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Storage Classes

This document describes storage classes in CSL.

Overview

By default, any variable declared in a CSL program is accessible only within the program itself. This means that when linking two separately-compiled objects A and B, variables declared in the source code for A will not be visible in the source code for B, and vice versa. It also means that tools manipulating symbols in the compiled binary (such as readelf or the Cerebras ELFLoader class in Python) will not necessarily be able to access the contents of the symbol by name.

CSL features two storage classes for global variables and functions that modify this default behavior: extern and export.

extern

The extern storage class declares that a symbol for a variable or function, with a certain type, is expected to be defined in an export declaration elsewhere. That export declaration may either be in the object file currently being compiled, or in another object file that will be linked with the object being compiled. Declaring a variable with extern makes that variable or function available for use within the current source file, under the declared name.

A variable declaration with the extern storage class must not have an initializer expression, and a function declaration with the extern storage class must not have a body block.

Declarations with extern storage must have export-compatible types. See Type restrictions for more details.

Currently, no calling convention for extern functions is formally defined. Calling extern functions at comptime is not allowed.

Examples:

extern var x: i16;
extern var y: *const fn(i16) i16;
extern const z: [*]f16;

extern fn f() void;
extern fn g(x: i16, y: [*]f16) i16;

export

The export storage class defines a variable or function with a certain name and type, and makes that variable or function available to other object files that are linked with the object being compiled.

A variable declaration with the export storage class may have an initializer expression, but it is not required. A function declaration with the export storage class must have a body block.

Declarations with export storage must have export-compatible types. See Type restrictions for more details.

Currently, no calling convention for export functions is formally defined.

In addition to making symbols accessible by other objects, export ensures that the symbol will be accessible via the Python ELFLoader class (or other tools that operate on ELF files). If a symbol is intended to be accessed either in the compiled binary or in a post-execution core dump via ELFLoader, that symbol must be declared export.

Examples:

fn internal_func(x: i16) i16 { return x+1; }

export var x: i16 = 42;
export var y: *const fn(i16) i16 = &internal_func;
export const z: [*]f16;

export fn f() void { x += 1; }
export fn g(x: i16) i16 { return x+2; }

Object file symbols for extern and export declarations

Unless a linkname is supplied (see below), the symbol name for an export or extern declaration will be the same as its CSL variable name, even if the variable is declared inside an imported module. Additionally, export or extern symbols will be part of a single global namespace shared with all other objects with which the compiled object is linked. For example:

///// Source file "root.csl"

// Symbol name in the compiled object file will be "x", and "x" will
// have global scope.
export var x: i16 = 32;

const m = @import_module("submodule.csl");


///// Source file "submodule.csl"

// Symbol name in the compiled object file will be "y", and "y" will
// have global scope.
export var y: i16 = 42;

// Symbol name in the compiled object file will be "z", and "z" will
// have global scope. Its actual definition is expected to be supplied by
// another object.
extern var z: i16;

Interaction with linkname

Both export and extern declarations may be assigned a linkname. If a linkname is present, that name is used for the symbol in the object file. For example:

// Symbol name in the compiled object will be "foo", and "foo" will have
// global scope, i.e., will be accessible by other objects.
export var x: i16 linkname("foo") = 99;

// Note that within the current program, the name "x" must still be used.
...
x += 1;    // valid
foo += 1;  // invalid
...

This can be useful when export declarations are present in code that is intended to be imported with @import_module. Here, comptime determination of the linkname can be used to avoid clashes among symbol names. For example:

///// Source file "root.csl"

const m1 = @import_module("submodule.csl", .{ .sym_name = "m1_foo" });
const m2 = @import_module("submodule.csl", .{ .sym_name = "m2_foo" });

...
m1.x  // corresponds to the object symbol "m1_foo"
m2.x  // corresponds to the object symbol "m2_foo"
...


///// Source file "submodule.csl"

param sym_name: comptime_string;

// Without the variable linkname, two imported copies of "submodule.csl"
// would always have symbol naming conflicts, since they would both
// attempt to define the object symbol "x".
export var x: i16 linkname(sym_name);

Mixing extern and export declarations

It is legal to have multiple extern declarations with the same object symbol name within a compiled program, and up to one export declaration, as long as all declarations sharing a symbol name agree on declaration kind (var, const, or fn) and have the same type. Neither an extern nor an export declaration may share a symbol name with a declaration that has no storage class (i.e., one which is neither export nor extern).

For example:

///// Source file "root.csl"

// The variable "x" will have a symbol name of "x", since no linkname is
// specified.
export var x: i16;

// The variable "y" will essentially be aliased to "x", since they share a
// symbol name.
extern var y: i16 linkname("x");

const m = @import_module("submodule.csl");


///// Source file "submodule.csl"

// The variable "x" will have a symbol name of "x", since no linkname is
// specified. Since it shares a symbol name with the variable "x" from
// root.csl, it will effectively be aliased to that variable.
extern var x: i16;

// The variable "z" will essentially be aliased to "x", since they share a
// symbol name.
extern var z: i16 linkname("x");

// Note that the following declarations would not be allowed.

// A declaration without a storage class cannot have the same symbol name
// as an 'export' or 'extern' declaration.
// var bad_var_decl: i16 linkname("x");

// All declarations sharing a symbol name must have the same type.
// extern var bad_type: i32 linkname("x");

// Variable declarations sharing a symbol name must match with respect to
// constness.
// extern const bad_constness: i16 linkname("x");

Type restrictions

Declarations with export or extern storage class must have a type that is export compatible.

For variable declarations, export compatibility is defined as follows:

• The built-in types i8, i16, i32, i64, u8, u16, u32, u64, f16, f32, bool, and color are export compatible.

• An array type of the form [N]T is export compatible if its base type T is export compatible.

• A pointer type of the form *T or *const T is export compatible if its base type is export compatible.

• Function pointers are a special case: a pointer type of the form *const T, where T is a function type, is export compatible if T is export compatible for a function declaration (see below). (The const qualifier is required.)

• enum types are export compatible.

In particular, note that comptime-only types, struct types, DSD and DSR types, the direction type, the range type, void (except in function return type position–see below), and function types (as opposed to function _pointer_ types) are not export compatible.

For function declarations, export compatibility is defined as follows:

• A function type of the form fn(t1, t2, ..., tn) t is export compatible if all of its argument types (t1, t2, ..., tn) are export compatible for variable declarations (see above) and are allowed for CSL function arguments, and its return type t either is export compatible and allowed for CSL function return types, or is void.

The restrictions here are motivated by ABI compatibility with C and assembly code. Note, however, that there is currently no specific calling convention defined for extern or export functions.

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