On Fri, Mar 16, 2018 at 12:08 AM, Miguel Ojeda
[off-list ref] wrote:

On Thu, Mar 15, 2018 at 11:58 PM, Miguel Ojeda
[off-list ref] wrote:

quoted

On Thu, Mar 15, 2018 at 11:46 PM, Kees Cook [off-list ref] wrote:

quoted

By using this eye-bleed:

size_t __error_not_const_arg(void) \
__compiletime_error("const_max() used with non-compile-time constant arg");
size_t __error_not_positive_arg(void) \
__compiletime_error("const_max() used with negative arg");
#define const_max(x, y)                                                 \
        __builtin_choose_expr(__builtin_constant_p(x) &&                \
                              __builtin_constant_p(y),                  \
                __builtin_choose_expr((x) >= 0 && (y) >= 0,             \
                                      (typeof(x))(x) > (typeof(y))(y) ? \
                                        (x) : (y),                      \
                                      __error_not_positive_arg()),      \
                __error_not_const_arg())

I was writing it like this:

#define const_max(a, b) \
    ({ \
        if ((a) < 0) \
            __const_max_called_with_negative_value(); \
        if ((b) < 0) \
            __const_max_called_with_negative_value(); \
        if (!__builtin_types_compatible_p(typeof(a), typeof(b))) \
            __const_max_called_with_incompatible_types(); \
        __builtin_choose_expr((a) > (b), (a), (b)); \
})

The full one, using your naming convention:

#define const_max(x, y)                                          \
    ({                                                           \
        if (!__builtin_constant_p(x))                            \
            __error_not_const_arg();                             \
        if (!__builtin_constant_p(y))                            \
            __error_not_const_arg();                             \
        if (!__builtin_types_compatible_p(typeof(x), typeof(y))) \
            __error_incompatible_types();                        \
        if ((x) < 0)                                             \
            __error_not_positive_arg();                          \
        if ((y) < 0)                                             \
            __error_not_positive_arg();                          \
        __builtin_choose_expr((x) > (y), (x), (y));              \
    })

Nevermind... gcc doesn't take that as a constant expr, even if it
compiles as one at -O0.