Julia ASTs

Julia has two representations of code. First there is a surface syntax AST returned by the parser (e.g. the Meta.parse function), and manipulated by macros. It is a structured representation of code as it is written, constructed by julia-parser.scm from a character stream. Next there is a lowered form, or IR (intermediate representation), which is used by type inference and code generation. In the lowered form there are fewer types of nodes, all macros are expanded, and all control flow is converted to explicit branches and sequences of statements. The lowered form is constructed by julia-syntax.scm.

First we will focus on the lowered form, since it is more important to the compiler. It is also less obvious to the human, since it results from a significant rearrangement of the input syntax.

Lowered form

The following data types exist in lowered form:

• Expr

  Has a node type indicated by the head field, and an args field which is a Vector{Any} of subexpressions. While almost every part of a surface AST is represented by an Expr, the IR uses only a limited number of Exprs, mostly for calls, conditional branches (gotoifnot), and returns.

• Slot

  Identifies arguments and local variables by consecutive numbering. Slot is an abstract type with subtypes SlotNumber and TypedSlot. Both types have an integer-valued id field giving the slot index. Most slots have the same type at all uses, and so are represented with SlotNumber. The types of these slots are found in the slottypes field of their MethodInstance object. Slots that require per-use type annotations are represented with TypedSlot, which has a typ field.

• CodeInfo

  Wraps the IR of a method. Its code field is an array of expressions to execute.

• GotoNode

  Unconditional branch. The argument is the branch target, represented as an index in the code array to jump to.

• QuoteNode

  Wraps an arbitrary value to reference as data. For example, the function f() = :a contains a QuoteNode whose value field is the symbol a, in order to return the symbol itself instead of evaluating it.

• GlobalRef

  Refers to global variable name in module mod.

• SSAValue

  Refers to a consecutively-numbered (starting at 1) static single assignment (SSA) variable inserted by the compiler. The number (id) of an SSAValue is the code array index of the expression whose value it represents.

• NewvarNode

  Marks a point where a variable (slot) is created. This has the effect of resetting a variable to undefined.

Expr types

These symbols appear in the head field of Exprs in lowered form.

• call

  Function call (dynamic dispatch). args[1] is the function to call, args[2:end] are the arguments.

• invoke

  Function call (static dispatch). args[1] is the MethodInstance to call, args[2:end] are the arguments (including the function that is being called, at args[2]).

• static_parameter

  Reference a static parameter by index.

• gotoifnot

  Conditional branch. If args[1] is false, goes to the index identified in args[2].

• =

  Assignment. In the IR, the first argument is always a Slot or a GlobalRef.

• method

  Adds a method to a generic function and assigns the result if necessary.

  Has a 1-argument form and a 4-argument form. The 1-argument form arises from the syntax function foo end. In the 1-argument form, the argument is a symbol. If this symbol already names a function in the current scope, nothing happens. If the symbol is undefined, a new function is created and assigned to the identifier specified by the symbol. If the symbol is defined but names a non-function, an error is raised. The definition of “names a function” is that the binding is constant, and refers to an object of singleton type. The rationale for this is that an instance of a singleton type uniquely identifies the type to add the method to. When the type has fields, it wouldn’t be clear whether the method was being added to the instance or its type.

  The 4-argument form has the following arguments:

  • args[1]

    A function name, or false if unknown. If a symbol, then the expression first behaves like the 1-argument form above. This argument is ignored from then on. When this is false, it means a method is being added strictly by type, (::T)(x) = x.

  • args[2]

    A SimpleVector of argument type data. args[2][1] is a SimpleVector of the argument types, and args[2][2] is a SimpleVector of type variables corresponding to the method’s static parameters.

  • args[3]

    A CodeInfo of the method itself. For “out of scope” method definitions (adding a method to a function that also has methods defined in different scopes) this is an expression that evaluates to a :lambda expression.

  • args[4]

    true or false, identifying whether the method is staged (@generated function).

• const

  Declares a (global) variable as constant.

• null

  Has no arguments; simply yields the value nothing.

