linux-kernel - RE: [RFC] tools/nolibc: replace duplicated -ENOSYS return with single -ENOSYS return

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Date:   Mon, 28 Aug 2023 09:46:14 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Zhangjin Wu' <falcon@...ylab.org>, "'w@....eu'" <w@....eu>
CC:     "'arnd@...db.de'" <arnd@...db.de>,
        "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
        "'linux-kselftest@...r.kernel.org'" <linux-kselftest@...r.kernel.org>,
        "'thomas@...ch.de'" <thomas@...ch.de>,
        "'tanyuan@...ylab.org'" <tanyuan@...ylab.org>
Subject: RE: [RFC] tools/nolibc: replace duplicated -ENOSYS return with single
 -ENOSYS return

From: David Laight
> Sent: 27 August 2023 22:52
> 
> ...
> > Of course, we can also use the __stringify() trick to do so, but it is
> > expensive (bigger size, worse performance) to unstringify and get the number
> > again, the expensive atoi() 'works' for the numeric __NR_*, but not work for
> > (__NR_*_base + offset) like __NR_* definitions (used by ARM and MIPS), a simple
> > interpreter is required for such cases and it is more expensive than atoi().
> >
> >     /* not for ARM and MIPS */
> >
> >     static int atoi(const char *s);
> >     #define __get_nr(name)          __nr_atoi(__stringify(__NR_##name))
> >     #define __nr_atoi(str)          (str[0] == '_' ? -1L : ___nr_atoi(str))
> >     #define ___nr_atoi(str)         (str[0] == '(' ? -1L : atoi(str))
> >
> > Welcome more discussion or let's simply throw away this direction ;-)
> 
> While it will look horrid the it ought to be possible to
> get the compiler to evaluate the string.
...
> So something that starts:
> #define dig(c) (c < '0' || c > '9' ? 999999 : c - '0')
> 	str[0] == '_' ? -1 :
> 	str[0] != '(' ? str[1] == ' ' ? dig(str[0]) :
> 		str[2] == '1' ? (dig(str[0]) * 10 + dig(str[1]) :
> Any unexpected character will expand the 99999 and generate
> an over-large result.

See https://godbolt.org/z/rear4c1hj

That will convert "1234" or "(1234 + 5678)" (or shorter numbers)
as a compile-time constant.

	David

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