Re: 1.3.10 memcmp() bug

[Tim Prince <tprince at computer dot org>]

On Tuesday 23 April 2002 22:04, Sami Korhonen wrote:
>  I wasnt sure wheter I should post about this on gcc bug report list or
> here. Anyways, it seems that using -O2 flag with gcc causes huge slowdown
> in memcmp(). However i dont see performance drop under linux, so I suppose
> it is cygwin issue.
>
> $ gcc memtest.c -O2 -o memtest ; ./memtest.exe
> Amount of memory to scan (mbytes)? 100
> Memory block size (default 1024)? 1024
> Allocating memory
> Testing memory - read (1 byte at time)
> Complete: 889.73MB/sec
> Testing memory - read (4 bytes at time)
> Complete: 3313.07MB/sec
> Freeing memory
>
> $ gcc memtest.c -o memtest ; ./memtest.exe
> Amount of memory to scan (mbytes)? 100
> Memory block size (default 1024)? 1024
> Allocating memory
> Testing memory - read (1 byte at time)
> Complete: 2517.94MB/sec
> Testing memory - read (4 bytes at time)
> Complete: 2933.50MB/sec
> Freeing memory
>
>
> '1 byte at time' is using memcmp() to compare two blocks.
You leave so many relevant considerations unspecified, that anything I say 
must be a stab in the dark.  I assume you have a standard cygwin 
installation, where binutils is built to honor only 4-byte alignments, while 
recent linux configurations provide for 16-byte alignments.  The significance 
of that is different on various CPU families, with code alignment being quite 
important on certain CPU's, and data alignment on others.  Do we assume that 
you are running on a 486, since you have not told gcc otherwise?  You may 
have fallen accidentally into good alignment in one case and bad in the 
other.  You might or might not be using similar versions of gcc in cygwin and 
linux.  If you would provide a test case, and mention some hardware 
parameters, some of the mystery could be eliminated; for example, we could 
find out whether memcmp() is code generated by gcc or from a library.  cygwin 
is not generally considered an important target for performance optimization, 
as you can see from the alignment considerations and the differences in the 
libraries.
-- 
Tim Prince

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