low level optimization

[Anders Andersson]

In <1991Apr11.150858.4242 at odin.diku.dk> juul at diku.dk (Anders Juul Munch) writes:

[ stuff deleted ]

>	I once read saw an implementation of quicksort in a 
>magazine, which contained code like:

>	array[index++] = something;

>They stated it was written that way for efficiency.

That's crazy. If you want efficiency in QSort you should use
pointer arithmetic like:

	*(pointer++) = something;  

Then it makes a difference...
OK, OK, not true an all machines but *absolutly* true on a
MC600x0. (Yes, I've tried it!)

>I tried compiling that code, together with the rewrite


>	array[index] = something;
>	index++;

>And then I looked at the code that my "real-world" compiler 
>(Turbo C++ 1.0) generated for these two code fragments  -
>	Identical!

I can't imagine a compiler where they wouldn't be. (Unless there is 
some strange addressing modes on the machine).

>The morale of the story is, don't use increment/decrement-operators
>within other expressions - it makes the code less readable
>and it's probably doesn't give you any performance gain anyway. 

The moral of my comment is, if you want speed you'd better not stop
half way - go for pointer arithmetic and make your code truly
unreadable. (But run a profiler first... You might not want it
everywhere...)

[ name and disclamer deleted ]

-Anders Andersson  (andand at cia.docs.uu.se)