Julia ASTs

Julia has two representations of code. First there is a surface syntax AST returned by the parser (e.g. the 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.

• 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.

• LineNumberNode

  Contains a single number, specifying the line number the next statement came from.

• LabelNode

  Branch target, a consecutively-numbered integer starting at 0.

• GotoNode

  Unconditional branch.

• 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 0) static single assignment (SSA) variable inserted by the compiler.

• NewvarNode

  Marks a point where a variable 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.

• line

  Line number and file name metadata. Unlike a LineNumberNode, can also contain a file name.

• gotoifnot

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

• =

  Assignment.

• 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. This is the value of the run time system variable jl_exception_in_transit.

• enter

  Enters an exception handler (setjmp). args[1] is the label of the catch block to jump to on error.

• leave

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

• 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

  Indicates the beginning or end of a section of code that performs a bounds check. Like inbounds, a stack is maintained, and the second argument can be one of: true, false, or :pop.

• 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.

  • :push_loc: enters a sequence of statements from a specified source location.

    • args[2] specifies a filename, as a symbol.

    • args[3] optionally specifies the name of an (inlined) function that originally contained the code.

  • :pop_loc: returns to the source location before the matching :push_loc.

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 #undef, if this is a top-level Lambda that 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 - JL_CALLABLE jl_value_t *(*)(jl_function_t *f, jl_value_t *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.

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

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) "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
    f
end

parses as:

(if a (block (line 2) b)
    (block (line 3) (if c (block (line 4) d)
                        (block (line 5) e (line 6) f))))

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 body (= var1 val1) (= var2 val2) ...).

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

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

parses as:

(function (call (curly f T) (parameters (kw k 1))
                (:: x T))
          (block (line 2 file.jl) (return (call + x 1))))

Type definition:

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

parses as:

(type #t (curly Foo (<: T S))
      (block (line 2 none) (:: 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.