Re: [musl] [PATCH] Use __builtin_FILE/__builtin_LINE if available

[Fangrui Song <i@maskray.me>]

On Mon, Feb 27, 2023 at 2:27 PM Szabolcs Nagy <nsz@port70.net> wrote:
>
> * Fangrui Song <i@maskray.me> [2023-02-21 11:09:14 -0800]:
>
> > On Sat, Feb 18, 2023 at 4:17 AM Jon Chesterfield
> > <jonathanchesterfield@gmail.com> wrote:
> > >
> > > On Sat, 18 Feb 2023, 02:54 Fangrui Song, <i@maskray.me> wrote:
> > >
> > > On Fri, Feb 17, 2023 at 6:03 PM Rich Felker <dalias@libc.org> wrote:
> > > >
> > > > On Fri, Feb 17, 2023 at 05:33:33PM -0800, Fangrui Song wrote:
> > > > > C++ inline functions are requred to have exact same sequence of tokens
> > > > > in every translation unit, but __FILE__ and __LINE__ may expand to
> > > > > different tokens. The ODR violatioin is usually benign, but it can lead
> > > > > to errors when C++20 modules are used.
> > >
> > >
> > >
> > > It is sad that C++ modules broke 'assert' but not surprising. Modules were largely created out of aversion to macros. This isn't something libc can fix though, I suggest a defect report against C++ instead.

To lichray: ^^

> > > Changing the semantics of assert in C seems like a bad thing to do.
> > >
> > > Thanks
> >
> > I disagree. This is a footgun where the right fix (or workaround, if
> > you prefer) is on the libc side. It is fairly reasonable for a header
> > to use assert and not expect two includes using different paths to not
> > cause C++ module problems.
> >
> > The current module behavior regarding macros is a reasonable
> > compromise. It can be evolved (e.g.
> > https://gracicot.github.io/modules/2018/05/14/modules-macro.html).
>
> i dont see how that solves the fundamental problem:
>
> the *behavior* of assert changes depending on which include path is
> used and thus inline functions that are supposed to be equivalent
> aren't. (__builtin_FILE makes the pp-token sequence the same across
> the instances, but the actual code will have different paths, which
> while not an odr violation per the literal words of the spec, it
> clearly violates the reason the rule is there in the first place.)
> libc can avoid printing the file path in the assert fail message for
> c++. this makes assert less useful but it solves the conformance issue.
> if c++ does not specify which path assert should print (or allow it to
> be unpredictable) then it is difficult to do better than this.
>
> it would have been more useful to have a __builtin_canonical_FILE()
> or similar that gives a path that is somehow independent of include
> path, but we don't have that now.

__FILE_NAME__ / __builtin_FILE_NAME just expands to the basename
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108978), which may help.

I created this musl patch as I saw glibc made a similar change on 2023-02-10.
Rejecting this patch is fine. It probably needs some time for standard
C++ modules to become mainstream to expose this deployment problem.