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

Tom Rini trini at kernel.crashing.org
Wed Nov 13 03:25:05 EST 2002

On Tue, Nov 12, 2002 at 05:09:11PM +0100, Joakim Tjernlund wrote:
> > On Tue, Nov 12, 2002 at 10:40:41AM +0100, Joakim Tjernlund wrote:
> >
> > > 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.
> >
> > I'm sort-of supprised that gcc-2.95.x (or gcc-*, for that matter) will
> > unroll some loops with only -O2 since the info page on gcc-3.2 and
> > gcc-2.95 both say that -funroll-loops isn't turned on my any of the -O
> > levels.
> >
> > So I suspect someone decided that small loops can safely be unrolled on
> > i386 at some optimization level, but that same decision (with possibly
> > good reason) was not made for PPC32.  So it's a gcc feature, not a
> > MVista-specific issue.
> Newer gcc(>=3.0) may do the same for PPC32. We only know that newer gcc's(Alan Cox knows more)
> will do it for x86 and 2.95.3 for ppc_8xx won't, so there is a big ? in the middle.

Did Alan say what version of gcc Alan was talking about?

> Now to the trick question(s):
> Where might it be suitable to add -funroll-loops or, better yet, can it be done
> with a pragma or attribute attached to the function in question? It's pretty
> hard to unroll inline functions otherwise (and only the inline function).

Well, to lib/Makefile:
ifeq ($(CONFIG_PPC32),y)
CFLAGS_crc32.o += -funroll-loops

Should work.  And it's not unheard of.

>  Any progress on the i2c-algo-8xx.c and/or 8xx_io/enet.c patches I sent earlier?

As I said privatly, Dan Malek is handling the enet patch, and I'm
looking for time to do the i2c one.  Right now I'm working on making the
kernel easier to tweak (in some ways) for 2.5.

Tom Rini (TR1265)

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