linux-kernel - Re: [PATCH v1 1/1] x86/percpu: Cast -1 to argument type when comparing in percpu_add

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Message-ID: <ZxJS0_DUfgi4mHe1@smile.fi.intel.com>
Date: Fri, 18 Oct 2024 15:21:39 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Dave Hansen <dave.hansen@...el.com>, Ingo Molnar <mingo@...nel.org>,
	Uros Bizjak <ubizjak@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
	Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>
Subject: Re: [PATCH v1 1/1] x86/percpu: Cast -1 to argument type when
 comparing in percpu_add_op()

On Thu, Oct 17, 2024 at 08:18:59PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 16, 2024 at 12:44:18PM -0700, Dave Hansen wrote:
> 
> > Would anybody hate if we broke this up a bit, like:
> > 
> >         const typeof(var) _val = val;
> >         const int paoconst = __builtin_constant_p(val);
> >         const int paoinc   = paoconst && ((_val) == 1);
> >         const int paodec   = paoconst && ((_val) == (typeof(var))-1);
> > 
> > and then did
> > 
> > 	if (paoinc)
> > 		percpu_unary_op(size, qual, "inc", var);
> > 	...
> 
> I think that is an overall improvement. Proceed! :-)

Wouldn't typeof(var) be a regression? The val can be wider (in term of bits)
than var and cutting it like this might bring different result depending on
the signedness. TL;DR: Whatever is done, please add more (corner) test cases
to the percpu_test.c.

-- 
With Best Regards,
Andy Shevchenko