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From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Tue, 12 Jul 2022 12:11:07 -0700
Message-ID: <CAMe9rOrkO0MyGt4MzePsQ52+=UYo9WhO9W25WBMgpxUYVLtoNw@mail.gmail.com>
Subject: Re: [PATCH 3/7] Remove atomic_bit_set/bit_test_set
To: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Cc: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>,
 Noah Goldstein <goldstein.w.n@gmail.com>, 
 GNU C Library <libc-alpha@sourceware.org>
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On Tue, Jul 12, 2022 at 11:44 AM Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
>
> Hi,
>
> I don't believe it could make a difference since it isn't performance critical code.
> Note that several similar fetch_or/fetch_and are used which do not optimize into a

Only the operations with test have this issue.  Simple fetch_or and
fetch_and are
OK.

> bitset operation, so even if we fix this particular case to use unsigned, there are many
> others. IIRC there is even a fetch_or with zero which is kind of useless!
>
> More importantly, it raises a question for locking: we currently use a compare-exchange
> for each lock and unlock. If compare-exchange has higher overheads than other atomics
> then we should investigate more efficient locking sequences.

compare-exchange can be very expensive under contention.

> Cheers,
> Wilco



-- 
H.J.