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 AST, since it is needed to write macros.
Surface syntax AST
Front end ASTs consist almost entirely of Expr
s 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
Input | AST |
---|---|
f(x) | (call f x) |
f(x, y=1, z=2) | (call f x (kw y 1) (kw z 2)) |
f(x; y=1) | (call f (parameters (kw y 1)) x) |
f(x...) | (call f (... x)) |
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.
Input | AST |
---|---|
x+y | (call + x y) |
a+b+c+d | (call + a b c d) |
2x | (call * 2 x) |
a&&b | (&& a b) |
x += 1 | (+= x 1) |
a ? 1 : 2 | (if a 1 2) |
a:b | (: a b) |
a:b:c | (: a b c) |
a,b | (tuple a b) |
a==b | (call == a b) |
1<i<=n | (comparison 1 < i <= n) |
a.b | (. a (quote b)) |
a.(b) | (. a (tuple b)) |
Bracketed forms
Input | AST |
---|---|
a[i] | (ref a i) |
t[i;j] | (typed_vcat t i j) |
t[i j] | (typed_hcat t i j) |
t[a b; c d] | (typed_vcat t (row a b) (row c d)) |
t[a b;;; c d] | (typed_ncat t 3 (row a b) (row c d)) |
a{b} | (curly a b) |
a{b;c} | (curly a (parameters c) b) |
[x] | (vect x) |
[x,y] | (vect x y) |
[x;y] | (vcat x y) |
[x y] | (hcat x y) |
[x y; z t] | (vcat (row x y) (row z t)) |
[x;y;; z;t;;;] | (ncat 3 (nrow 2 (nrow 1 x y) (nrow 1 z t))) |
[x for y in z, a in b] | (comprehension x (= y z) (= a b)) |
T[x for y in z] | (typed_comprehension T x (= y z)) |
(a, b, c) | (tuple a b c) |
(a; b; c) | (block a (block b c)) |
Macros
Input | AST |
---|---|
@m x y | (macrocall @m (line) x y) |
Base.@m x y | (macrocall (. Base (quote @m)) (line) x y) |
@Base.m x y | (macrocall (. Base (quote @m)) (line) x y) |
Strings
Input | AST |
---|---|
"a" | "a" |
x"y" | (macrocall @x_str (line) "y") |
x"y"z | (macrocall @x_str (line) "y" "z") |
"x = $x" | (string "x = " x) |
`a b c` | (macrocall @cmd (line) "a b c") |
Doc string syntax:
"some docs"
f(x) = x
parses as (macrocall (|.| Core '@doc) (line) "some docs" (= (call f x) (block x)))
.
Imports and such
Input | AST |
---|---|
import a | (import (. a)) |
import a.b.c | (import (. a b c)) |
import ...a | (import (. . . . a)) |
import a.b, c.d | (import (. a b) (. c d)) |
import Base: x | (import (: (. Base) (. x))) |
import Base: x, y | (import (: (. Base) (. x) (. y))) |
export a, b | (export a b) |
using
has the same representation as import
, but with expression head :using
instead of :import
.
Numbers
Julia supports more number types than many scheme implementations, so not all numbers are represented directly as scheme numbers in the AST.
Input | AST |
---|---|
11111111111111111111 | (macrocall @int128_str (null) "11111111111111111111") |
0xfffffffffffffffff | (macrocall @uint128_str (null) "0xfffffffffffffffff") |
1111...many digits... | (macrocall @big_str (null) "1111....") |
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 LineNumberNode
s 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.
Lowered form
Lowered form (IR) is more important to the compiler, since it is used for type inference, optimizations like inlining, and code generation. It is also less obvious to the human, since it results from a significant rearrangement of the input syntax.
In addition to Symbol
s and some number types, the following data types exist in lowered form:
Expr
Has a node type indicated by the
head
field, and anargs
field which is aVector{Any}
of subexpressions. While almost every part of a surface AST is represented by anExpr
, the IR uses only a limited number ofExpr
s, mostly for calls and some top-level-only forms.Slot
Identifies arguments and local variables by consecutive numbering.
Slot
is an abstract type with subtypesSlotNumber
andTypedSlot
. Both types have an integer-valuedid
field giving the slot index. Most slots have the same type at all uses, and so are represented withSlotNumber
. The types of these slots are found in theslottypes
field of theirCodeInfo
object. Slots that require per-use type annotations are represented withTypedSlot
, which has atyp
field.Argument
The same as
SlotNumber
, but appears only post-optimization. Indicates that the referenced slot is an argument of the enclosing function.CodeInfo
Wraps the IR of a group of statements. 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.
GotoIfNot
Conditional branch. If the
cond
field evaluates to false, goes to the index identified by thedest
field.ReturnNode
Returns its argument (the
val
field) as the value of the enclosing function. If theval
field is undefined, then this represents an unreachable statement.QuoteNode
Wraps an arbitrary value to reference as data. For example, the function
f() = :a
contains aQuoteNode
whosevalue
field is the symbola
, in order to return the symbol itself instead of evaluating it.GlobalRef
Refers to global variable
name
in modulemod
.SSAValue
Refers to a consecutively-numbered (starting at 1) static single assignment (SSA) variable inserted by the compiler. The number (
id
) of anSSAValue
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 Expr
s 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, atargs[2]
).static_parameter
Reference a static parameter by index.
