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, 13 insertions, 2 deletions

diff --git a/clang/lib/CodeGen/ItaniumCXXABI.cpp b/clang/lib/CodeGen/ItaniumCXXABI.cpp
index 5c33f8c1afa..d78c032cec8 100644
--- a/clang/lib/CodeGen/ItaniumCXXABI.cpp
+++ b/clang/lib/CodeGen/ItaniumCXXABI.cpp
@@ -62,9 +62,20 @@ public:
 
   bool classifyReturnType(CGFunctionInfo &FI) const override;
 
+  bool passClassIndirect(const CXXRecordDecl *RD) const {
+    // Clang <= 4 used the pre-C++11 rule, which ignores move operations.
+    // The PS4 platform ABI follows the behavior of Clang 3.2.
+    if (CGM.getCodeGenOpts().getClangABICompat() <=
+            CodeGenOptions::ClangABI::Ver4 ||
+        CGM.getTriple().getOS() == llvm::Triple::PS4)
+      return RD->hasNonTrivialDestructor() ||
+             RD->hasNonTrivialCopyConstructor();
+    return !canCopyArgument(RD);
+  }
+
   RecordArgABI getRecordArgABI(const CXXRecordDecl *RD) const override {
     // If C++ prohibits us from making a copy, pass by address.
-    if (!canCopyArgument(RD))
+    if (passClassIndirect(RD))
       return RAA_Indirect;
     return RAA_Default;
   }
@@ -1012,7 +1023,7 @@ bool ItaniumCXXABI::classifyReturnType(CGFunctionInfo &FI) const {
     return false;
 
   // If C++ prohibits us from making a copy, return by address.
-  if (!canCopyArgument(RD)) {
+  if (passClassIndirect(RD)) {
     auto Align = CGM.getContext().getTypeAlignInChars(FI.getReturnType());
     FI.getReturnInfo() = ABIArgInfo::getIndirect(Align, /*ByVal=*/false);
     return true;