linux-kernel - Re: [PATCH 01/16] math128: Introduce various 128bit primitives

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Date:	Thu, 25 Oct 2012 15:38:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Juri Lelli <juri.lelli@...il.com>, tglx@...utronix.de,
	mingo@...hat.com, rostedt@...dmis.org, oleg@...hat.com,
	fweisbec@...il.com, darren@...art.com, johan.eker@...csson.com,
	p.faure@...tech.ch, linux-kernel@...r.kernel.org,
	claudio@...dence.eu.com, michael@...rulasolutions.com,
	fchecconi@...il.com, tommaso.cucinotta@...up.it,
	nicola.manica@...i.unitn.it, luca.abeni@...tn.it,
	dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...csson.com, liming.wang@...driver.com,
	jkacur@...hat.com, harald.gustafsson@...csson.com,
	vincent.guittot@...aro.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 01/16] math128: Introduce various 128bit primitives

On Thu, 2012-10-25 at 07:21 +0200, Geert Uytterhoeven wrote:
> On Wed, Oct 24, 2012 at 11:53 PM, Juri Lelli <juri.lelli@...il.com> wrote:
> > +#ifdef __SIZEOF_INT128__ /* gcc-4.6+ */
> > +       unsigned __int128 val;
> > +#endif
> 
> So the definition of val depends on (gcc) __SIZEOF_INT128__...
> 
> > +/*
> > + * Make usage of __int128 dependent on arch code so they can
> > + * judge if gcc is doing the right thing for them and can over-ride
> > + * any funnies.
> > + */
> > +
> > +#ifndef ARCH_HAS_INT128
> 
> ... but all generic users depend on (Kconfig) ARCH_HAS_INT128?

Ah, you're saying both should depend on the same thing. I fear there's a
chicken-egg problem in the code as it is now, the asm/math128.h thing
needs the data structure but is also the one setting ARCH_HAS_INT128.

So its not Kconfig.

> How can Kconfig know if gcc supports this?

It cannot, its per the asm/math128.h header to opt-in on using it. This
so archs can make sure gcc doesn't generate broken code or relies on
libgcc for its __int128 implementation.

Now, if we do as Linus suggests and push the data structure definition
into a separate header we could possibly avoid this.
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