ed
> and eax, 0x07 // eax = x
> shr ebx, 3 // ebx = y
> mov ecx, 0
> sub eax, ebx // eax = x - y
> rol ecx, 4 // ecx = 8 * (x < y)
> add eax, ecx // eax = x - y + 8 * (x < y)
>}
>
>
>>#else
>> int x, y;
>> x = iSqNr%8;
>> y = iSqNr/8;
>> return x-y + 8*(x-y < 0);
>>#endif
>
>It may help if you get your compiler to output the assembly code, or if you can
>look at it in a debugger, to see what it does when compiling the above code.
>
>Cheers, Tim.
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