Re: Patch: improve doloop recognition

[Dale Johannesen <dalej at apple dot com>]

On Monday, September 16, 2002, at 04:18 PM, Geoff Keating wrote:
> Dale Johannesen <dalej@apple.com> writes:
> > This addresses the optimization problem I reported here
> > http://gcc.gnu.org/ml/gcc-patches/2002-09/msg00614.html
> > in which CFG's forwarding forces a loop into a form which
> > can't be recognized as a doloop.  Obviously the test could
> > be more extensive, but AFAIK this is sufficient to catch
> > all the cases that can be recognized as doloops.  Bootstrapped
> > and tested on Darwin, and I've checked that real branches
> > out of loops (break, etc.) do get forwarded on a later pass.
> The patch itself is good, but:
>
> - please write a ChangeLog entry
I do put them in, I just usually forget to post them in the patches.  
Here:
2002-09-16  Dale Johannesen  <dalej@apple.com>
        * cfgcleanup.c (try_forward_edges):  Do not forward a
        branch to just after a loop exit before loop optimization;
        this interfered with doloop detection.

> - say what platform you tested it on, see
>   http://gcc.gnu.org/cvswrite.html#testing
Darwin isn't enough?  OK, Darwin on PowerPC.

> - please write a testcase.  If you can't think of a better way, write
>   a gcc.dg testcase that is powerpc-specific and uses scan-assembler
>   to look for a decrement-and-branch instruction.
I can't think of a better way.  This patch is not really target-specific
so I wasn't crazy about this approach, but I suppose it is better than
nothing.  (The test doesn't specify Darwin; I believe the relevant
assembler mnemonics are the same on other OSs?)

/* Make sure both loops are recognized as doloops.
   If so, "bdnz" will be generated on ppc; if not,
   you will get "blt". */

/* { dg-do compile { target powerpc-*-* } } */
/* { dg-options "-O2" } */
void foo (int count, char* pca, char* pcb) {
    int i;
    if (count > 10)
        for (i = 0; i < count; ++i)
            pcb += i;
    else
        for (i = 0; i < count; ++i)
            pca += i;
    *pca = *pcb;
}
/* { dg-final { scan-assembler "bdnz" } } */
/* { dg-final { scan-assembler-not "blt" } } */