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

[Bob Ippolito]

On Monday, Sep 29, 2003, at 09:13 America/New_York, Christian Tismer 
wrote:

> 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.

Ah, the joys of x86.  It's amazing that so many people still use such a 
braindead ISA :)

-bob