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

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Date:	Thu, 28 Apr 2016 17:23:31 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Chris Metcalf <cmetcalf@...lanox.com>
Cc:	Martin Jambor <mjambor@...e.cz>,
	"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

On Thursday 28 April 2016 10:58:43 Chris Metcalf wrote:
> (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 

How about figuring out whether you hit the gcc bug on tile as a
first step?

Another idea would be to adapt this section in include/linux/compiler-gcc.h:

#if !defined(CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING) ||                \
    !defined(CONFIG_OPTIMIZE_INLINING) || (__GNUC__ < 4)
#define inline          inline          __attribute__((always_inline)) notrace
#define __inline__      __inline__      __attribute__((always_inline)) notrace
#define __inline        __inline        __attribute__((always_inline)) notrace
#else
/* A lot of inline functions can cause havoc with function tracing */
#define inline          inline          notrace
#define __inline__      __inline__      notrace
#define __inline        __inline        notrace
#endif

to work around the issue. We already check for gcc before 4.0, and
we could also check for the affected releases (4.9, 5.x, 6.1) in the
same place, possibly conditional on ARCH_USE_BUILTIN_BSWAP with
a comment pointing to the gcc bug tracker.

	Arnd