Re: Patch: improve doloop recognition

[Geoff Keating <geoffk at geoffk dot org>]

> Date: Mon, 16 Sep 2002 17:13:27 -0700
> Cc: Dale Johannesen <dalej@apple.com>, gcc-patches@gcc.gnu.org
> From: Dale Johannesen <dalej@apple.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.

[FYI, you don't need to say why a patch is made in the ChangeLog
entry; if you think some chunk of code needs explanation, the best
thing is to add a comment in the actual code.]

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

Oh, sorry, I missed that part of your original message.

> > - 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?)

Yes, the mnemonics don't differ between powerpc-* targets.

> /* 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" } } */

Thanks!  With these additions, the patch is OK, please commit it.

-- 
- Geoffrey Keating <geoffk@geoffk.org>