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

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Message-ID: <aR85mS5UaBeyDrr_@yury>
Date: Thu, 20 Nov 2025 10:54:01 -0500
From: Yury Norov <yury.norov@...il.com>
To: david.laight.linux@...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 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.

Thanks,
Yury