PR18097: Support initializing an _Atomic(T) from an object of C++ class type T

or a class derived from T. We already supported this when initializing
_Atomic(T) from T for most (and maybe all) other reasonable values of T.

llvm-svn: 214390

1 files changed, 19 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index b2f08c32d92..b508dcb446f 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -1045,6 +1045,25 @@ llvm::Constant *CodeGenModule::EmitConstantExpr(const Expr *E,
 llvm::Constant *CodeGenModule::EmitConstantValue(const APValue &Value,
                                                  QualType DestType,
                                                  CodeGenFunction *CGF) {
+  // For an _Atomic-qualified constant, we may need to add tail padding.
+  if (auto *AT = DestType->getAs<AtomicType>()) {
+    QualType InnerType = AT->getValueType();
+    auto *Inner = EmitConstantValue(Value, InnerType, CGF);
+
+    uint64_t InnerSize = Context.getTypeSize(InnerType);
+    uint64_t OuterSize = Context.getTypeSize(DestType);
+    if (InnerSize == OuterSize)
+      return Inner;
+
+    assert(InnerSize < OuterSize && "emitted over-large constant for atomic");
+    llvm::Constant *Elts[] = {
+      Inner,
+      llvm::ConstantAggregateZero::get(
+          llvm::ArrayType::get(Int8Ty, (OuterSize - InnerSize) / 8))
+    };
+    return llvm::ConstantStruct::getAnon(Elts);
+  }
+
   switch (Value.getKind()) {
   case APValue::Uninitialized:
     llvm_unreachable("Constant expressions should be initialized.");