Add flag to request Clang is ABI-compatible with older versions of itself

This patch adds a flag -fclang-abi-compat that can be used to request that
Clang attempts to be ABI-compatible with some older version of itself.

This is provided on a best-effort basis; right now, this can be used to undo
the ABI change in r310401, reverting Clang to its prior C++ ABI for pass/return
by value of class types affected by that change, and to undo the ABI change in
r262688, reverting Clang to using integer registers rather than SSE registers
for passing <1 x long long> vectors. The intent is that we will maintain this
backwards compatibility path as we make ABI-breaking fixes in future.

The reversion to the old behavior for r310401 is also applied to the PS4 target
since that change is not part of its platform ABI (which is essentially to do
whatever Clang 3.2 did).

llvm-svn: 311823

1 files changed, 12 insertions, 3 deletions

diff --git a/clang/lib/CodeGen/TargetInfo.cpp b/clang/lib/CodeGen/TargetInfo.cpp
index 64ee31f..77b9b4c 100644
--- a/clang/lib/CodeGen/TargetInfo.cpp
+++ b/clang/lib/CodeGen/TargetInfo.cpp
@@ -184,7 +184,11 @@ const TargetInfo &ABIInfo::getTarget() const {
   return CGT.getTarget();
 }
 
-bool ABIInfo:: isAndroid() const { return getTarget().getTriple().isAndroid(); }
+const CodeGenOptions &ABIInfo::getCodeGenOpts() const {
+  return CGT.getCodeGenOpts();
+}
+
+bool ABIInfo::isAndroid() const { return getTarget().getTriple().isAndroid(); }
 
 bool ABIInfo::isHomogeneousAggregateBaseType(QualType Ty) const {
   return false;
@@ -2112,9 +2116,14 @@ class X86_64ABIInfo : public SwiftABIInfo {
     return !getTarget().getTriple().isOSDarwin();
   }
 
-  /// GCC classifies <1 x long long> as SSE but compatibility with older clang
-  // compilers require us to classify it as INTEGER.
+  /// GCC classifies <1 x long long> as SSE but some platform ABIs choose to
+  /// classify it as INTEGER (for compatibility with older clang compilers).
   bool classifyIntegerMMXAsSSE() const {
+    // Clang <= 3.8 did not do this.
+    if (getCodeGenOpts().getClangABICompat() <=
+        CodeGenOptions::ClangABI::Ver3_8)
+      return false;
+
     const llvm::Triple &Triple = getTarget().getTriple();
     if (Triple.isOSDarwin() || Triple.getOS() == llvm::Triple::PS4)
       return false;