From b67f62b3d506113c2e3da1af67bd0c60497fc97e Mon Sep 17 00:00:00 2001
From: Michal Gorny <mgorny@gentoo.org>
Date: Tue, 6 Mar 2018 21:26:28 +0000
Subject: [PATCH] [FrontEnd] Allow overriding the default C/C++ -std via CMake
 vars

Provide two new CMake cache variables -- CLANG_DEFAULT_STD_C
and CLANG_DEFAULT_STD_CXX -- that can be used to override the default
C/ObjC and C++/ObjC++ standards appropriately. They can be set to one of
the identifiers from LangStandards.def, or left unset (the default) to
respect the current platform default.

This option is mostly intended for compiler vendors that may wish
to adjust the defaults their compilers are using. For example, Gentoo
planned to use it to set clang and gcc to matching standards, so that
we could maintain as much compatibility between different compilers
as possible.

The code relies on explicit identifiers rather than the string aliases
for simplicity. This saves us from the necessity of parsing aliases at
build-time or adding additional processing at runtime. For the latter
case, it also adds trivial value check -- if incorrect value is passed,
the code simply fails to compile through referencing an undefined
constant.

If the variable is used to redefine the default standard, the explicit
value overrides the special case for PS4. It is done this way mostly
following other kinds of variables where 'platform defaults' are
redefined.

Differential Revision: https://reviews.llvm.org/D34365

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@326836 91177308-0d34-0410-b5e6-96231b3b80d8
---
 CMakeLists.txt                      |  6 ++++++
 include/clang/Config/config.h.cmake |  6 ++++++
 lib/Frontend/CompilerInvocation.cpp | 12 ++++++++++++
 3 files changed, 24 insertions(+)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index a2fcaec9c3..3bf3cfdf33 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -212,6 +212,12 @@ set(ENABLE_LINKER_BUILD_ID OFF CACHE BOOL "pass --build-id to ld")
 set(ENABLE_X86_RELAX_RELOCATIONS OFF CACHE BOOL
     "enable x86 relax relocations by default")
 
+# TODO: verify the values against LangStandards.def?
+set(CLANG_DEFAULT_STD_C "" CACHE STRING
+  "Default standard to use for C/ObjC code (IDENT from LangStandards.def, empty for platform default)")
+set(CLANG_DEFAULT_STD_CXX "" CACHE STRING
+  "Default standard to use for C++/ObjC++ code (IDENT from LangStandards.def, empty for platform default)")
+
 set(CLANG_DEFAULT_LINKER "" CACHE STRING
   "Default linker to use (linker name or absolute path, empty for platform default)")
 
diff --git a/include/clang/Config/config.h.cmake b/include/clang/Config/config.h.cmake
index 5f420195e8..5822619a37 100644
--- a/include/clang/Config/config.h.cmake
+++ b/include/clang/Config/config.h.cmake
@@ -11,6 +11,12 @@
 /* Default linker to use. */
 #define CLANG_DEFAULT_LINKER "${CLANG_DEFAULT_LINKER}"
 
+/* Default C/ObjC standard to use. */
+#cmakedefine CLANG_DEFAULT_STD_C LangStandard::lang_${CLANG_DEFAULT_STD_C}
+
+/* Default C++/ObjC++ standard to use. */
+#cmakedefine CLANG_DEFAULT_STD_CXX LangStandard::lang_${CLANG_DEFAULT_STD_CXX}
+
 /* Default C++ stdlib to use. */
 #define CLANG_DEFAULT_CXX_STDLIB "${CLANG_DEFAULT_CXX_STDLIB}"
 
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 23f3c8fbbd..ff8db474e1 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -1809,18 +1809,30 @@ void CompilerInvocation::setLangDefaults(LangOptions &Opts, InputKind IK,
       break;
     case InputKind::Asm:
     case InputKind::C:
+#if defined(CLANG_DEFAULT_STD_C)
+      LangStd = CLANG_DEFAULT_STD_C;
+#else
       // The PS4 uses C99 as the default C standard.
       if (T.isPS4())
         LangStd = LangStandard::lang_gnu99;
       else
         LangStd = LangStandard::lang_gnu11;
+#endif
       break;
     case InputKind::ObjC:
+#if defined(CLANG_DEFAULT_STD_C)
+      LangStd = CLANG_DEFAULT_STD_C;
+#else
       LangStd = LangStandard::lang_gnu11;
+#endif
       break;
     case InputKind::CXX:
     case InputKind::ObjCXX:
+#if defined(CLANG_DEFAULT_STD_CXX)
+      LangStd = CLANG_DEFAULT_STD_CXX;
+#else
       LangStd = LangStandard::lang_gnucxx14;
+#endif
       break;
     case InputKind::RenderScript:
       LangStd = LangStandard::lang_c99;
-- 
2.40.0