Re: [patch] Use __builtin_FILE and __builtin_LINE in assert implementation in C++

[Jonathan Wakely <jwakely@redhat.com>]

On Tue, 24 Jan 2023 at 12:08, Florian Weimer <fweimer@redhat.com> wrote:
>
> * Jonathan Wakely:
>
> >>>>> I think __builtin_FILE and __builtin_LINE are farily recent GCC/Clang
> >
> > Added to GCC 4.8.0 in 2012. I don't know about Clang.
>
> Oh, I didn't realize it's been this long.
>
> >> > __FILE__ expansion needs to be delayed, otherwise it refers to assert.h.
> >> >
> >> > I think the builtins also have the advantage that they avoid ODR
> >> > violations because the definition is the same no matter what the file is
> >> > called.
> >
> > It's not that simple though, the definition consists of the same
> > sequence of tokens, but can still result in incompatible definitions:
> >
> > char* f()
> > {
> >   static char array[__builtin_LINE()];
> >   return array;
> > }
> >
> > c.f. https://cplusplus.github.io/CWG/issues/2678.html
>
> Ugh, I was worried about something like that.  And the location
> functions need to be constexpr.

Yes. But that's not a problem for assert because it can't be used to
define objects or constants that would vary between TUs.

> There's also the issue that <cassert> is defined in terms of ISO C,
> and ISO C specifies that __FILE__ and __LINE__ must be used.  Is that
> another defect?  But maybe the difference is not observable because
> __FILE__ cannot be redefined?

I don't think it's observable. C requires the text printed by a failed
assert to include "the values of the preprocessing macros __FILE__ and
__LINE__ " but it doesn't require the actual tokens to appear in the
definition. The built-ins do return "the values" of those macros, so
that seems OK to me.