Re: [RFC PATCH] compiler.h: give up __compiletime_assert_fallback()

[Daniel Santos <daniel.santos@pobox.com>]

On 08/19/2018 03:25 PM, Nick Desaulniers wrote:
> + gbiv who wrote this cool paste (showing alternatives to
> _Static_assert, which is supported by both compilers in -std=gnu89,
> but not until gcc 4.6): https://godbolt.org/g/DuLsxu
>
> I can't help but think that BUILD_BUG_ON_MSG should use
> _Static_assert, then have fallbacks for gcc < 4.6.

Unfortunately _Static_assert is a woefully inadequate replacement
because it requires a C constant expression.  Example:

    int a = 1;
    _Static_assert(a == 1, "a != 1");

results in "error: expression in static assertion is not constant." 
Language standards tend to shy away from defining implementation details
like optimizations, but we need to have completed a good data flow
analysis and constant propagation in order to do BUILD_BUG_ON_MSG, et.
al.; this is why they only work when optimizations are enabled.  As the
optimizer improves, new expressions can be used with BUILD_BUG_ON*.  I
did an analysis of this back in 2012 of how various types of variables
could be resolved to constants at compile-time and how that evolved from
gcc 3.4 to 4.7:

https://drive.google.com/open?id=1cQRAAOzjFy6Aw7CDc4QauHvd_spVkd5a

This changed again when -findirect-inline was added -- i.e.,
BUILD_BUG_ON could be used on parameters of inline functions even when
called by pointer, although the caller needed __flatten in some cases --
a bit messy.

Daniel