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

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Date:   Sun, 27 Aug 2023 21:51:35 +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

...
> 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.

Since "abc"[2] (etc) is converted to a constant (by gcc and clang
except at -O0) and you only need to process "n" "nn" "nnn"
"(n + m)" (with variable length n and m) then append some spaces
and convert the characters back to digits.

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.
I'm not sure how constant the array index need to be.
They may well have to be 'integer constant expressions'
so cant depend on a previous str[const] value.

I just found a(nother) clang bug:
	int f(void) { return "a"[2]; }
compiles to just a 'return'.

	David

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