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, 28 insertions, 5 deletions

diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 22b830ee838..154e3a86429 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -4875,13 +4875,36 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     CmdArgs.push_back("-fuse-line-directives");
 
   // -fms-compatibility=0 is default.
-  if (Args.hasFlag(options::OPT_fms_compatibility,
-                   options::OPT_fno_ms_compatibility,
-                   (IsWindowsMSVC &&
-                    Args.hasFlag(options::OPT_fms_extensions,
-                                 options::OPT_fno_ms_extensions, true))))
+  bool IsMSVCCompat = Args.hasFlag(
+      options::OPT_fms_compatibility, options::OPT_fno_ms_compatibility,
+      (IsWindowsMSVC && Args.hasFlag(options::OPT_fms_extensions,
+                                     options::OPT_fno_ms_extensions, true)));
+  if (IsMSVCCompat)
     CmdArgs.push_back("-fms-compatibility");
 
+  // Handle -fgcc-version, if present.
+  VersionTuple GNUCVer;
+  if (Arg *A = Args.getLastArg(options::OPT_fgnuc_version_EQ)) {
+    // Check that the version has 1 to 3 components and the minor and patch
+    // versions fit in two decimal digits.
+    StringRef Val = A->getValue();
+    Val = Val.empty() ? "0" : Val; // Treat "" as 0 or disable.
+    bool Invalid = GNUCVer.tryParse(Val);
+    unsigned Minor = GNUCVer.getMinor().getValueOr(0);
+    unsigned Patch = GNUCVer.getSubminor().getValueOr(0);
+    if (Invalid || GNUCVer.getBuild() || Minor >= 100 || Patch >= 100) {
+      D.Diag(diag::err_drv_invalid_value)
+          << A->getAsString(Args) << A->getValue();
+    }
+  } else if (!IsMSVCCompat) {
+    // Imitate GCC 4.2.1 by default if -fms-compatibility is not in effect.
+    GNUCVer = VersionTuple(4, 2, 1);
+  }
+  if (!GNUCVer.empty()) {
+    CmdArgs.push_back(
+        Args.MakeArgString("-fgnuc-version=" + GNUCVer.getAsString()));
+  }
+
   VersionTuple MSVT = TC.computeMSVCVersion(&D, Args);
   if (!MSVT.empty())
     CmdArgs.push_back(