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Date: Fri, 28 Jul 2023 15:24:15 +0200
From: Jan Hubicka <hubicka@ucw.cz>
To: Richard Biener <richard.guenther@gmail.com>
Cc: gcc-patches@gcc.gnu.org
Subject: Re: Loop-split improvements, part 3
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> On Fri, Jul 28, 2023 at 2:57 PM Jan Hubicka via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > Hi,
> > This patch extends tree-ssa-loop-split to understand test of the form
> >  if (i==0)
> > and
> >  if (i!=0)
> > which triggers only during the first iteration.  Naturally we should
> > also be able to trigger last iteration or split into 3 cases if
> > the test indeed can fire in the middle of the loop.
> >
> > Last iteration is bit trickier pattern matching so I want to do it
> > incrementally, but I implemented easy case using value range that handled
> > loops with constant iterations.
> >
> > The testcase gets misupdated profile, I will also fix that incrementally.
> >
> > Bootstrapped/regtested x86_64-linux, OK?
> 
> OK, though I think we can handle more loops by simply conservatively peeling
> one iteration at the beginning/end with such conditions and would be not subject
> to all other limitations the loop splitting pass has?

I was also thinking of extending loop peeling heuristics by this.
Loop-ch already can handle case where the static test exits loop, so we
could get this if I figure out how to merge the analysis.

To handle last iteration (like in hmmer), we would need to extend loop
peeling to support that.

Even with that tree-ssa-loop-split has chance to be more informed and
have better cost model.  Let me see how many restrictions can be dropped
it.

Honza