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Date:   Thu, 25 Jul 2019 08:22:32 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Michael Ellerman <mpe@ellerman.id.au>
Cc:     Nathan Chancellor <natechancellor@gmail.com>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com
Subject: Re: [PATCH v2] powerpc: slightly improve cache helpers
Message-ID: <20190725132232.GQ20882@gate.crashing.org>
References: <a5864549-40c3-badd-8c41-d5b7bf3c4f3c@c-s.fr> <20190709064952.GA40851@archlinux-threadripper> <20190719032456.GA14108@archlinux-threadripper> <20190719152303.GA20882@gate.crashing.org> <20190719160455.GA12420@archlinux-threadripper> <20190721075846.GA97701@archlinux-threadripper> <20190721180150.GN20882@gate.crashing.org> <87imru74ul.fsf@concordia.ellerman.id.au> <20190722151801.GC20882@gate.crashing.org> <875znt7izy.fsf@concordia.ellerman.id.au>
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On Tue, Jul 23, 2019 at 09:21:53AM +1000, Michael Ellerman wrote:
> Segher Boessenkool <segher@kernel.crashing.org> writes:
> >> can use both RA and RB to compute the address, rather than us forcing RA
> >> to 0.
> >> 
> >> But at least with my compiler here (GCC 8 vintage) I don't actually see
> >> GCC ever using both GPRs even with the patch. Or at least, there's no
> >> difference before/after the patch as far as I can see.
> >
> > The benefit is small, certainly.
> 
> Zero is small, but I guess some things are smaller? :P

Heh.  0 out of 12 is small.

It actually is quite easy to do trigger the macros to generate two-reg
dcb* instructions; but all the places where that is especially useful,
in loops for example, already use hand-written assembler code (and yes,
using two-reg forms).

You probably will not want to write those routines as plain C ever
given how important those are for performance (memset, clear-a-page),
so the dcb* macros won't ever be very hot, oh well.

> >> So my inclination is to revert the original patch. We can try again in a
> >> few years :D
> >> 
> >> Thoughts?
> >
> > I think you should give the clang people time to figure out what is
> > going on.
> 
> Yeah fair enough, will wait and see what their diagnosis is.

Thanks!


Segher