> 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. 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). > > -- > Tom Rini (TR1265) > http://gate.crashing.org/~trini/ > [ disclaimer: I work for MVista. ] Yeah, I know :-) Jocke PS. Any progress on the i2c-algo-8xx.c and/or 8xx_io/enet.c patches I sent earlier? ** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/
