[pypy-dev] Re: OT: abs(x) with 4 assembly insns

[Christian Tismer]

Michael Hudson wrote:

> Christian Tismer <tismer at tismer.com> writes:
...

>>Hmm. 4 insns.
> 
> 
> Or you can cheat, and remember that you saw something like this in
> some psyco header file somewhere...

No, I couldn't. Armin was referring to it, and I wanted
to find out by myself. He told me about the exhaustive
search, which I found funny.

>   /* as you can check the following takes the absolute value of (say) EAX:      \
>        ADD EAX, EAX                                                             \
>        SBB EAX, sourcecopy                                                      \
>        SBB EDX, EDX                                                             \
>        XOR EAX, EDX                                                             \
>     (note: although the idea is not original, the above code might be           \
>      original as it has been found by an exhaustive search on *all*             \
>      short codes :-)                                                            \
>   */                                                                            \

Actually, the above is not correct since it needs
5 insns. Armin didn't count the sourcecopy.

My solution really has four insns, but it is
cheating since it *needs* the register eax.

> I don't know x86 assembler, so I have no idea which of the two code
> sequences are actually likely to run quicker on a modern processor.

I guess they are almost the same speed.

ciao - chris

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