[avr-chat] ensuring that _delay_us() gets a compile-time double constant

[Britton Kerin]

I'd like to say:

  #define USUALLY_INT_CONSTANT 42

  int val_for_expression_needing_to_stay_int = USUALLY_INT_CONSTANT * foo_int;

  _delay_us (USUALLY_INT_CONSTANT);

So the questions:

1.  Does _delay_us() need a double const, or is an int const guaranteed
    to work?    The avr libc manual just says "known const" in one place,
    but the signature     calls for a double.

2.  Can the compiler be relied on to turn USUALLY_INT_CONSTANT to a double
    at compile time?  Where is this guarantee written down?

3.  Is there something I can write in the code to check that things are
    correct at compile-time?  I've looked a bit at _Static_assert() and
    __builtin_constant_p() but I'm not sure what I want that is gauranteed to
    work, or if the builtin check that the modern GCC is doing is gauranteed
    to work?  It generates output like this:

    In file included from one_wire_slave.c:8:0:
    /home/bkerin/opt/avr/avr/include/util/delay.h: In function ‘ows_write_bit’:
    /home/bkerin/opt/avr/avr/include/util/delay.h:163:28: error:
__builtin_avr_delay_cycles expects a compile time integer constant
      __builtin_avr_delay_cycles(__ticks_dc);
                                ^
    make: *** [one_wire_slave.o] Error 1