llvm-project/mlir/docs/SymbolsAndSymbolTables.md

Symbols and Symbol Tables

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With Regions, the multi-level aspect of MLIR is structural in the IR. A lot of infrastructure within the compiler is built around this nesting structure; including the processing of operations within the pass manager. One advantage of the MLIR design is that it is able to process operations in parallel, utilizing multiple threads. This is possible due to a property of the IR known as IsolatedFromAbove.

Without this property, any operation could affect or mutate the use-list of operations defined above. Making this thread-safe requires expensive locking in some of the core IR data structures, which becomes quite inefficient. To enable multi-threaded compilation without this locking, MLIR uses local pools for constant values as well as Symbol accesses for global values and variables. This document details the design of Symbols, what they are and how they fit into the system.

The Symbol infrastructure essentially provides a non-SSA mechanism in which to refer to an operation symbolically with a name. This allows for referring to operations defined above regions that were defined as IsolatedFromAbove in a safe way. It also allows for symbolically referencing operations define below other regions as well.

Symbol

A Symbol is a named operation that resides immediately within a region that defines a SymbolTable. The name of a symbol must be unique within the parent SymbolTable. This name is semantically similarly to an SSA result value, and may be referred to by other operations to provide a symbolic link, or use, to the symbol. An example of a Symbol operation is func. func defines a symbol name, which is referred to by operations like std.call.

Defining a Symbol

A Symbol operation should use the SymbolOpInterface interface to provide the necessary verification and accessors; it also supports operations, such as module, that conditionally define a symbol. Symbols must have the following properties:

• A StringAttr attribute named 'SymbolTable::getSymbolAttrName()'(sym_name).
  • This attribute defines the symbolic 'name' of the operation.
• An optional StringAttr attribute named 'SymbolTable::getVisibilityAttrName()'(sym_visibility)
  • This attribute defines the visibility of the symbol, or more specifically in-which scopes it may be accessed.
• No SSA results
  • Intermixing the different ways to use an operation quickly becomes unwieldy and difficult to analyze.

Symbol Table

Described above are Symbols, which reside within a region of an operation defining a SymbolTable. A SymbolTable operation provides the container for the Symbol operations. It verifies that all Symbol operations have a unique name, and provides facilities for looking up symbols by name. Operations defining a SymbolTable must use the OpTrait::SymbolTable trait.

Referencing a Symbol

Symbols are referenced symbolically by name via the SymbolRefAttr attribute. A symbol reference attribute contains a named reference to an operation that is nested within a symbol table. It may optionally contain a set of nested references that further resolve to a symbol nested within a different symbol table. When resolving a nested reference, each non-leaf reference must refer to a symbol operation that is also a symbol table.

Below is an example of how an operation can reference a symbol operation:

// This `func` operation defines a symbol named `symbol`.
func @symbol()

// Our `foo.user` operation contains a SymbolRefAttr with the name of the
// `symbol` func.
"foo.user"() {uses = [@symbol]} : () -> ()

// Symbol references resolve to the nearest parent operation that defines a
// symbol table, so we can have references with arbitrary nesting levels.
func @other_symbol() {
  affine.for %i0 = 0 to 10 {
    // Our `foo.user` operation resolves to the same `symbol` func as defined
    // above.
    "foo.user"() {uses = [@symbol]} : () -> ()
  }
  return
}

// Here we define a nested symbol table. References within this operation will
// not resolve to any symbols defined above.
module {
  // Error. We resolve references with respect to the closest parent operation
  // that defines a symbol table, so this reference can't be resolved.
  "foo.user"() {uses = [@symbol]} : () -> ()
}

// Here we define another nested symbol table, except this time it also defines
// a symbol.
module @module_symbol {
  // This `func` operation defines a symbol named `nested_symbol`.
  func @nested_symbol()
}

// Our `foo.user` operation may refer to the nested symbol, by resolving through
// the parent.
"foo.user"() {uses = [@module_symbol::@nested_symbol]} : () -> ()

Using an attribute, as opposed to an SSA value, has several benefits:

• References may appear in more places than the operand list; including nested attribute dictionaries, array attributes, etc.

• Handling of SSA dominance remains unchanged.

  • If we were to use SSA values, we would need to create some mechanism in which to opt-out of certain properties of it such as dominance. Attributes allow for referencing the operations irregardless of the order in which they were defined.
  • Attributes simplify referencing operations within nested symbol tables, which are traditionally not visible outside of the parent region.

The impact of this choice to use attributes as opposed to SSA values is that we now have two mechanisms with reference operations. This means that some dialects must either support both SymbolRefs and SSA value references, or provide operations that materialize SSA values from a symbol reference. Each has different trade offs depending on the situation. A function call may directly use a SymbolRef as the callee, whereas a reference to a global variable might use a materialization operation so that the variable can be used in other operations like std.addi. llvm.mlir.addressof is one example of such an operation.

See the LangRef definition of the SymbolRefAttr for more information about the structure of this attribute.

Operations that reference a Symbol and want to perform verification and general mutation of the symbol should implement the SymbolUserOpInterface to ensure that symbol accesses are legal and efficient.

Manipulating a Symbol

As described above, SymbolRefs act as an auxiliary way of defining uses of operations to the traditional SSA use-list. As such, it is imperative to provide similar functionality to manipulate and inspect the list of uses and the users. The following are a few of the utilities provided by the SymbolTable:

• SymbolTable::getSymbolUses

  • Access an iterator range over all of the uses on and nested within a particular operation.

• SymbolTable::symbolKnownUseEmpty

  • Check if a particular symbol is known to be unused within a specific section of the IR.

• SymbolTable::replaceAllSymbolUses

  • Replace all of the uses of one symbol with a new one within a specific section of the IR.

• SymbolTable::lookupNearestSymbolFrom

  • Lookup the definition of a symbol in the nearest symbol table from some anchor operation.

Symbol Visibility

Along with a name, a Symbol also has a visibility attached to it. The visibility of a symbol defines its structural reachability within the IR. A symbol has one of the following visibilities:

• Public (Default)

  • The symbol may be referenced from outside of the visible IR. We cannot assume that all of the uses of this symbol are observable.

• Private

  • The symbol may only be referenced from within the current symbol table.

• Nested

  • The symbol may be referenced by operations outside of the current symbol table, but not outside of the visible IR, as long as each symbol table parent also defines a non-private symbol.

A few examples of what this looks like in the IR are shown below:

module @public_module {
  // This function can be accessed by 'live.user', but cannot be referenced
  // externally; all uses are known to reside within parent regions.
  func @nested_function() attributes { sym_visibility = "nested" }

  // This function cannot be accessed outside of 'public_module'.
  func @private_function() attributes { sym_visibility = "private" }
}

// This function can only be accessed from within the top-level module.
func @private_function() attributes { sym_visibility = "private" }

// This function may be referenced externally.
func @public_function()

"live.user"() {uses = [
  @public_module::@nested_function,
  @private_function,
  @public_function
]} : () -> ()