I was thinking about int approximations of square roots, and thought about the current guess and check methods and how they would fair speed wise with a 64bit integer.


IntSqrtSSE:
  mov eax,
  cvtsi2sd xmm0,eax
  sqrtsd xmm0,xmm0
  cvtsd2si eax,xmm0
  ret 4

Is very fast, BUT you can't CVT 64bit integers into double FP values. Using the FPU would just make the function slow. Also because it's SIGNED integer the highest value you can do is 7FFFFFFFh.

Here's a square root for all unsigned 32bit values.

IntSqrt:
  push esi
  push edi
  mov esi, ; IN number to find sqr root of
  xor ecx,ecx        ;zero out
  xor eax,eax        ;
  mov edi,10h      ;loop counter , use 32 for 64bit int
  mov edx,esi
.LP:
  shr edx,1eh
  shl eax,1
  lea ecx,
  lea edx, ;2x + 1 part of the binomial expand of (x+1)^2
  shl esi,2
  cmp edx,ecx
  ja .SKIP
  sub ecx,edx
  inc eax
.SKIP:
  mov edx,esi
;unroll        15% speed boost
  shr edx,1eh
  shl eax,1
  lea ecx,
  lea edx,
  shl esi,2
  cmp edx,ecx
  ja .SKIP1
  sub ecx,edx
  inc eax
.SKIP1:
  mov edx,esi
  sub edi,2
  jnz .LP
  pop edi
  pop esi
  ret 4

Surprisingly it's only 2x slower than the SSE version (after the 15% speed increase from the unroll).

Also in 64bit world the function can be easily modified to get the int sqrt of 64bit integers.

I usually like to comment my code, but unless your familiar with the ambiguous math involved it really wouldn't help much. It relies on the bond and bit patterns between square root, and remaining numbers.

With the extra registers on a 64bit system this function has a lot of room for optimization.

BEFORE SOMEONE ASKS - Why would you need an integer square root? I don't know! :P
Posted on 2005-08-12 18:54:43 by r22
Why would you need an integer square root?

i have 2 386-based pc without fpu :)
Posted on 2005-08-31 07:05:17 by Shoo
Replace shl eax,1 with add eax,eax.
This should improve pairability and speed !
Posted on 2005-09-19 06:54:25 by MCoder
i found this by tran , written a loong time ago...

ftp://hornet.madtracker.org/mirrors/hornet/code/demosrc/demos/as.zip

quote: (end of file)




; Hey you... Yeah, you reading this message... Ignore this.
;
; Square root
; In:
;  EAX - number to take root of
; Out:
;  EAX - root
;
sqrtbasetbl    db      0,1,4,9,16,25,36,49,64,81,100,121,144,169,196,225
_sqrt:
        pushad
        mov ebp,eax
        bsr ebx,eax
        jnz short sqrtf0
        xor ebx,ebx
sqrtf0:
        shr ebx,3
        lea eax,
        mov cl,32
        sub cl,al
        rol ebp,cl
        mov eax,ebp
        movzx eax,al
        mov edi,offset sqrtbasetbl
        mov ecx,10h
sqrtl0:
        scasb
        je short sqrtl0d
        jb short sqrtl0d2
        loop sqrtl0
        inc edi
sqrtl0d2:
        dec edi
        inc cl
sqrtl0d:
        movzx edx,byte ptr
        dec cl
        xor cl,0fh
        mov edi,ecx
        mov ecx,ebx
        jecxz short sqrtdone
        sub eax,edx
sqrtml:
        shld eax,ebp,8
        rol ebp,8
        mov ebx,edi
        shl ebx,5
        xor edx,edx
        mov esi,eax
        div ebx
        rol edi,4
        add edi,eax
        add ebx,eax
sqrtf2:
        imul eax,ebx
        mov edx,eax
        mov eax,esi
        sub eax,edx
        jc short sqrtf1
        loop sqrtml
sqrtdone:
        mov ,edi
        popad
        ret
sqrtf1:
        dec ebx
        dec edi
        movzx eax,bl
        and al,1fh
        jmp sqrtf2

Posted on 2005-09-22 12:30:14 by HeLLoWorld
Posted a realy shorter implementation there:

http://www.asmcommunity.net/board/index.php?topic=25521.msg186474#msg186474
Posted on 2006-11-09 22:47:01 by llaurrentt