[BusyBox] [PATCH] Re: Sigh... this time i attached the file (bunzip-4.c)

Rob Landley rob at landley.net
Wed Oct 22 10:23:16 UTC 2003


I'm going through Manuel's bunzip-4 optimization drop, and whipping up a patch 
against CVS.  I haven't really changed Manuel's code much, but I cleaned up 
and fluffed out the comments tremendously. :)

I did a couple cleanups that shouldn't affect functionality, but should make 
it easier to read.  (Famous last words.)  The one I'm not quite sure about is 
how the assembly works out turning an if(--x) into if(x--)...  that might 
wind up being one extra instruction and slightly slower, I'm not sure.  (If 
so, it's easily revertible.  It's just that when trying to detect a run 
length of 3, counting down from 3 makes more sense than counting down from 4.  
But it's not worth taking a speed or size hit...)

About the #warnings:

#warning should probably be "ssize_t read_bunzip(bunzip_data *,void *,size_t)"

No, it shouldn't.  This is not an offset into a file, this is the size of a 
memory buffer.  A platform that can index a block of memory larger than its 
integer size is _badly_designed_.  We shouldn't need funky typedefs designed 
to disguise the fact that ancient HP mainframes, which did all math as binary 
coded decimal, also had to sacrifice a live chicken to access memory.  So I 
say "int" is correct.

Feel free to disagree with this, but I'd like to hear the rationale. :)

#warning check for EOF and trailing trash

I put that into the sample main, but really it belongs in the busybox 
bunzip/bzcat command code that calls it.  (Not really a decompress engine 
issue.)  A to-do item. :)

Here's the patch against CVS.  It compiles to 6520 bytes, and decompresses the 
kernel tarball on my laptop in a little under 65 seconds vs a little under 76 
seconds for the original. :)

Rob

P.S.  I also #ifdefed out the simple main so this can be used as a library 
without having te edit the file.  Compile ala "gcc -Os -D TESTING bunzip.c" 
to enable standalone testing.
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