[clang] [CIR] Inline trivial copy/move assignment at call sites (PR #198918)

[Andy Kaylor via cfe-commits]

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@@ -901,6 +901,33 @@ void CIRGenFunction::emitImplicitAssignmentOperatorBody(FunctionArgList &args) {
   assert(!cir::MissingFeatures::incrementProfileCounter());
   assert(!cir::MissingFeatures::runCleanupsScope());
 
+  // For a memcpy-equivalent special member (a union, or a trivially-copyable
+  // record) the synthesized body either copies nothing -- a union body is just
+  // `return *this` -- or relies on a memcpy the AST does not spell out as
+  // field assignments.  Mirror classic CodeGen's AssignmentMemcpyizer: copy
+  // the whole object once, then fall through to emit the trailing
+  // `return *this`.  Emitting the copy but skipping the return would leave the
+  // result reference uninitialized.
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andykaylor wrote:

```suggestion
  // For a memcpy-equivalent assignment operator, copy the whole object,
  // then fall through to emit the trailing `return *this`.
```

https://github.com/llvm/llvm-project/pull/198918