linux-kernel - Re: [PATCH] scsi: fc: force inlining of wwn conversion functions

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Message-ID: <fb8e623c-9a4f-655e-0670-2f8fa82021a7@mellanox.com>
Date:	Thu, 28 Apr 2016 10:58:43 -0400
From:	Chris Metcalf <cmetcalf@...lanox.com>
To:	Arnd Bergmann <arnd@...db.de>, Martin Jambor <mjambor@...e.cz>
CC:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	<linux-scsi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Thomas Graf <tgraf@...g.ch>,
	Peter Zijlstra <peterz@...radead.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Himanshu Madhani <himanshu.madhani@...gic.com>,
	<qla2xxx-upstream@...gic.com>, Jan Hubicka <hubicka@....cz>
Subject: Re: [PATCH] scsi: fc: force inlining of wwn conversion functions

(Resending as text/plain)

On 4/27/2016 5:34 PM, Arnd Bergmann wrote:
> This won't help on TILE, which is the one architecture that sets
> ARCH_SUPPORTS_OPTIMIZED_INLINING but does not set ARCH_USE_BUILTIN_BSWAP.
> Chris Metcalf should be able to figure out whether we can just
> set ARCH_USE_BUILTIN_BSWAP for tile as well.

We certainly could enable ARCH_USE_BUILTIN_BSWAP.  The only problem is
that we never added explicit support for bswap16() in gcc, which is
efficiently done on tilegx via the "revbytes" instruction and a 48-bit
right-shift.  So gcc instead does a generic thing with four
instructions in three bundles, so really not as good as our asm/swab.h.

I'm not sure how to weigh the implications of converting to
builtin_bswap16 (and possibly upstreaming a better implementation to
gcc), vs. disabling ARCH_SUPPORTS_OPTIMIZED_INLINING (which no one
else but x86 uses anyway), vs. just ignoring the compiler bug and
hoping it's not an issue in practice :-)
-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com