linux-kernel - Re: [PATCH v3 01/16] bitops: Change parity8() return type to bool

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Message-ID: <Z8tJNt83uVBca0cj@thinkpad>
Date: Fri, 7 Mar 2025 14:30:08 -0500
From: Yury Norov <yury.norov@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Ingo Molnar <mingo@...nel.org>, Jiri Slaby <jirislaby@...nel.org>,
	Kuan-Wei Chiu <visitorckw@...il.com>, tglx@...utronix.de,
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	Yu-Chun Lin <eleanor15x@...il.com>
Subject: Re: [PATCH v3 01/16] bitops: Change parity8() return type to bool

On Fri, Mar 07, 2025 at 04:14:34AM -0800, H. Peter Anvin wrote:
> On March 7, 2025 4:13:26 AM PST, Ingo Molnar <mingo@...nel.org> wrote:
> >
> >* Jiri Slaby <jirislaby@...nel.org> wrote:
> >
> >> On 07. 03. 25, 12:38, Ingo Molnar wrote:
> >> > 
> >> > * Jiri Slaby <jirislaby@...nel.org> wrote:
> >> > 
> >> > > On 06. 03. 25, 17:25, Kuan-Wei Chiu wrote:
> >> > > > Change return type to bool for better clarity. Update the kernel doc
> >> > > > comment accordingly, including fixing "@value" to "@val" and adjusting
> >> > > > examples. Also mark the function with __attribute_const__ to allow
> >> > > > potential compiler optimizations.
> >> > > > 
> >> > > > Co-developed-by: Yu-Chun Lin <eleanor15x@...il.com>
> >> > > > Signed-off-by: Yu-Chun Lin <eleanor15x@...il.com>
> >> > > > Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
> >> > > > ---
> >> > > >    include/linux/bitops.h | 10 +++++-----
> >> > > >    1 file changed, 5 insertions(+), 5 deletions(-)
> >> > > > 
> >> > > > diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> >> > > > index c1cb53cf2f0f..44e5765b8bec 100644
> >> > > > --- a/include/linux/bitops.h
> >> > > > +++ b/include/linux/bitops.h
> >> > > > @@ -231,26 +231,26 @@ static inline int get_count_order_long(unsigned long l)
> >> > > >    /**
> >> > > >     * parity8 - get the parity of an u8 value
> >> > > > - * @value: the value to be examined
> >> > > > + * @val: the value to be examined
> >> > > >     *
> >> > > >     * Determine the parity of the u8 argument.
> >> > > >     *
> >> > > >     * Returns:
> >> > > > - * 0 for even parity, 1 for odd parity
> >> > > > + * false for even parity, true for odd parity
> >> > > 
> >> > > This occurs somehow inverted to me. When something is in parity means that
> >> > > it has equal number of 1s and 0s. I.e. return true for even distribution.
> >> > > Dunno what others think? Or perhaps this should be dubbed odd_parity() when
> >> > > bool is returned? Then you'd return true for odd.
> >> > 
> >> > OTOH:
> >> > 
> >> >   - '0' is an even number and is returned for even parity,
> >> >   - '1' is an odd  number and is returned for odd  parity.
> >> 
> >> Yes, that used to make sense for me. For bool/true/false, it no longer does.
> >> But as I wrote, it might be only me...
> >
> >No strong opinion on this from me either, I'd guess existing practice 
> >with other parity functions should probably control. (If a coherent 
> >praxis exists.).
> >
> >Thanks,
> >
> >	Ingo
> 
> Instead of "bool" think of it as "bit" and it makes more sense

So, to help people thinking that way we can introduce a corresponding
type:
        typedef unsigned _BitInt(1) u1;

It already works for clang, and GCC is going to adopt it with std=c23.
We can make u1 an alias to bool for GCC for a while. If you guys like
it, I can send a patch.

For clang it prints quite a nice overflow warning:

tst.c:59:9: warning: implicit conversion from 'int' to 'u1' (aka 'unsigned _BitInt(1)') changes value from 2 to 0 [-Wconstant-conversion]
   59 |         u1 r = 2;
      |            ~   ^

Thanks,
Yury