=
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 3-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 3-argument form has the following arguments:
args[1]
A function name, or
nothing
if unknown or unneeded. If a symbol, then the expression first behaves like the 1-argument form above. This argument is ignored from then on. It can benothing
when methods are added strictly by type,(::T)(x) = x
, or when a method is being added to an existing function,MyModule.f(x) = x
.args[2]
A
SimpleVector
of argument type data.args[2][1]
is aSimpleVector
of the argument types, andargs[2][2]
is aSimpleVector
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.
struct_type
A 7-argument expression that defines a new
struct
:args[1]
The name of the
struct
args[2]
A
call
expression that creates aSimpleVector
specifying its parametersargs[3]
A
call
expression that creates aSimpleVector
specifying its fieldnamesargs[4]
A
Symbol
,GlobalRef
, orExpr
specifying the supertype (e.g.,:Integer
,GlobalRef(Core, :Any)
, or:(Core.apply_type(AbstractArray, T, N))
)args[5]
A
call
expression that creates aSimpleVector
specifying its fieldtypesargs[6]
A Bool, true if
mutable
args[7]
The number of arguments to initialize. This will be the number of fields, or the minimum number of fields called by an inner constructor's
new
statement.
abstract_type
A 3-argument expression that defines a new abstract type. The arguments are the same as arguments 1, 2, and 4 of
struct_type
expressions.primitive_type
A 4-argument expression that defines a new primitive type. Arguments 1, 2, and 4 are the same as
struct_type
. Argument 3 is the number of bits.Julia 1.5 struct_type
,abstract_type
, andprimitive_type
were removed in Julia 1.5 and replaced by calls to new builtins.global
Declares a global binding.
const
Declares a (global) variable as constant.
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 arbitrarynew
expressions can easily segfault.splatnew
Similar to
new
, except field values are passed as a single tuple. Works similarly toBase.splat(new)
ifnew
were a first-class function, hence the name.isdefined
Expr(:isdefined, :x)
returns a Bool indicating whetherx
has already been defined in the current scope.the_exception
Yields the caught exception inside a
catch
block, as returned byjl_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 bypop_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 associatedenter
.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 valuetrue
.loopinfo
Marks the end of the a loop. Contains metadata that is passed to
LowerSimdLoop
to either mark the inner loop of@simd
expression, or to propagate information to LLVM loop passes.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.
foreigncall
Statically-computed container for
ccall
information. The fields are:args[1]
: nameThe expression that'll be parsed for the foreign function.
args[2]::Type
: RTThe (literal) return type, computed statically when the containing method was defined.
args[3]::SimpleVector
(of Types) : ATThe (literal) vector of argument types, computed statically when the containing method was defined.
args[4]::Int
: nreqThe number of required arguments for a varargs function definition.
args[5]::QuoteNode{Symbol}
: calling conventionThe calling convention for the call.
args[6:length(args[3])]
: argumentsThe values for all the arguments (with types of each given in args[3]).
args[(length(args[3]) + 1):end]
: gc-rootsThe additional objects that may need to be gc-rooted for the duration of the call. See Working with LLVM for where these are derived from and how they get handled.
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 (if available, usually compressed).
generator
A callable object which can be executed to get specialized source for a specific method signature.
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.
primary_world
The world age that "owns" this Method.
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 aModule
, 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 bydef.sparam_syms
. For theMethodInstance
atMethod.unspecialized
, this is the emptySimpleVector
. But for a runtimeMethodInstance
from theMethodTable
cache, this will always be defined and indexable.uninferred
The uncompressed source code for a toplevel thunk. Additionally, for a generated function, this is one of many places that the source code might be found.
backedges
We store the reverse-list of cache dependencies for efficient tracking of incremental reanalysis/recompilation work that may be needed after a new method definitions. This works by keeping a list of the other
MethodInstance
that have been inferred or optimized to contain a possible call to thisMethodInstance
. Those optimization results might be stored somewhere in thecache
, or it might have been the result of something we didn't want to cache, such as constant propagation. Thus we merge all of those backedges to various cache entries here (there's almost always only the one applicable cache entry with a sentinel value for max_world anyways).cache
Cache of
CodeInstance
objects that share this template instantiation.
CodeInstance
def
The
MethodInstance
that this cache entry is derived from.
rettype
/rettype_const
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 it could be set to
nothing
to just indicaterettype
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], uint32t nargs)`
- 2 - Constant (value stored in
rettype_const
) - 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. If max_world is the special token value
-1
, the value is not yet known. It may continue to be used until we encounter a backedge that requires us to reconsider.
CodeInfo
A (usually temporary) container for holding lowered source code.
code
An
Any
array of statementsslotnames
An array of symbols giving names for each slot (argument or local variable).
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 (the length ofcode
array). If an array, specifies a type for each location.ssaflags
Statement-level flags for each expression in the function. Many of these are reserved, but not yet implemented:
- 0 = inbounds
- 1,2 = <reserved> inlinehint,always-inline,noinline
- 3 = <reserved> strict-ieee (strictfp)
- 4-6 = <unused>
- 7 = <reserved> has out-of-band info
linetable
An array of source location objects
codelocs
An array of integer indices into the
linetable
, giving the location associated with each statement.
Optional Fields:
slottypes
An array of types for the slots.
rettype
The inferred return type of the lowered form (IR). Default value is
Any
.method_for_inference_limit_heuristics
The
method_for_inference_heuristics
will expand the given method's generator if necessary during inference.parent
The
MethodInstance
that "owns" this object (if applicable).min_world
/max_world
The range of world ages for which this code was valid at the time when it had been inferred.
Boolean properties:
inferred
Whether this has been produced by type inference.
inlineable
Whether this should be eligible for inlining.
propagate_inbounds
Whether this 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.