================
@@ -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(),
+ std::string("emitImplicitAssignmentOperatorBody: ") +
+ s->getStmtClassName());
----------------
andykaylor wrote:
I thought the whole point of this change was that the AST didn't contain the
necessary expressions. For example, for the union test case, I'm seeing this:
```
| |-CXXMethodDecl <line:1:7> col:7 implicit used constexpr operator= 'YYSTYPE
&(const YYSTYPE &) noexcept' inline default trivial external-linkage
| | |-ParmVarDecl <col:7> col:7 'const YYSTYPE &'
| | `-CompoundStmt <col:7>
| | `-ReturnStmt <col:7>
| | `-UnaryOperator <col:7> 'YYSTYPE' lvalue prefix '*' cannot overflow
| | `-CXXThisExpr <col:7> 'YYSTYPE *' this
```
What I'm wondering is whether the real fix for this should be in the AST rather
than in CIR codegen. I do see that the array case you added contains a builtin
memcpy call, but that just reinforces my belief that the union case should have
a more complete body in the AST. The fact that we're assuming that an aggregate
copy is needed in all cases and ignoring something the AST provided in a subset
of those cases just seems entirely wrong.
https://github.com/llvm/llvm-project/pull/198918
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