Hi all,

First: THANK YOU all for an excellent forum! :)

Second:

I want to optimize a 64-bit integer division by a 32 bit integer.
The result is allowed to be a 64 bit rounded integer.
If possible show its rounded with flag set.

I've got the 64 bit stored as 2 Dword, LoOperand and HiOperand.

The result as integer 2 Dword as LoResult and HiResult.

Any trick from the gurus??

SgtPepper
Posted on 2002-11-02 08:24:23 by SgtPepper
The first trick is......

Post some code.
Posted on 2002-11-02 11:00:43 by ThoughtCriminal
Assuming your divisor is 32 bit unsigned. Signed and/or 64bit divisor makes it much more complicated.

``````[size=12];In <-  edx:eax = 64 bit dividend
;   <-  ecx = 32bit divisor

push eax
mov  eax, edx
xor  edx, edx
div  ecx
xchg eax, [esp]
div  ecx
pop  edx

;Out -> edx:eax = 64 bit quotient.[/size]``````

Of course this could all be done better with MMX.
Posted on 2002-11-02 11:08:27 by iblis
Of course this could all be done better with MMX

MMX?
Did you mean FPU?
Of course there might be some several algos to do it with MMX, but I haven't seen one at least as fast as FPU.
Posted on 2002-11-02 19:05:19 by The Svin
The only other algo I've seen to do it used MMX. I haven't seen an FPU version although I imagine it would be relatively easy to do.
Posted on 2002-11-02 19:40:00 by iblis
FPU version is too easy.
``````
fild qword ptr [64bit int]
fild dword ptr [32bit int]
fdivp  ; fdiv if MASM
fistp qword ptr [64bit result]
``````

However, if you want 'unsigned' operation, it is not a solution. fild always treats the MSB as the sign bit.

Anyway, it is usually much faster than integer ops + conditional jumps. I once timed it against __divdi3() in libgcc (with 64bit divisor), and FPU version is at least twice as fast as __divdi3(). (Of course, this is not a fair comparison to an optimized integer ops version if there is one.)
Posted on 2002-11-02 21:33:19 by Starless
Thanks!

This is my final implementation.
Reminder is in ecx.

;In <- edx:eax = 64 bit dividend
; <- ecx = 32bit divisor
push eax
mov eax, edx
xor edx, edx
div ecx
xchg eax,
div ecx
xchg edx,
pop ecx
;Out -> edx:eax = 64 bit quotient.
;Out -> ecx = reminder

/SgtPepper
Posted on 2002-11-04 10:43:41 by SgtPepper