[SPEC] Constexpr Specification

gazprea/index.rst

@@ -21,6 +21,7 @@ Hardware Acceleration Laboratory in Markham, ON.
    spec/namespaces
    spec/comments
    spec/declarations
+   spec/constexpr
    spec/type_qualifiers
    spec/types
    spec/type_inference

gazprea/spec/constexpr.rst

@@ -0,0 +1,146 @@
+.. _sec:constexpr:
+
+Constant Expressions
+====================
+
+A constant expression (sometimes called a constexpr) is an expression that can
+be fully evaluated by the compiler at compile time. This feature is primarily
+for specifying the size of
+:ref:`statically-sized arrays <ssec:array>`.
+
+In *Gazprea*, a ``constexpr`` is not a keyword, but a property of a ``const``
+variable. A ``const`` variable is considered a ``constexpr`` if and only if its
+initializer expression meets a strict set of criteria:
+
+.. _ssec:constexpr_rules:
+
+Rules for Constant Expressions
+------------------------------
+
+An expression is a valid ``constexpr`` if it is composed exclusively of:
+
+1.  Literals of base types (``boolean``, ``integer``, ``real``, ``character``).
+2.  The operators ``+``, ``-``, ``*``, ``/``, ``not``, ``and``, ``or``,
+    between two or more ``constexpr``\ s.
+3.  Constructors for aggregate types, provided that the aggregate is const and
+    all members are ``constexpr``\ s.
+4.  Index or field access on ``constexpr`` aggregate types.
+5.  Other variables that are themselves valid ``constexpr``\ s.
+
+An expression is **not** a ``constexpr`` if it contains:
+
+1.  References to ``var`` variables.
+2.  Function or procedure calls.
+3.  Any I/O operations (``<-``).
+
+The compiler must perform this validation recursively. When checking if a
+variable is a ``constexpr``, the compiler must trace its entire dependency
+chain. If the chain ever depends on a runtime value, the check fails.
+
+The only expressions that *must* be ``constexpr`` are global constants. Other
+constexprs arising from constants inside function scope may also be constexprs
+but the implementation does not need to enforce or necessarily identify this.
+Students should also note that MLIR has a constant propagation pass built in,
+so doing constant folding yourself may not be necessary depending on your
+implementation.
+
+**Examples:**
+
+**Note**: we will annotate the scope explicitly in these examples. Some
+'illegal' examples here would be legal within a non-global scope.
+
+::
+
+    // ----------------------------
+    // in global scope
+    // ----------------------------
+
+    // Legal Global Constant Expressions
+    const A = 10;
+    const B = A * 2; // Depends on another constexpr
+    const C = B + 5; // C is 25
+
+    // Illegal Global Constant Expressions
+    var x = 10;
+    const Y = x + 5; // Not a constexpr: depends on a 'var'
+
+    function get_val() returns integer { return 100; }
+    const Z = get_val(); // Not a constexpr: depends on a function call
+
+.. _ssec:constexpr_aggregates:
+
+Constant Expressions with Aggregate Types
+-----------------------------------------
+
+Arrays and tuples can also be ``constexpr``\ s if they meet specific criteria,
+allowing them to be used to define other constants.
+
+#. Arrays
+
+   A ``const`` array is a ``constexpr`` if:
+
+   1. Its size is a valid ``constexpr``.
+   2. All of its element initializers are valid ``constexpr``\ s.
+
+   A ``vector`` (the dynamically-sized type) can never be a ``constexpr``
+   aggregate, since its size is determined at runtime. An inferred-size array
+   such as ``integer[*] X = [1, 2, 3]`` must be a ``constexpr``, meaning its
+   initializer is itself a ``constexpr``: ``[*]`` denotes an inferred size, not
+   a dynamic one.
+
+   ::
+
+        // ----------------------------
+        // in global scope
+        // ----------------------------
+
+        const WIDTH = 5;
+        const integer[WIDTH] LOOKUP_TABLE = [10, 20, 30, 40, 50]; // Legal constexpr array
+
+        const ELEMENT = LOOKUP_TABLE[3];          // Legal: ELEMENT is a constexpr with value 30
+        integer[ELEMENT] my_array = 0;            // Legal: static array of size 30, zero-filled
+
+        const integer[2] BAD_TABLE = [10, get_val()]; // Illegal: initializer is not a constexpr
+                                                      //  also illegal if a procedure since
+                                                      //  procedure calls are not allowed
+                                                      //  within declarations
+
+   A ``constexpr`` can appear anywhere a ``const`` declaration is legal,
+   including inside functions, procedures, and control-flow blocks. However,
+   **not every** ``const`` variable is a ``constexpr``. ``const`` means only
+   that the variable is immutable within its scope; ``constexpr`` is the
+   stronger property that the value is fully known at compile time. For
+   example:
+
+   ::
+
+        // ----------------------------------
+        // in local/function/non-global scope
+        // ----------------------------------
+        var integer x;
+        x <- std_input;
+        const integer y = x; // Legal: y is immutable, but NOT a constexpr
+                             // because its value depends on runtime input.
+        integer[y] arr;      // Illegal: an explicit array size must be a
+                             // constexpr, and y is not a constexpr.
+        vector<integer> v;   // Legal: a vector is the dynamically-sized type;
+                             // use it when the size is only known at runtime.
+
+   The compiler propagates the constexpr property through local scopes
+   normally; there is no restriction on where in a block the declaration
+   appears, as long as its entire dependency chain satisfies the rules above.
+
+#. Tuples
+
+   A ``const`` tuple is a ``constexpr`` if all of its fields are initialized
+   with valid constant expressions.
+
+   ::
+
+        // ----------------------------
+        // in global scope
+        // ----------------------------
+        const CONFIG = (true, 10 * 2); // Legal constexpr tuple
+
+        const IS_ENABLED = CONFIG.1; // Legal: IS_ENABLED is a constexpr with value 'true'
+        const VALUE = CONFIG.2;      // Legal: VALUE is a constexpr with value 20

