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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Zhangjin Wu' <falcon@tinylab.org>
CC:     "ammarfaizi2@gnuweeb.org" <ammarfaizi2@gnuweeb.org>,
        "arnd@arndb.de" <arnd@arndb.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-kselftest@vger.kernel.org" <linux-kselftest@vger.kernel.org>,
        "tanyuan@tinylab.org" <tanyuan@tinylab.org>,
        "thomas@t-8ch.de" <thomas@t-8ch.de>, "w@1wt.eu" <w@1wt.eu>
Subject: RE: [RFC] tools/nolibc: replace duplicated -ENOSYS return with single
 -ENOSYS return
Thread-Topic: [RFC] tools/nolibc: replace duplicated -ENOSYS return with
 single -ENOSYS return
Thread-Index: AdnbXfVru8RTG9gTQ+SsAG5ax7hYowAeEOWAAANtgiA=
Date:   Thu, 31 Aug 2023 08:37:11 +0000
Message-ID: <f7f06eb4a5b44ce29718341e28c823f8@AcuMS.aculab.com>
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 <20230831074151.7900-1-falcon@tinylab.org>
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...
> We also need this style of checking for the delta logic in __atoi_add(). have
> randomly tried different clang and gcc versions, seems all of them work
> correctly, but the compiling speed is not that good if we want to support the
> worst cases like "((0x900000 + 0x0f0000) + 5)", the shorter one
> "((0x900000+0x0f0000)+5)" is used by ARM+OABI (not supported by nolibc
> currently), therefore, we can strip some tailing branches but it is either not
> that fast, of course, the other architectures/variants can use faster
> __atoi_add() versions with less branches and without hex detection, comparison
> and calculating.

If there are only a few prefix offsets then the code can be optimised
to explicitly detect them - rather than decoding arbitrary hex values.
After all it only needs to decode the values that actually appear.

The code also needs a compile-time assert that the result
is constant (__buitin_constant_p() will do the check.
But you can't use _Static_assert() to report the error
because that requires an 'integer constant expression'.

	David

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