> > Jaap-Jan, > > 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.
yes (e.g. in fs/jffs2/Makefile) > > Where in the PPC FAQ did you read that -Os for 8xx processors? http://penguinppc.org/embedded/howto/x1273.html#AEN1346 > > Joakim > > > Joakim, > > > > 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. > > > > Jaap-Jan > > > > > > > > > > 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? > > > > > > > > > > > > > > > > > > ** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/
