ppc_8xx-gcc 2.95.3 Monta Vista does not do ANY loop unrolling

Jaap-Jan Boor jjboor at lucent.com
Tue Nov 12 20:56:17 EST 2002


Enable loop unrolling by default for 8xx processors is not
a good idea because of the limited instructions cache size.

I think that's also what is recommended in the ppc faq: enable
size optimization (-Os) for 8xx processors gives better performance.

For a 750 or so, it would be good to enable loop unrolling.


> Hi
> I optimized the crc32() in JFFS2(fs/jffs2/crc.h) by manually unrolling
> the crc32 loop. This gave me a speed increase of 22% in mounting JFFS2 FS
> Later Alan Cox pointed out that my changes makes x86 run slower and it turns
> out that on x86 and a fairly new gcc will automatically unroll loops 'where appropriate'
> Removed my hand coded unrolling and added -funroll-loops to the JFFS2 Makefile,
> I got similar results as my hand coded unrolling (a little better).
> I therefore conclude that ppc_8xx-gcc 2.95.3 from Monta Vista does not do ANY unrolling
> unless you specify -funroll-loops. Doing this for the whole kernel is NOT a good idea,
> it will run slower due to big increase of size.
> Now I wonder:
> Is this a gcc 2.95.3, PPC or Monta Vista limitation?
> Which compiler will do unrolling 'where appropriate' for 8xx PPC and
> Where can I get a precompiled version?
> The short term solution is to specify -funroll-loops for individual files/directories.
> Obviously JFFS2 should be included, but what else?

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