gazprea/spec/globals.rst

@@ -24,11 +24,19 @@ place since mutable global variables would ruin functional purity.
 If functions have access to mutable global state then we can not guarantee
 their purity.
 
-Globals must be initialized, but the initialization expressions may only contain
-a single _scalar_ literal. That means that functions and even previously defined globals may not
-appear on the RHS of a global declaration. The reason is because it is very difficult to
-evaluate variables and functions at compile time. Global expression evaluation could
-be deferred to runtime, but that has the disadvantage of changing errors from compile
-time to run time.
+Globals must be initialized with a valid
+:ref:`constant expression <sec:constexpr>`. A global initializer may therefore
+reference other globals and use arithmetic and constexpr aggregates, but it must
+be fully evaluable by the compiler before the program runs. This preserves
+functional purity and enables compile-time optimizations. As a consequence:
+
+*   Functions, procedures, and I/O operations may not appear in a global's
+    initializer.
+*   A global may not have a ``vector`` type (the dynamically-sized type),
+    because a vector's size is determined at runtime. An inferred-size array
+    such as ``const integer[*] X = [1, 2, 3]`` *is* permitted: ``[*]`` denotes
+    an inferred size that is fixed by its ``constexpr`` initializer at compile
+    time.
+*   All globals are implicitly ``constexpr``.
 
 

gazprea/spec/typedef.rst

@@ -61,18 +61,27 @@ Duplicate alias names should raise a `SymbolError`
   typealias integer ty;
   typealias character ty;
 
-Some type aliases may be parameterized with an expression, such as with arrays,
-such expressions are restricted to be composed exclusively from arithmetic
-operations on scalar literals. Practically speaking, this requires constant
-folding but *not* constant propagation.
+Some type aliases may be parameterized with an expression, such as the size of
+an array. Such size expressions must be valid
+:ref:`constant expressions <sec:constexpr>`. This permits not only constant
+folding of scalar literals but also constant propagation through other
+``constexpr`` values, such as global constants.
 
 ::
 
   typealias integer[1 + 3 - 2] vec_of_two;
   procedure main() returns integer {
-    vec_of_two v = 1..3;   
+    vec_of_two v = 1..3;
   }
 
 Should raise a ``SizeError`` on line 3 since the ``vec_of_two`` type has a size
 of 2 and an array of size 3 is being assigned.
 
+Because the size may be any ``constexpr``, it can reference other constant
+expressions rather than being limited to literals:
+
+::
+
+  const WIDTH = 4;
+  typealias integer[WIDTH] row;   // legal: WIDTH is a constexpr
+

gazprea/spec/types/array.rst

@@ -26,9 +26,11 @@ array instead of a ``real`` array.
 
 #. Explicit Size Declarations
 
-   When an array is declared it may be explicitly given a size. This
-   size can be given as any integer expression, thus the size of the
-   array may not be known until runtime.
+   When an array is declared it may be explicitly given a size. This size
+   must be a :ref:`constant expression <sec:constexpr>`. Every array, whether
+   explicitly or implicitly sized, has a size that is known at compile time; a
+   collection whose size is only known at runtime requires a
+   :ref:`vector <ssec:vector>`.
 
    ::
 
@@ -77,8 +79,9 @@ array instead of a ``real`` array.
 
 
    In this example the compiler can infer both the size and the type of
-   ``w`` from ``v``. The size may not always be known at compile time, so this
-   may need to be handled during runtime.
+   ``w`` from ``v``. As with any array, this inferred size is known at compile
+   time; a collection whose size is only known at runtime must be a
+   :ref:`vector <ssec:vector>`.
 
 .. _sssec:array_constr: