[Bug c/47781] warnings from custom printf format specifiers

["manu at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47781

--- Comment #14 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Tom Tromey from comment #13)
> I have been reconsidering the plugin approach given some new things
> I learned about the details of the firefox code (namely that it doesn't
> faithfully follow printf semantics, sigh).
> 
> One additional note for this bug is that it would be nice if any
> such addition by a plugin worked properly with -Wmissing-format-attribute.

Note that plugins can define attributes. Perhaps one way to go about this would
be to create a plugin that parsed some kind of GCC_printf_format_info attribute
that matches GCC internal printf checking. Then move GCC own format checking to
use this attribute and enable the plugin by default when building GCC.

This will give you as much flexibility as GCC format checking supports, and the
plugin will be developed, build, tested and distributed alongside GCC. Users
outside GCC just need to use the plugin and add the attributes to their own
printf-style functions. Moreover, since the plugin is developed alongside GCC,
it would be logical to add whatever hooks the plugin needs.

Moreover, nothing stops users from creating some kind of intermediate language
that simplifies custom printf attribute syntax. Probably some C preprocessor
magic could be enough.

The challenge is the define the syntax of the attribute, but I think this
challenge is unavoidable for whoever wants to implement this. You may present a
simplified syntax to the user, but you still need to handle correctly all the
complexity and corner cases in c-format.c.
>From gcc-bugs-return-476017-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Wed Feb 04 18:45:31 2015
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From: "pault at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/64921] [4.9/5 Regression] FAIL: gfortran.dg/class_allocate_18.f90
Date: Wed, 04 Feb 2015 18:45:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd921

--- Comment #8 from Paul Thomas <pault at gcc dot gnu.org> ---
(In reply to H.J. Lu from comment #7)
> (In reply to Paul Thomas from comment #6)
> > (In reply to H.J. Lu from comment #2)
> > > I got
> > >
> > > Program received signal SIGSEGV: Segmentation fault - invalid memory
> > > reference.
> > >
> > > Backtrace for this error:
> > > #0  0xF763FACE
> > > #1  0xF763EBDE
> > > #2  0xF773CBBF
> > > #3  0x8048BA5 in __final_main_T2.3337 at class_allocate_18.f90:?
> > > #4  0x8048D68 in __final_main_T3.3328 at class_allocate_18.f90:?
> > > #5  0x8048A59 in MAIN__ at class_allocate_18.f90:?
> > > FAIL: gfortran.dg/class_allocate_18.f90   -O1  execution test
> > >
> > > But I couldn't reproduce it on another machine. I will keep an eye on it.
> >
> > Hi HJ,
> >
> > Given that the error is sporadic, are you sure that the offending revisions
> > are not 220125 and 220130 - PR64230. The error messages that you are getting
> > are remarkably similar to the original report for this PR.
> >
>
> I don't know for sure.  This failure seems more consistent on 4.9 branch
> than on trunk.

Dear HJ,

I can confirm that the part of resolve.c modified in r220191 is not visited by
the compilation of class_allocate_18.f90.

If I have understood correctly, -fPIC is not supported on x86_64 and so, unless
I am mistaken, I cannot help you further.

Please let me know if there is anything that I can do to help.

Best regards

Paul