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How not to optimize with assembly language

Time to imitate Alex Papadimoulis.

While researching some algorithms on the web, I saw a forum poster "optimize" a routine written in a high-level language into assembly language. I won't post the whole function, but here's part of it:

; s1 = seed & 0xFFFF
    xor   eax, eax
    add   eax, seed
    mov   ebx, eax
    and   eax, 0FFFFh
    mov   eax, ebx
    mov   s1, eax
; s2 = (seed / 65536) & 0xFFFF
    mov   eax, seed
    mov   ebx, 10000h
    xor   edx, edx
    div   ebx
    and   eax, 0FFFFh
    mov   s2, eax

Now, I like assembly language, and the asm code is indeed correct compared to the original high-level code. However, it's a perfect example of a bad use of assembly language to optimize. Looking at it, there are a ton of missed opportunities, such as changing the divide to a shift, removing the and operation that's a no-op, and so on. Well, except that the statement immediately before the block is this:

    mov   seed, 1

Basically, the fragment above computes two constant expressions -- and no, there isn't a branch target in between. And actually, due to a bug, the code wouldn't actually work otherwise. (Hint: Errant mov.) Makes you wonder why the coder didn't just use this:

    mov   s1, 1
mov s2, 0

The rest of the translated function, by the way, was equally faithful and awful. The worst part was that the original source had a big comment block indicating how the algorithm could be easily sped up by an order of magnitude, which was of course ignored.

If you're going to go to assembly language for speed, your job is to optimize based on knowledge that the compiler lacks, such as restricted values in data, and not to act as a really slow compiler with no optimizer. Merely translating a routine verbatim into asm (and in this case, really bad asm) is a total waste of time.


This blog was originally open for comments when this entry was first posted, but was later closed and then removed due to spam and after a migration away from the original blog software. Unfortunately, it would have been a lot of work to reformat the comments to republish them. The author thanks everyone who posted comments and added to the discussion.