Add support for nested symbol references.

This change allows for adding additional nested references to a SymbolRefAttr to allow for further resolving a symbol if that symbol also defines a SymbolTable. If a referenced symbol also defines a symbol table, a nested reference can be used to refer to a symbol within that table. Nested references are printed after the main reference in the following form:

  symbol-ref-attribute ::= symbol-ref-id (`::` symbol-ref-id)*

Example:

  module @reference {
    func @nested_reference()
  }

  my_reference_op @reference::@nested_reference

Given that SymbolRefAttr is now more general, the existing functionality centered around a single reference is moved to a derived class FlatSymbolRefAttr. Followup commits will add support to lookups, rauw, etc. for scoped references.

PiperOrigin-RevId: 279860501

1 files changed, 4 insertions, 2 deletions

diff --git a/mlir/g3doc/LangRef.md b/mlir/g3doc/LangRef.md
index 391f77325d4..3409b9fac83 100644
--- a/mlir/g3doc/LangRef.md
+++ b/mlir/g3doc/LangRef.md
@@ -1367,13 +1367,15 @@ A string attribute is an attribute that represents a string literal value.
 Syntax:
 
 ``` {.ebnf}
-symbol-ref-attribute ::= symbol-ref-id
+symbol-ref-attribute ::= symbol-ref-id (`::` symbol-ref-id)*
 ```
 
 A symbol reference attribute is a literal attribute that represents a named
 reference to an operation that is nested within an operation with the
 `OpTrait::SymbolTable` trait. As such, this reference is given meaning by the
-nearest parent operation containing the `OpTrait::SymbolTable` trait.
+nearest parent operation containing the `OpTrait::SymbolTable` trait. It may
+optionally contain a set of nested references that further resolve to a symbol
+nested within a different symbol table.
 
 This attribute can only be held internally by
 [array attributes](#array-attribute) and