[clang] [CIR] Upstream support for operator assign (PR #145979)

[Erich Keane via cfe-commits]

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@@ -85,23 +85,24 @@ RValue CIRGenFunction::emitCXXMemberOrOperatorMemberCallExpr(
     return RValue::get(nullptr);
   }
 
-  bool trivialForCodegen =
-      md->isTrivial() || (md->isDefaulted() && md->getParent()->isUnion());
-  bool trivialAssignment =
-      trivialForCodegen &&
-      (md->isCopyAssignmentOperator() || md->isMoveAssignmentOperator()) &&
-      !md->getParent()->mayInsertExtraPadding();
-  (void)trivialAssignment;
+  // Note on trivial assignment
+  // --------------------------
+  // Classic codegen avoids generating the trivial copy/move assignment operator
+  // when it isn't necessary, choosing instead to just produce IR with an
+  // equivalent effect. We have chosen not to do that in CIR, instead emitting
+  // trivial copy/move assignment operators and allowing later transformations
+  // to optimize them away if appropriate.
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erichkeane wrote:

I like this FWIW, and I think we should do MORE of this if we can get away with it.  The 'premature optimizations' in the FE are only there because LLVM-IR isn't expressive enough to make it clear it is legal.  CIR should make sure it is expressive enough for these situations.

https://github.com/llvm/llvm-project/pull/145979