Re: [PATCH] Fix wrong code on aarch64 due to paradoxical subreg

[Jeff Law <law at redhat dot com>]

On 08/01/2016 05:31 PM, Segher Boessenkool wrote:
> Hi,
>
> On Mon, Aug 01, 2016 at 06:52:54PM +0000, Bernd Edlinger wrote:
> > On 08/01/16 19:54, Jeff Law wrote:
> > > Looks like you've probably nailed it.  It'll be interesting see if
> > > there's any fallout (though our RTL optimizer testing is pretty weak, so
> > > even if there were, I doubt we'd catch it).
> >
> > If there is, it will probably a performance regression...
>
> I tested building Linux with and without the patch, on many archs.
> The few that show differences are:
>
>        alpha   6148872   6148776
>         ia64  16946958  16946670
>         s390  12345770  12345850
>         tile  12016086  12016070
>
> (left before, right after; arm and aarch64 did not build, kernel problems).
>
> So all except s390 generate smaller code even.
They're all deep enough in the noise that I wouldn't care either way :-)

> > However I think there are more paradoxical subregs generated all over,
> > but the aarch64 insv code pattern did trigger more hidden bugs than
> > any other port.  It is certainly unfortunate that the major source
> > of paradoxical subreg is in a target-dependent code path :(
>
> It is certainly unfortunate that paradoxical subregs exist at all!  :-)
Yea.  It probably seemed like a good idea 25-30 years ago, but I always 
cringe when I see them being used.  Yea it gives the compiler some more 
freedom, but more often than not I think we'd be better off with real 
extensions.

jeff