linux-kernel - Re: [PATCH 2/3] s390/bitops: Limit return value range of _

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Message-ID: <4f3557f5-d8e0-4948-a653-ad48cd08331a-jchrist@linux.ibm.com>
Date: Thu, 11 Sep 2025 09:44:05 +0200
From: Juergen Christ <jchrist@...ux.ibm.com>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: Nathan Chancellor <nathan@...nel.org>, Miguel Ojeda <ojeda@...nel.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        Sven Schnelle <svens@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>
Subject: Re: [PATCH 2/3] s390/bitops: Limit return value range of __flogr()

> With the recent ffs() and ffs64() optimization a logical and operation was
> removed, which allows the compiler to tell that the return value range of
> both functions. This may lead to compile warnings as reported by the kernel
> test robot.
> 
> Instead of only adding the not needed mask again, also add an __assume()
> statement to tell newer compilers that they can assume a specific value
> range. This allows newer compilers to optimize the not-needed logical and
> operation away.
> 
> Also change the return type of flogr() to unsigned long and add the const
> attribute to the function.
> 
> With this the reported warning is away, and in addition the kernel image
> size is reduced by ~4kb.
> 
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202508211859.UoYsJbLN-lkp@intel.com/
> Fixes: de88e74889a3 ("s390/bitops: Slightly optimize ffs() and fls64()")
> Suggested-by: Juergen Christ <jchrist@...ux.ibm.com>
> Signed-off-by: Heiko Carstens <hca@...ux.ibm.com>

Reviewed-by: Juergen Christ <jchrist@...ux.ibm.com>