Assume `__cxa_allocate_exception` returns an under-aligned memory on

Darwin if the version of libc++abi isn't new enough to include the fix
in r319123

This patch resurrects r264998, which was committed to work around a bug
in libc++abi that was causing _cxa_allocate_exception to return a memory
that wasn't double-word aligned.

http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20160328/154332.html

In addition, this patch makes clang issue a warning if the type of the
thrown object requires an alignment that is larger than the minimum
guaranteed by the target C++ runtime.

rdar://problem/49864414

Differential Revision: https://reviews.llvm.org/D61667

llvm-svn: 360404

1 files changed, 1 insertions, 14 deletions

diff --git a/clang/lib/CodeGen/ItaniumCXXABI.cpp b/clang/lib/CodeGen/ItaniumCXXABI.cpp
index d46130432b8..3fac8d60fd2 100644
--- a/clang/lib/CodeGen/ItaniumCXXABI.cpp
+++ b/clang/lib/CodeGen/ItaniumCXXABI.cpp
@@ -154,19 +154,6 @@ public:
                                Address Ptr, QualType ElementType,
                                const CXXDestructorDecl *Dtor) override;
 
-  /// Itanium says that an _Unwind_Exception has to be "double-word"
-  /// aligned (and thus the end of it is also so-aligned), meaning 16
-  /// bytes.  Of course, that was written for the actual Itanium,
-  /// which is a 64-bit platform.  Classically, the ABI doesn't really
-  /// specify the alignment on other platforms, but in practice
-  /// libUnwind declares the struct with __attribute__((aligned)), so
-  /// we assume that alignment here.  (It's generally 16 bytes, but
-  /// some targets overwrite it.)
-  CharUnits getAlignmentOfExnObject() {
-    auto align = CGM.getContext().getTargetDefaultAlignForAttributeAligned();
-    return CGM.getContext().toCharUnitsFromBits(align);
-  }
-
   void emitRethrow(CodeGenFunction &CGF, bool isNoReturn) override;
   void emitThrow(CodeGenFunction &CGF, const CXXThrowExpr *E) override;
 
@@ -1191,7 +1178,7 @@ void ItaniumCXXABI::emitThrow(CodeGenFunction &CGF, const CXXThrowExpr *E) {
   llvm::CallInst *ExceptionPtr = CGF.EmitNounwindRuntimeCall(
       AllocExceptionFn, llvm::ConstantInt::get(SizeTy, TypeSize), "exception");
 
-  CharUnits ExnAlign = getAlignmentOfExnObject();
+  CharUnits ExnAlign = CGF.getContext().getExnObjectAlignment();
   CGF.EmitAnyExprToExn(E->getSubExpr(), Address(ExceptionPtr, ExnAlign));
 
   // Now throw the exception.