ppc_8xx-gcc 2.95.3 Monta Vista does not do ANY loop unrolling
joakim.tjernlund at lumentis.se
Tue Nov 12 21:42:22 EST 2002
Yes, it's not a good ide to enable -funroll-loops for the whole
kernel, but some functions should be unrolled anyway, like the crc32()
function since it won't increase the size very much but will yield
a significant speed increase.
So maybe the right way is to identify some loops and enable unrolling on them.
Where in the PPC FAQ did you read that -Os for 8xx processors?
> 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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