[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.
+  if (assignOp->isMemcpyEquivalentSpecialMember(getContext())) {
+    CanQualType recordTy =
+        getContext().getCanonicalTagType(assignOp->getParent());
+    LValue dest = makeNaturalAlignAddrLValue(loadCXXThis(), recordTy);
+    // The source is the trailing reference parameter; load it to get the
+    // referent's address before copying (mirrors the copy-constructor path).
+    mlir::Value srcPtr = builder.createLoad(getLoc(assignOp->getLocation()),
+                                            getAddrOfLocalVar(args.back()));
+    LValue src = makeNaturalAlignAddrLValue(srcPtr, recordTy);
+    emitAggregateAssign(dest, src, recordTy);
+
+    for (Stmt *s : rootCS->body())
+      if (isa<ReturnStmt>(s))
+        if (emitStmt(s, /*useCurrentScope=*/true).failed())
+          cgm.errorNYI(s->getSourceRange(),
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andykaylor wrote:

This isn't actually something that's not implemented. It's just an error. In practice, it shouldn't happen, and if it does happen because of something that's NYI, we should have emitted an error earlier. You should just call `cgm.error()` here and give it a more specific message.

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