linux-kernel - Re: [PATCH v2] byteswap: try to avoid __builtin_constant

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Message-ID: <7892604.CjmUVh55Wd@wuerfel>
Date:	Wed, 22 Jun 2016 14:25:11 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Tomas Winkler <tomasw@...il.com>
Cc:	"Levy, Amir (Jer)" <amir.jer.levy@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"mm-commits@...r.kernel.org" <mm-commits@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"jpoimboe@...hat.com" <jpoimboe@...hat.com>,
	Martin Jambor <mjambor@...e.cz>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Thomas Graf <tgraf@...g.ch>,
	Peter Zijlstra <peterz@...radead.org>,
	David Rientjes <rientjes@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Himanshu Madhani <himanshu.madhani@...gic.com>,
	Dept-Eng QLA2xxx Upstream <qla2xxx-upstream@...gic.com>,
	Jan Hubicka <hubicka@....cz>
Subject: Re: [PATCH v2] byteswap: try to avoid __builtin_constant_p gcc bug

On Wednesday, June 22, 2016 2:44:21 PM CEST Tomas Winkler wrote:
> >
> > There are more than 20 files that have the statement: case cpu_to_...
> > Sparse complains about: case __builtin_bswap, not about __builtin_constant_p.
> 
> There is even much more in the header files used in  initializers,
> which also require constants.  I wonder if __builtin_bswap produces
> constant expression correctly under gcc?

In gcc-4.8 or later yes. gcc-4.6/4.7 on powerpc was a special case that we
have worked around now, as the 16-bit byteswap there was not a constant
expression, unlike the 32-bit and 64-bit ones.

	Arnd