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Re: "better" version of Iscale functions [message #15159 is a reply to message #15140]

Sun, 06 April 2008 04:47

mirek
Messages: 14265
Registered: November 2005

Ultimate Member

mdelfede wrote on Wed, 02 April 2008 10:48

Back to Iscale, I don't know about modern processors that does have an hardware ftp and doesn't have 32x32->64 bit mul and 64/32 ->32 bit div core instructions... but I can be wrong.
Yet, I don't remember if intel ones works just with unsigned or signed or both integers...
BTW, I noticed that my iscale needs to work with 32 bit result; if not it'll use full 64 bit math for the multiply (in iscalefloor and iscaleceil) which can be slow.
I guess that using 32x32 multiply and 64/32 division, GCC translates it directly in DIV and MUL, but I've not checked yet.

Well, this is what MSC does seem to do to divide these numbers:

0041A920  push edi 
0041A921  push esi 
0041A922  push ebx 
0041A923  xor edi,edi 
0041A925  mov eax,[esp+0x14] 
0041A929  or eax,eax 
0041A92B  jnl 0x41a941 
0041A92D  inc edi 
0041A92E  mov edx,[esp+0x10] 
0041A932  neg eax 
0041A934  neg edx 
0041A936  sbb eax,byte +0x0 
0041A939  mov [esp+0x14],eax 
0041A93D  mov [esp+0x10],edx 
0041A941  mov eax,[esp+0x1c] 
0041A945  or eax,eax 
0041A947  jnl 0x41a95d 
0041A949  inc edi 
0041A94A  mov edx,[esp+0x18] 
0041A94E  neg eax 
0041A950  neg edx 
0041A952  sbb eax,byte +0x0 
0041A955  mov [esp+0x1c],eax 
0041A959  mov [esp+0x18],edx 
0041A95D  or eax,eax 
0041A95F  jnz 0x41a979 
0041A961  mov ecx,[esp+0x18] 
0041A965  mov eax,[esp+0x14] 
0041A969  xor edx,edx 
0041A96B  div ecx 
0041A96D  mov ebx,eax 
0041A96F  mov eax,[esp+0x10] 
0041A973  div ecx 
0041A975  mov edx,ebx 
0041A977  jmp short 0x41a9ba 
0041A979  mov ebx,eax 
0041A97B  mov ecx,[esp+0x18] 
0041A97F  mov edx,[esp+0x14] 
0041A983  mov eax,[esp+0x10] 
0041A987  shr ebx,1 
0041A989  rcr ecx,1 
0041A98B  shr edx,1 
0041A98D  rcr eax,1 
0041A98F  or ebx,ebx 
0041A991  jnz 0x41a987 
0041A993  div ecx 
0041A995  mov esi,eax 
0041A997  mul dword [esp+0x1c] 
0041A99B  mov ecx,eax 
0041A99D  mov eax,[esp+0x18] 
0041A9A1  mul esi 
0041A9A3  add edx,ecx 
0041A9A5  jc 0x41a9b5 
0041A9A7  cmp edx,[esp+0x14] 
0041A9AB  ja 0x41a9b5 
0041A9AD  jc 0x41a9b6 
0041A9AF  cmp eax,[esp+0x10] 
0041A9B3  jna 0x41a9b6 
0041A9B5  dec esi 
0041A9B6  xor edx,edx 
0041A9B8  mov eax,esi 
0041A9BA  dec edi 
0041A9BB  jnz 0x41a9c4 
0041A9BD  neg edx 
0041A9BF  neg eax 
0041A9C1  sbb edx,byte +0x0 
0041A9C4  pop ebx 
0041A9C5  pop esi 
0041A9C6  pop edi 
0041A9C7  ret 0x10

I would say we should better learn GCC assembly syntax Smile

Mirek

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