• new

  Allocates a new struct-like object. First argument is the type. The new pseudo-function is lowered to this, and the type is always inserted by the compiler. This is very much an internal-only feature, and does no checking. Evaluating arbitrary new expressions can easily segfault.

• return

  Returns its argument as the value of the enclosing function.

• the_exception

  Yields the caught exception inside a catch block, as returned by jl_current_exception().

• enter

  Enters an exception handler (setjmp). args[1] is the label of the catch block to jump to on error. Yields a token which is consumed by pop_exception.

• leave

  Pop exception handlers. args[1] is the number of handlers to pop.

• pop_exception

  Pop the stack of current exceptions back to the state at the associated enter when leaving a catch block. args[1] contains the token from the associated enter.

  Julia 1.1

  pop_exception is new in Julia 1.1.

• inbounds

  Controls turning bounds checks on or off. A stack is maintained; if the first argument of this expression is true or false (true means bounds checks are disabled), it is pushed onto the stack. If the first argument is :pop, the stack is popped.

• boundscheck

  Has the value false if inlined into a section of code marked with @inbounds, otherwise has the value true.

• copyast

  Part of the implementation of quasi-quote. The argument is a surface syntax AST that is simply copied recursively and returned at run time.

• meta

  Metadata. args[1] is typically a symbol specifying the kind of metadata, and the rest of the arguments are free-form. The following kinds of metadata are commonly used:

  • :inline and :noinline: Inlining hints.

Method

A unique’d container describing the shared metadata for a single method.

• name, module, file, line, sig

  Metadata to uniquely identify the method for the computer and the human.

• ambig

  Cache of other methods that may be ambiguous with this one.

• specializations

  Cache of all MethodInstance ever created for this Method, used to ensure uniqueness. Uniqueness is required for efficiency, especially for incremental precompile and tracking of method invalidation.

• source

  The original source code (usually compressed).

• roots

  Pointers to non-AST things that have been interpolated into the AST, required by compression of the AST, type-inference, or the generation of native code.

• nargs, isva, called, isstaged, pure

  Descriptive bit-fields for the source code of this Method.

• min_world / max_world

  The range of world ages for which this method is visible to dispatch.

MethodInstance

A unique’d container describing a single callable signature for a Method. See especially Proper maintenance and care of multi-threading locks for important details on how to modify these fields safely.

• specTypes

  The primary key for this MethodInstance. Uniqueness is guaranteed through a def.specializations lookup.

• def

  The Method that this function describes a specialization of. Or a Module, if this is a top-level Lambda expanded in Module, and which is not part of a Method.

• sparam_vals

  The values of the static parameters in specTypes indexed by def.sparam_syms. For the MethodInstance at Method.unspecialized, this is the empty SimpleVector. But for a runtime MethodInstance from the MethodTable cache, this will always be defined and indexable.

• rettype

  The inferred return type for the specFunctionObject field, which (in most cases) is also the computed return type for the function in general.

• inferred

  May contain a cache of the inferred source for this function, or other information about the inference result such as a constant return value may be put here (if jlcall_api == 2), or it could be set to nothing to just indicate rettype is inferred.

• ftpr

  The generic jlcall entry point.

• jlcall_api

  The ABI to use when calling fptr. Some significant ones include:

  • 0 - Not compiled yet
  • 1 - JLCALLABLE `jlvaluet ()(jlfunctiont \f, jlvaluet *args[nargs], uint32*t nargs)`
  • 2 - Constant (value stored in inferred)
  • 3 - With Static-parameters forwarded jl_value_t *(*)(jl_svec_t *sparams, jl_function_t *f, jl_value_t *args[nargs], uint32_t nargs)
  • 4 - Run in interpreter jl_value_t *(*)(jl_method_instance_t *meth, jl_function_t *f, jl_value_t *args[nargs], uint32_t nargs)

• min_world / max_world

  The range of world ages for which this method instance is valid to be called.

CodeInfo

A temporary container for holding lowered source code.

• code

  An Any array of statements

• slotnames

  An array of symbols giving the name of each slot (argument or local variable).

• slottypes

  An array of types for the slots.

