A study in code optimization in C

[Joe Huffman]

In article <1990Jul26.144134.16053 at ux1.cso.uiuc.edu>, mcdonald at aries.scs.uiuc.edu (Doug McDonald) writes:
> In article <133 at smds.UUCP> rh at smds.UUCP (Richard Harter) writes:
> >
> >The macro shown below is an optimized memory to memory copy macro.
> >It is probably faster than memcopy on your machine -- I have checked
> >it on several machines and have always found it to be faster.
>                                  !!!!!!
> >
> Oh My!.
> 
> Time on my computer, in seconds, for 1000 copies of a 20 kilobyte array:
> 
>                           His code                   library memcpy            
> Compiler 1:
>                (chars)     12.6                            2.7
>                (ints)       6.9                            2.7
> 
> 
> Compiler 2:
>                (chars)     23.6                            1.3
>                (ints)       6.9                            1.3
> 
[Stuff deleted... compilers were Microsoft and Microway NDPC, machine was
20 MHz 386]

I just ran it on a 20 MHz 386 running SCO UNIX.  The timing were done with 
5000 copies but then divided by 5 to make the numbers comparable.


			   His code		       library memcpy
SCO supplied MSC 5.1
		(chars)	    14.0		             2.05

Zortech
	     386 code generator not available		     1.80


The reason why the macro is so much slower is (as Doug alluded to and I 
deleted) is that the library routines were written in assembly language and
tweaked for near optimum speed and/or size.  Not something you usually should
do if you are trying to write portable code.  Hence let the compiler writers
do it -- usually they can be trusted to squeeze the most performance out of the
machine.  Especially in a competitive marketplace.
---
Zortech is my major source of income.  Statements about them or their 
competitors cannot be totally without bias.
-- 
joe at proto.com
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