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

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Message-ID: <aBT5C81se-z-eQMe@yury>
Date: Fri, 2 May 2025 12:55:39 -0400
From: Yury Norov <yury.norov@...il.com>
To: Ian Rogers <irogers@...gle.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 02, 2025 at 09:43:12AM -0700, Ian Rogers wrote:
> 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.

Not sure I understand... 

I mean just 
        return (unsigned int)hweight_long(...);

because the bitmap_weight return type is unsigned int. Do I miss
something?
 
> > 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.

Yes, I'm in CC for that series, but I didn't find the changes for
bitmap_weight(), fls64() and other functions you touch in this series.
Anyways, it would be logical to sync tools with the mother kernel in
the same series.