linux-kernel - Re: [PATCH 1/2] x86/bootflag: Change some static functions to bool

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Message-ID: <Z7zBXyywUEC2ieiR@gmail.com>
Date: Mon, 24 Feb 2025 19:58:39 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Jiri Slaby <jirislaby@...nel.org>
Cc: Uros Bizjak <ubizjak@...il.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/2] x86/bootflag: Change some static functions to bool


* Jiri Slaby <jirislaby@...nel.org> wrote:

> On 24. 02. 25, 8:39, Uros Bizjak wrote:
> > The internal compiler representation of the following testcase:
> > 
> > _Bool foo (int x) { return x; }
> > 
> > is:
> > 
> > --cut here--
> > _Bool foo (int x)
> > {
> >   _Bool _2;
> > 
> >   <bb 2> [local count: 1073741824]:
> >   _2 = x_1(D) != 0;
> >   return _2;
> 
> Yes, exactly as dictated by the C99 standard.
> 
> > }
> > --cut here--
> > 
> > For me, !!x in the source means that the change of types was
> > intentional. Surely, the compiler can do it by itself, so at the end
> > of the day, it is just a matter of personal taste.
> 
> I've just learnt, that we even have that in CodingStyle:
> ===
> > 17) Using bool
> > --------------
> > 
> > The Linux kernel bool type is an alias for the C99 _Bool type. bool values can
> > only evaluate to 0 or 1, and implicit or explicit conversion to bool
> > automatically converts the value to true or false. When using bool types the
> > !! construction is not needed, which eliminates a class of bugs.
> ===

This rule doesn't apply here, because the !! operation isn't done on 
bool types: 'x' in the parity() function is an 'int'...

So this CodingStyle entry is a red herring, and the !! is absolutely 
used in the kernel as an explicit marker of intentional type conversion 
to bool.

Thanks,

	Ingo