linux-kernel - Re: [PATCH 01/44] x86/asm/bitops: Change the return type of variable_

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Message-ID: <20251120211810.60f53cb7@pumpkin>
Date: Thu, 20 Nov 2025 21:18:10 +0000
From: David Laight <david.laight.linux@...il.com>
To: Yury Norov <yury.norov@...il.com>
Cc: linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>, Dave
 Hansen <dave.hansen@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>,
 Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org
Subject: Re: [PATCH 01/44] x86/asm/bitops: Change the return type of
 variable__ffs() to unsigned int

On Thu, 20 Nov 2025 13:33:11 -0500
Yury Norov <yury.norov@...il.com> wrote:

> On Thu, Nov 20, 2025 at 06:29:09PM +0000, David Laight wrote:
> > On Thu, 20 Nov 2025 10:54:01 -0500
> > Yury Norov <yury.norov@...il.com> wrote:
> >   
> > > On Wed, Nov 19, 2025 at 10:40:57PM +0000, david.laight.linux@...il.com wrote:  
> > > > From: David Laight <david.laight.linux@...il.com>
> > > > 
> > > > The return type of variable__ffs() is currently 'unsigned long'.
> > > > This makes the x86 __ffs() be 'unsigned long' whereas the generic
> > > > version is 'unsigned int'.
> > > > 
> > > > Similarly change variable_ffz() and ffz().
> > > > 
> > > > This may save some REX prefix on 64bit.
> > > > 
> > > > Detected by some extra checks added to min_t() to detect possible
> > > > truncation of large values.
> > > > 
> > > > Signed-off-by: David Laight <david.laight.linux@...il.com>
> > > > ---
> > > >  arch/x86/include/asm/bitops.h | 18 +++++++-----------
> > > >  1 file changed, 7 insertions(+), 11 deletions(-)
> > > > 
> > > > diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> > > > index c2ce213f2b9b..2e8a954d2e2d 100644
> > > > --- a/arch/x86/include/asm/bitops.h
> > > > +++ b/arch/x86/include/asm/bitops.h
> > > > @@ -240,7 +240,7 @@ arch_test_bit_acquire(unsigned long nr, const volatile unsigned long *addr)
> > > >  					  variable_test_bit(nr, addr);
> > > >  }
> > > >  
> > > > -static __always_inline __attribute_const__ unsigned long variable__ffs(unsigned long word)
> > > > +static __always_inline __attribute_const__ unsigned int variable__ffs(unsigned long word)    
> > > 
> > > There's a mismatch with the generic ffs() in asm-generic/bitops/ffs.h.
> > > 
> > > The generic_ffs() returns int. There is another variable__ffs() defined
> > > in arch/risk, also returning int.
> > > 
> > > So I believe, the correct fix would be to switch x86 to int as well.
> > > And anyways, I believe this deserves a separate series.  
> > 
> > It is a single patch, do you want me to resend the patch on its own?
> > (With a different commit message)  
> 
> The patch looks wrong to me. All the flavors of ffs(), ffz() and
> others should have the same signature.

I only changed 'long' to 'int', all the generic (and builtin) ones are 'int'.
So I'm not sure which difference you are talking about.
IIRC the __attribute_const__ just lets the compiler do CSE.

	David