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

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Date:	Wed, 22 Jun 2016 10:25:57 +0000
From:	"Levy, Amir (Jer)" <amir.jer.levy@...el.com>
To:	Arnd Bergmann <arnd@...db.de>, Tomas Winkler <tomasw@...il.com>
CC:	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 2016-06-22 Arnd Bergmann wrote:
> On Wednesday, June 22, 2016 11:24:50 AM CEST Tomas Winkler wrote:
> > On Tue, Jun 21, 2016 at 12:02 PM, Tomas Winkler <tomasw@...il.com>
> wrote:
> > > On Tue, May 3, 2016 at 9:36 AM, Arnd Bergmann <arnd@...db.de>
> wrote:
> > >> On Monday 02 May 2016 16:32:25 Andrew Morton wrote:
> 
> > >>  #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)) ?     \
> > >
> > >>
> > >
> > > I wonder if this doesn't break switch statement that requires a
> > > constant expression, there few cases like this over the kernel.
> > >
> > > switch(val) {
> > > case cpu_to_le32(IXGBE_RXDADV_STAT_FCSTAT_FCPRSP):
> > >
> > > http://lxr.free-electrons.com/source/drivers/net/ethernet/intel/ixgb
> > > e/ixgbe_fcoe.c#L458
> > >
> >
> > I'm asking because sparse and checkpatch doesn't agree on that ping
> > sparse issues
> > 'error: bad constant expression'
> > When changing to __constant_cpu_to_le32 sparse is happy but
> > checkpatch.ps is complaining
> > __constant_cpu_to_le32 should be cpu_to_le32
> >
> 
> That is an interesting problem, as both seem to be reasonable:
> sparse probably doesn't understand __builtin_constant_p() enough, while
> avoiding __constant_cpu_to_le32() is a good recommendation in general.
> 
> How many instances of this do you see in the kernel? If ixgbe is the only one,
> I'd just move the byteswap up into the switch statement:
> 
> 	switch (le32_to_cpu(val)) {
> 	case IXGBE_RXDADV_STAT_FCSTAT_FCPRSP:
> 
> which may cost one or two cycles for the non-constant byteswap, but is also
> easier to read than the current code.
> 
> 	Arnd

There are more than 20 files that have the statement: case cpu_to_...
Sparse complains about: case __builtin_bswap, not about __builtin_constant_p.

	Amir