Add -fgnuc-version= to control __GNUC__ and other GCC macros

I noticed that compiling on Windows with -fno-ms-compatibility had the
side effect of defining __GNUC__, along with __GNUG__, __GXX_RTTI__, and
a number of other macros for GCC compatibility. This is undesirable and
causes Chromium to do things like mix __attribute__ and __declspec,
which doesn't work. We should have a positive language option to enable
GCC compatibility features so that we can experiment with
-fno-ms-compatibility on Windows. This change adds -fgnuc-version= to be
that option.

My issue aside, users have, for a long time, reported that __GNUC__
doesn't match their expectations in one way or another. We have
encouraged users to migrate code away from this macro, but new code
continues to be written assuming a GCC-only environment. There's really
nothing we can do to stop that. By adding this flag, we can allow them
to choose their own adventure with __GNUC__.

This overlaps a bit with the "GNUMode" language option from -std=gnu*.
The gnu language mode tends to enable non-conforming behaviors that we'd
rather not enable by default, but the we want to set things like
__GXX_RTTI__ by default, so I've kept these separate.

Helps address PR42817

Reviewed By: hans, nickdesaulniers, MaskRay

Differential Revision: https://reviews.llvm.org/D68055

llvm-svn: 374449

1 files changed, 1 insertions, 1 deletions

diff --git a/clang/test/Sema/atomic-ops.c b/clang/test/Sema/atomic-ops.c
index 638c30f278c..0340e233489 100644
--- a/clang/test/Sema/atomic-ops.c
+++ b/clang/test/Sema/atomic-ops.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 %s -verify -ffreestanding -fsyntax-only -triple=i686-linux-gnu -std=c11
+// RUN: %clang_cc1 %s -verify -fgnuc-version=4.2.1 -ffreestanding -fsyntax-only -triple=i686-linux-gnu -std=c11
 
 // Basic parsing/Sema tests for __c11_atomic_*