Author: Tony Werten
Date: 21:56:19 01/22/02
Go up one level in this thread
On January 22, 2002 at 16:06:19, Ed Schröder wrote:
>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.
Yes, this way they can be paired, but iirc this needs to be followed by a third
instruction (not using eax or ebx ) to (be sure to) prevent a stall.
Tony
>
>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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