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| author | Reid Kleckner <rnk@google.com> | 2019-10-10 21:04:25 +0000 |
|---|---|---|
| committer | Reid Kleckner <rnk@google.com> | 2019-10-10 21:04:25 +0000 |
| commit | c677d420e10769596cb21a859d7896139adc8be5 (patch) | |
| tree | bd2fc50b3194f838a51656f8191b7ebd99fb401e /docs | |
| parent | 3ee1fe728c53ee07305dcb33177f7de493d108c2 (diff) | |
| download | clang-c677d420e10769596cb21a859d7896139adc8be5.tar.gz | |
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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374449 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/ReleaseNotes.rst | 5 | ||||
| -rw-r--r-- | docs/UsersManual.rst | 7 |
2 files changed, 11 insertions, 1 deletions
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst index 8e9d298c95..7f823e3826 100644 --- a/docs/ReleaseNotes.rst +++ b/docs/ReleaseNotes.rst @@ -80,7 +80,10 @@ Non-comprehensive list of changes in this release New Compiler Flags ------------------ -- ... +- The -fgnuc-version= flag now controls the value of ``__GNUC__`` and related + macros. This flag does not enable or disable any GCC extensions implemented in + Clang. Setting the version to zero causes Clang to leave ``__GNUC__`` and + other GNU-namespaced macros, such as ``__GXX_WEAK__``, undefined. Deprecated Compiler Flags ------------------------- diff --git a/docs/UsersManual.rst b/docs/UsersManual.rst index 419714d38c..de28d77671 100644 --- a/docs/UsersManual.rst +++ b/docs/UsersManual.rst @@ -701,6 +701,13 @@ Other Options ------------- Clang options that don't fit neatly into other categories. +.. option:: -fgnuc-version= + + This flag controls the value of ``__GNUC__`` and related macros. This flag + does not enable or disable any GCC extensions implemented in Clang. Setting + the version to zero causes Clang to leave ``__GNUC__`` and other + GNU-namespaced macros, such as ``__GXX_WEAK__``, undefined. + .. option:: -MV When emitting a dependency file, use formatting conventions appropriate |