...
> 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
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
