linux-kernel - RE: drivers/hid/intel-thc-hid/intel-thc/intel-thc-dma.c:298:38: sparse: sparse: dubious: !x

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Message-ID: <IA1PR11MB6098B4AFF412D34D2F62C535F4F62@IA1PR11MB6098.namprd11.prod.outlook.com>
Date: Thu, 6 Feb 2025 02:50:17 +0000
From: "Xu, Even" <even.xu@...el.com>
To: David Laight <david.laight.linux@...il.com>
CC: lkp <lkp@...el.com>, "oe-kbuild-all@...ts.linux.dev"
	<oe-kbuild-all@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Jiri Kosina <jikos@...nel.org>, "Sun,
 Xinpeng" <xinpeng.sun@...el.com>, Srinivas Pandruvada
	<srinivas.pandruvada@...ux.intel.com>, Mark Pearson
	<mpearson-lenovo@...ebb.ca>
Subject: RE: drivers/hid/intel-thc-hid/intel-thc/intel-thc-dma.c:298:38:
 sparse: sparse: dubious: !x | !y



> -----Original Message-----
> From: David Laight <david.laight.linux@...il.com>
> Sent: Thursday, February 6, 2025 5:57 AM
> To: Xu, Even <even.xu@...el.com>
> Cc: lkp <lkp@...el.com>; oe-kbuild-all@...ts.linux.dev; linux-
> kernel@...r.kernel.org; Jiri Kosina <jikos@...nel.org>; Sun, Xinpeng
> <xinpeng.sun@...el.com>; Srinivas Pandruvada
> <srinivas.pandruvada@...ux.intel.com>; Mark Pearson <mpearson-
> lenovo@...ebb.ca>
> Subject: Re: drivers/hid/intel-thc-hid/intel-thc/intel-thc-dma.c:298:38: sparse:
> sparse: dubious: !x | !y
> 
> On Wed, 5 Feb 2025 02:56:05 +0000
> "Xu, Even" <even.xu@...el.com> wrote:
> 
> > > -----Original Message-----
> > > From: David Laight <david.laight.linux@...il.com>
> > > Sent: Thursday, January 30, 2025 6:22 AM
> > > To: lkp <lkp@...el.com>
> > > Cc: Xu, Even <even.xu@...el.com>; oe-kbuild-all@...ts.linux.dev;
> > > linux- kernel@...r.kernel.org; Jiri Kosina <jikos@...nel.org>; Sun,
> > > Xinpeng <xinpeng.sun@...el.com>; Srinivas Pandruvada
> > > <srinivas.pandruvada@...ux.intel.com>; Mark Pearson <mpearson-
> > > lenovo@...ebb.ca>
> > > Subject: Re: drivers/hid/intel-thc-hid/intel-thc/intel-thc-dma.c:298:38:
> sparse:
> > > sparse: dubious: !x | !y
> > >
> > > On Wed, 29 Jan 2025 22:02:59 +0800
> > > kernel test robot <lkp@...el.com> wrote:
> > >
> > > > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> master
> > > > head:   05dbaf8dd8bf537d4b4eb3115ab42a5fb40ff1f5
> > > > commit: a688404b2e20f00cce6d0a2b888ef4ca9154e144 HID: intel-thc-hid:
> > > intel-thc: Add THC DMA interfaces
> > > > date:   3 weeks ago
> > > > config: x86_64-allyesconfig
> > > > (https://download.01.org/0day-ci/archive/20250129/202501292144.eFD
> > > > q4ov
> > > > r-lkp@...el.com/config)
> > > > compiler: gcc-12 (Debian 12.2.0-14) 12.2.0 reproduce (this is a
> > > > W=1
> > > > build):
> > > > (https://download.01.org/0day-ci/archive/20250129/202501292144.eFD
> > > > q4ov
> > > > r-lkp@...el.com/reproduce)
> > > >
> > > > If you fix the issue in a separate patch/commit (i.e. not just a
> > > > new version of the same patch/commit), kindly add following tags
> > > > | Reported-by: kernel test robot <lkp@...el.com>
> > > > | Closes:
> > > > | https://lore.kernel.org/oe-kbuild-all/202501292144.eFDq4ovr-lkp@
> > > > | inte
> > > > | l.com/
> > > >
> > > > sparse warnings: (new ones prefixed by >>)
> > > > >> drivers/hid/intel-thc-hid/intel-thc/intel-thc-dma.c:298:38: sparse:
> > > > >> sparse: dubious: !x | !y
> > > >    drivers/hid/intel-thc-hid/intel-thc/intel-thc-dma.c: note: in
> > > > included file
> > > (through include/linux/mmzone.h, include/linux/gfp.h,
> include/linux/mm.h, ...):
> > > >    include/linux/page-flags.h:237:46: sparse: sparse:
> > > > self-comparison always
> > > evaluates to false
> > > >    include/linux/page-flags.h:237:46: sparse: sparse:
> > > > self-comparison always evaluates to false
> > > >
> > > > vim +298 drivers/hid/intel-thc-hid/intel-thc/intel-thc-dma.c
> > > ...
> > > >    297		for (i = 0; i < config->prd_tbl_num; i++) {
> > > >  > 298			if (!config->sgls[i] | !config->sgls_nent[i])
> > > >    299				continue;
> > > ...
> > >
> > > If zeros are unlikely the bit-wise 'or' is pretty likely to generate
> > > better code than a logical 'or'.
> > >
> > > 	David
> >
> > Good suggestion!
> > So the code can be optimized to:
> > 	for (i = 0; i < config->prd_tbl_num; i++) {
> > 	         if (config->sgls[i] && config->sgls_nent[i]) {
> >                                     ......
> >                        }
> 
> That just adds a level of indentation to the source.
> It'll generate much the same code as !x || !y.
> Both will generate two conditional branches.
> 
> It almost certainly makes in immeasurable difference here, but !x | !y can be
> generated using (on x86) the 'sete' instruction to get a 0/1 from the x == 0
> compare (that sets the flags), and then doing an 'or' and a single jump.

You are right.
I do a simple testing:

83 7d f8 00             cmpl   $0x0,-0x8(%rbp)
0f 94 c2                sete   %dl
83 7d fc 00             cmpl   $0x0,-0x4(%rbp)
0f 94 c0                sete   %al
09 d0                   or     %edx,%eax
84 c0                   test   %al,%al
74 0f                   je     1186 <main+0x3d>

Compare to:

83 7d f8 00             cmpl   $0x0,-0x8(%rbp)
74 06                   je     116f <main+0x26>
83 7d fc 00             cmpl   $0x0,-0x4(%rbp)
75 0f                   jne    117e <main+0x35>

So you suggest I'd better keep the original (!x | !y) code, if just for unlikely Zero comparing?

Thanks!

Best Regards,
Even Xu

> 
> 	David
> 
> > 	}
> >
> > Will create a patch for this.
> >
> > Best Regards,
> > Even Xu
> >
> >