[clang] Add -fc++-abi= flag for specifying which C++ ABI to use

This implements the flag proposed in RFC http://lists.llvm.org/pipermail/cfe-dev/2020-August/066437.html.

The goal is to add a way to override the default target C++ ABI through
a compiler flag. This makes it easier to test and transition between different
C++ ABIs through compile flags rather than build flags.

In this patch:
- Store `-fc++-abi=` in a LangOpt. This isn't stored in a
  CodeGenOpt because there are instances outside of codegen where Clang
  needs to know what the ABI is (particularly through
  ASTContext::createCXXABI), and we should be able to override the
  target default if the flag is provided at that point.
- Expose the existing ABIs in TargetCXXABI as values that can be passed
  through this flag.
  - Create a .def file for these ABIs to make it easier to check flag
    values.
  - Add an error for diagnosing bad ABI flag values.

Differential Revision: https://reviews.llvm.org/D85802

1 files changed, 9 insertions, 0 deletions

diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index a4c56cc..13320eb 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -3513,6 +3513,15 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
       Args.hasFlag(OPT_fexperimental_relative_cxx_abi_vtables,
                    OPT_fno_experimental_relative_cxx_abi_vtables,
                    /*default=*/false);
+
+  // The value can be empty, which indicates the system default should be used.
+  StringRef CXXABI = Args.getLastArgValue(OPT_fcxx_abi_EQ);
+  if (!CXXABI.empty()) {
+    if (!TargetCXXABI::isABI(CXXABI))
+      Diags.Report(diag::err_invalid_cxx_abi) << CXXABI;
+    else
+      Opts.CXXABI = TargetCXXABI::getKind(CXXABI);
+  }
 }
 
 static bool isStrictlyPreprocessorAction(frontend::ActionKind Action) {