[PATCH] combine: Allow combining two insns to two insns

[Richard Biener]

On Wed, Jul 25, 2018 at 11:50 AM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Wed, Jul 25, 2018 at 10:28:30AM +0200, Richard Biener wrote:
> > On Tue, Jul 24, 2018 at 7:18 PM Segher Boessenkool
> > <segher@kernel.crashing.org> wrote:
> > >
> > > This patch allows combine to combine two insns into two.  This helps
> > > in many cases, by reducing instruction path length, and also allowing
> > > further combinations to happen.  PR85160 is a typical example of code
> > > that it can improve.
> > >
> > > This patch does not allow such combinations if either of the original
> > > instructions was a simple move instruction.  In those cases combining
> > > the two instructions increases register pressure without improving the
> > > code.  With this move test register pressure does no longer increase
> > > noticably as far as I can tell.
> > >
> > > (At first I also didn't allow either of the resulting insns to be a
> > > move instruction.  But that is actually a very good thing to have, as
> > > should have been obvious).
> > >
> > > Tested for many months; tested on about 30 targets.
> > >
> > > I'll commit this later this week if there are no objections.
> >
> > Sounds good - but, _any_ testcase?  Please! ;)
>
> I only have target-specific ones.

Works for me.

>  Most *simple* ones will already be
> optimised by current code (via 3->2 combination).  But I've now got one
> that trunk does not optimise, and it can be confirmed with looking at
> the resulting machine code even (no need to look at the combine dump,
> which is a very good thing).  And it is a proper thing to test even: it
> tests that some source is compiled to properly optimised machine code.
>
> Any other kind of testcase is worse than useless, of course.
>
> Testing it results in working code isn't very feasible or useful either.
>
>
> Segher