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Re: [PATCH RFC] rangeset: mark a few functions pure
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 29 Sep 2022 10:20:31 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Thu, 29 Sep 2022 08:20:37 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 28.09.2022 16:27, Roger Pau Monné wrote:
> On Wed, Sep 28, 2022 at 02:12:30PM +0200, Jan Beulich wrote:
>> While for some of the functions there's locking involved, the acquiring
>> and releasing of a lock doesn't alter program state when comparing
>> "before" and "after" the function invocations. Furthermore without
>> (further) locking by callers, return values are stale anyway by the time
>> they can be evaluated. Hence both CSE and DCE are okay to occur for
>> invocations of these functions.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> RFC for the (imo) ambiguous wording in documentation.
>
> What happens if the contents of the rangeset is modified outside of
> the scope of the function. Ie: would for example:
>
> while ( !rangeset_is_empty(...) );
>
> Become an infinite loop then, without taking into account that the
> rangeset can be externally modified?
>
> It's not clear to me whether the elimination of repeated calls take
> into account that the observed variable can be externally modified?
Hmm, right, this actually matches the feof() counter-example that the
gcc doc has. So I guess I need to withdraw the patch and we will need
to live with these functions not being possible to eliminate even in
cases where it would be okay (and helpful).
Jan
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