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Re: [PATCH 2/7] xen/bitops: Implement ffs() in common logic
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 14 Mar 2024 17:35:02 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>, "consulting @ bugseng . com" <consulting@xxxxxxxxxxx>, Simone Ballarin <simone.ballarin@xxxxxxxxxxx>, Federico Serafini <federico.serafini@xxxxxxxxxxx>, Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 14 Mar 2024 16:35:08 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 14.03.2024 17:23, Andrew Cooper wrote:
> On 14/03/2024 2:16 pm, Jan Beulich wrote:
>> On 13.03.2024 18:27, Andrew Cooper wrote:
>>> --- a/xen/arch/arm/include/asm/bitops.h
>>> +++ b/xen/arch/arm/include/asm/bitops.h
>>> @@ -157,7 +157,7 @@ static inline int fls(unsigned int x)
>>> }
>>>
>>>
>>> -#define ffs(x) ({ unsigned int __t = (x); fls(ISOLATE_LSB(__t)); })
>>> +#define arch_ffs(x) ({ unsigned int __t = (x); fls(ISOLATE_LSB(__t)); })
>> The way the macro is invoked, I don't think the helper local variable
>> is then needed anymore?
>
> I strongly suspect It is still needed. ISOLATE_LSB() double-expands its
> parameter.
Even that double evaluation doesn't matter when the invoking entity is an
inline function, and it doesn't use any non-trivial expression as argument.
>>> --- a/xen/include/xen/bitops.h
>>> +++ b/xen/include/xen/bitops.h
>>> @@ -110,6 +110,21 @@ static inline int generic_flsl(unsigned long x)
>>>
>>> #include <asm/bitops.h>
>>>
>>> +/*
>>> + * Find First Set bit. Bits are labelled from 1.
>>> + */
>>> +static always_inline __pure unsigned int ffs(unsigned int x)
>> Why always_inline?
>
> For all the normal reasons to counter Clang and GCC doing stupid things
> with inlines that contain assembly.
Hmm, there are issues when the asm() would look "complex" to the compiler,
but that's not the case here. I was asking because, as you imply by how
you responded, we may need to gain many more always_inline when at some
time even you were arguing against overriding compiler decisions like this
(unless I'm mis-remembering).
Jan
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