• slotflags

  A UInt8 array of slot properties, represented as bit flags:

  • 2 - assigned (only false if there are no assignment statements with this var on the left)
  • 8 - const (currently unused for local variables)
  • 16 - statically assigned once
  • 32 - might be used before assigned. This flag is only valid after type inference.

• ssavaluetypes

  Either an array or an Int.

  If an Int, it gives the number of compiler-inserted temporary locations in the function. If an array, specifies a type for each location.

• linetable

  An array of source location objects

• codelocs

  An array of integer indices into the linetable, giving the location associated with each statement.

Boolean properties:

• inferred

  Whether this has been produced by type inference.

• inlineable

  Whether this should be inlined.

• propagate_inbounds

  Whether this should should propagate @inbounds when inlined for the purpose of eliding @boundscheck blocks.

• pure

  Whether this is known to be a pure function of its arguments, without respect to the state of the method caches or other mutable global state.

Surface syntax AST

Front end ASTs consist almost entirely of Exprs and atoms (e.g. symbols, numbers). There is generally a different expression head for each visually distinct syntactic form. Examples will be given in s-expression syntax. Each parenthesized list corresponds to an Expr, where the first element is the head. For example (call f x) corresponds to Expr(:call, :f, :x) in Julia.

Calls

do syntax:

f(x) do a,b
    body
end

parses as (do (call f x) (-> (tuple a b) (block body))).

Operators

Most uses of operators are just function calls, so they are parsed with the head call. However some operators are special forms (not necessarily function calls), and in those cases the operator itself is the expression head. In julia-parser.scm these are referred to as “syntactic operators”. Some operators (+ and *) use N-ary parsing; chained calls are parsed as a single N-argument call. Finally, chains of comparisons have their own special expression structure.

Bracketed forms

Macros

Strings

Doc string syntax:

"some docs"
f(x) = x

parses as (macrocall (|.| Core '@doc) (line) "some docs" (= (call f x) (block x))).

Imports and such

Numbers

Julia supports more number types than many scheme implementations, so not all numbers are represented directly as scheme numbers in the AST.

Block forms

A block of statements is parsed as (block stmt1 stmt2 ...).

If statement:

if a
    b
elseif c
    d
else
    e
end

parses as:

(if a (block (line 2) b)
    (elseif (block (line 3) c) (block (line 4) d)
            (block (line 5 e))))

A while loop parses as (while condition body).

A for loop parses as (for (= var iter) body). If there is more than one iteration specification, they are parsed as a block: (for (block (= v1 iter1) (= v2 iter2)) body).

break and continue are parsed as 0-argument expressions (break) and (continue).

let is parsed as (let (= var val) body) or (let (block (= var1 val1) (= var2 val2) ...) body), like for loops.

A basic function definition is parsed as (function (call f x) body). A more complex example:

function f(x::T; k = 1) where T
    return x+1
end

parses as:

(function (where (call f (parameters (kw k 1))
                       (:: x T))
                 T)
          (block (line 2) (return (call + x 1))))

Type definition:

mutable struct Foo{T<:S}
    x::T
end

parses as:

(struct true (curly Foo (<: T S))
        (block (line 2) (:: x T)))

The first argument is a boolean telling whether the type is mutable.

try blocks parse as (try try_block var catch_block finally_block). If no variable is present after catch, var is #f. If there is no finally clause, then the last argument is not present.

Quote expressions

Julia source syntax forms for code quoting (quote and :( )) support interpolation with $. In Lisp terminology, this means they are actually “backquote” or “quasiquote” forms. Internally, there is also a need for code quoting without interpolation. In Julia’s scheme code, non-interpolating quote is represented with the expression head inert.

inert expressions are converted to Julia QuoteNode objects. These objects wrap a single value of any type, and when evaluated simply return that value.

A quote expression whose argument is an atom also gets converted to a QuoteNode.

Line numbers

Source location information is represented as (line line_num file_name) where the third component is optional (and omitted when the current line number, but not file name, changes).

These expressions are represented as LineNumberNodes in Julia.

Macros

Macro hygiene is represented through the expression head pair escape and hygienic-scope. The result of a macro expansion is automatically wrapped in (hygienic-scope block module), to represent the result of the new scope. The user can insert (escape block) inside to interpolate code from the caller.