linux-kernel - Re: [PATCH v2 2/5] bitmap: Silence a clang -Wshorten-64-to-32 warning

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Message-ID: <CAP-5=fXqLh7RdUok5oqVwyGOWCH3fktmVeECdi4ENBWnEHeJYQ@mail.gmail.com>
Date: Fri, 2 May 2025 09:43:12 -0700
From: Ian Rogers <irogers@...gle.com>
To: Yury Norov <yury.norov@...il.com>
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>, Arnd Bergmann <arnd@...db.de>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
	Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
	Adrian Hunter <adrian.hunter@...el.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Jakub Kicinski <kuba@...nel.org>, Jacob Keller <jacob.e.keller@...el.com>, linux-arch@...r.kernel.org, 
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, Leo Yan <leo.yan@....com>
Subject: Re: [PATCH v2 2/5] bitmap: Silence a clang -Wshorten-64-to-32 warning

On Fri, May 2, 2025 at 9:03 AM Yury Norov <yury.norov@...il.com> wrote:
>
> Hi Ian,
>
> On Wed, Apr 30, 2025 at 10:15:31AM -0700, Ian Rogers wrote:
> > The clang warning -Wshorten-64-to-32 can be useful to catch
> > inadvertent truncation. In some instances this truncation can lead to
> > changing the sign of a result, for example, truncation to return an
> > int to fit a sort routine. Silence the warning by making the implicit
> > truncation explicit. This isn't to say the code is currently incorrect
> > but without silencing the warning it is hard to spot the erroneous
> > cases.
> >
> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > ---
> >  include/linux/bitmap.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> > index 595217b7a6e7..4395e0a618f4 100644
> > --- a/include/linux/bitmap.h
> > +++ b/include/linux/bitmap.h
> > @@ -442,7 +442,7 @@ static __always_inline
> >  unsigned int bitmap_weight(const unsigned long *src, unsigned int nbits)
> >  {
> >       if (small_const_nbits(nbits))
> > -             return hweight_long(*src & BITMAP_LAST_WORD_MASK(nbits));
> > +             return (int)hweight_long(*src & BITMAP_LAST_WORD_MASK(nbits));
>
> This should return unsigned int, I guess?

Hi Yury, I don't disagree. The issue there is that this could break
printf flags, etc. reliant on the return type. I've tried to keep the
patch minimal in this regard.

> Also, most of the functions you touch here have their copies in tools.
> Can you please keep them synchronized?

Yes, I do most of my work on the perf tool in the tools directory and
these patches come from adding -Wshorten-64-to-32 there due to a bug
found in ARM code that -Wshorten-64-to-32 would have caught:
https://lore.kernel.org/lkml/20250331172759.115604-1-leo.yan@arm.com/
The most recent patch series for tools is:
https://lore.kernel.org/linux-perf-users/20250430175036.184610-1-irogers@google.com/
However, I wanted to get the kernel versions of these headers agreed
before syncing them into the tools directory.

Thanks,
Ian



> Thanks,
> Yury
>
> >       return __bitmap_weight(src, nbits);
> >  }
> >
> > --
> > 2.49.0.906.g1f30a19c02-goog