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

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Date:	Tue, 03 May 2016 08:36:53 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	mm-commits@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, 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 Monday 02 May 2016 16:32:25 Andrew Morton wrote:
> On Tue, 03 May 2016 01:10:16 +0200 Arnd Bergmann <arnd@...db.de> wrote:
> 
> > On Monday 02 May 2016 16:02:18 Andrew Morton wrote:
> > > On Mon, 02 May 2016 23:48:19 +0200 Arnd Bergmann <arnd@...db.de> wrote:
> > > 
> > > > This is another attempt to avoid a regression in wwn_to_u64() after
> > > > that started using get_unaligned_be64(), which in turn ran into a
> > > > bug on gcc-4.9 through 6.1.
> > > 
> > > I'm still getting a couple screenfuls of things like
> > > 
> > > net/tipc/name_distr.c: In function 'tipc_named_process_backlog':
> > > net/tipc/name_distr.c:330: warning: format '%u' expects type 'unsigned int', but argument 3 has type 'unsigned int'
> > > net/tipc/name_distr.c:330: warning: format '%u' expects type 'unsigned int', but argument 4 has type 'unsigned int'
> > > net/tipc/name_distr.c:330: warning: format '%u' expects type 'unsigned int', but argument 5 has type 'unsigned int'
> > > net/tipc/name_distr.c:330: warning: format '%u' expects type 'unsigned int', but argument 7 has type 'unsigned int'
> > 
> > I've built a few thousand kernels (arm32 with gcc-6.1) with the patch applied,
> > but didn't see this one. What target architecture and compiler version produced
> > this? Does it go away if you add a (__u32) cast? I don't even know what the
> > warning is trying to tell me.
> 
> heh, I didn't actually read it.
> 
> Hopefully we can write this off as a gcc-4.4.4 glitch. 4.8.4 is OK.

Ah, old compiler. I've tried gcc-4.3 now on ARM, and I don't get this warning
(just a lot "may be used uninitialized"), but unlike gcc-4.4, my version doesn't
actually get into the code path I have changed because __builtin_bswap32 was only
introduced with 4.4.

I don't have gcc-4.4 and 4.5 here, but the warning does show up with 4.6, 4.7
and 4.8:

drivers/soc/sunxi/sunxi_sram.c: In function ‘sunxi_sram_show’:
drivers/soc/sunxi/sunxi_sram.c:103:7: warning: format ‘%x’ expects argument of type ‘unsigned int’, but argument 3 has type ‘unsigned int’ [-Wformat=]

4.8 is probably still common enough that we should try to address this.
This change addresses the problem for me with ARM gcc-4.8, but adding
two more type casts. This also makes the 16/32/64 bit swaps all
look the same. I would expect this to also have the same effect on 4.4.

Please fold into the previous patch.

Signed-off-by: Arnd Bergmann <arnd@...db.de>

diff --git a/include/uapi/linux/swab.h b/include/uapi/linux/swab.h
index d737804af181..8f3a8f606fd9 100644
--- a/include/uapi/linux/swab.h
+++ b/include/uapi/linux/swab.h
@@ -97,7 +97,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)
  * @x: value to byteswap
  */
 #ifdef __HAVE_BUILTIN_BSWAP16__
-#define __swab16(x) __builtin_bswap16((__u16)(x))
+#define __swab16(x) (__u16)__builtin_bswap16((__u16)(x))
 #else
 #define __swab16(x)				\
 	(__builtin_constant_p((__u16)(x)) ?	\
@@ -110,7 +110,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)
  * @x: value to byteswap
  */
 #ifdef __HAVE_BUILTIN_BSWAP32__
-#define __swab32(x) __builtin_bswap32((__u32)(x))
+#define __swab32(x) (__u32)__builtin_bswap32((__u32)(x))
 #else
 #define __swab32(x)				\
 	(__builtin_constant_p((__u32)(x)) ?	\