Re: [PATCH v2] c: Silently ignore pragma region [PR85487]

[David Malcolm <dmalcolm@redhat.com>]

On Fri, 2020-11-13 at 09:57 -0500, Austin Morton via Gcc-patches wrote:
> On the contrary, as a user of GCC I would much prefer a consistent
> behavior for #pragma region based purely on GCC version.
> 
> IE, so you can tell people:
>     "just update to GCC X.Y and those warnings will go away"
> rather than:
>     "update to GCC X.Y and pass some new flags - but make sure
>      not to pass them to old GCC versions, since that will generate
>      a new warning"
> 
> I do agree it may be generally useful to have a configurable way to
> specify pragmas to ignore at runtime, but that is not what I was
> trying
> to accomplish here.
> 
> Both clang and MSVC handle this pragma without any runtime
> configuration, and I think GCC should as well.

FWIW I like the patch (but I don't think I can approve it).

How much does this pragma get used "in the wild"?

Thinking aloud, I wonder if it would be useful to capture regions in
the diagnostic subsystem, and emit "In region <FOO>..." messages,
rather like we emit "In function <FOO>..." when first emitting a
diagnostic within a region?  (not sure if good idea, just
brainstorming)

Dave

> Austin
> 
> On Thu, Nov 12, 2020 at 11:25 PM Jeff Law <law@redhat.com> wrote:
> > 
> > On 9/2/20 6:59 PM, Austin Morton via Gcc-patches wrote:
> > > #pragma region is a feature introduced by Microsoft in order to
> > > allow
> > > manual grouping and folding of code within Visual Studio.  It is
> > > entirely ignored by the compiler.  Clang has supported this
> > > feature
> > > since 2012 when in MSVC compatibility mode, and enabled it across
> > > the
> > > board in 2018.
> > > 
> > > As it stands, you cannot use #pragma region within GCC without
> > > disabling unknown pragma warnings, which is not advisable.
> > > 
> > > I propose GCC adopt "#pragma region" and "#pragma endregion" in
> > > order
> > > to alleviate these issues.  Because the pragma has no purpose at
> > > compile time, the implementation is trivial.
> > > 
> > > 
> > > Microsoft Documentation on the feature:
> > > https://docs.microsoft.com/en-us/cpp/preprocessor/region-endregion
> > > 
> > > LLVM change which enabled pragma region across the board:
> > > https://reviews.llvm.org/D42248
> > > ---
> > >  gcc/ChangeLog                        |  5 +++++
> > >  gcc/c-family/ChangeLog               |  5 +++++
> > >  gcc/c-family/c-pragma.c              | 10 ++++++++++
> > >  gcc/doc/cpp.texi                     |  6 ++++++
> > >  gcc/testsuite/ChangeLog              |  5 +++++
> > >  gcc/testsuite/gcc.dg/pragma-region.c | 21 +++++++++++++++++++++
> > >  6 files changed, 52 insertions(+)
> > >  create mode 100644 gcc/testsuite/gcc.dg/pragma-region.c
> > 
> > I'm not sure that this is really the way we want to handle this
> > stuff.
> > I understand the problem you're trying to solve, but embedding a
> > list of
> > pragmas to ignore into the compiler itself just seems like the
> > wrong
> > approach -- it bakes that set of pragmas to ignore into the
> > compiler.
> > 
> > 
> > ISTM that we'd be better off either having a command line option to
> > list
> > the set of pragmas to ignore, or they should be pulled from a file
> > specified on the command line.   That would seem to be a lot more
> > friendly to downstream users since each project could set the list
> > of
> > pragmas to ignore on their own and have that set updated
> > dynamically
> > over time without having to patch and update GCC.
> > 
> > 
> > Any chance you would be willing to work on that?
> > 
> > Jeff
> >