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

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Date:	Wed, 22 Jun 2016 14:44:21 +0300
From:	Tomas Winkler <tomasw@...il.com>
To:	"Levy, Amir (Jer)" <amir.jer.levy@...el.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	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 Wed, Jun 22, 2016 at 1:25 PM, Levy, Amir (Jer)
<amir.jer.levy@...el.com> wrote:
> 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.

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?

Thanks
Tomas