Re: [PATCH v2 4/8] minmax: Simplify signedness check

[Linus Torvalds <torvalds@linuxfoundation.org>]

On Sun, 28 Jul 2024 at 07:21, David Laight <David.Laight@aculab.com> wrote:
>
> +/* Allow if both x and y are valid for either signed or unsigned compares. */
> +#define __types_ok(x, y)                               \
> +       ((__is_ok_signed(x) && __is_ok_signed(y)) ||    \
> +        (__is_ok_unsigned(x) && __is_ok_unsigned(y)))

This seems horrendous, exactly because it expands both x and y twice.
And the "expand multiple times" was really the fundamental problem.

Why not just change the model to say it's a bitmask of "signedness
bits", the bits are "signed ok" and "unsigned ok", and turn it into

  /* Signedness matches? */
  #define __types_ok(x, y) \
     (__signedness_bits(x) & __signedness_bits(y))

and __signedness_ok() simply does something like "1 if unsigned type,
2 if signed type, 3 if signed positive integer".

Something like (very very handwavy, very very untested):

   __builtin_choose_expr(is_signed_type(typeof(x)),
        2+__if_constexpr(x,(x)>0,0),
        1)

Actually, I think that "__if_constexpr()" could very well be "if known
positive value", ie 'x' itself doesn't have to be constant, but "x>0"
has to be a constant (the difference being that the compiler may be
able to tell that some variable is always positive, even if it's a
variable):

  #define statically_true(x) __builtin_constant_p((x),(x),0)
  #define is_positive_value(x) statically_true((x)>=0)

and then use

   __builtin_choose_expr(is_signed_type(typeof(x)),
        2+is_positive_value(x), 1)

and yes, I realize I count zero as a positive value, but writing out
"nonnegative()" is annoying and we never care.

I guess we could say "is_unsigned_value()"?

       Linus