Syntax of CSL¶

This document describes the basic structures of the CSL language.

Type system overview¶

The basic types of CSL are:

void type (void)
signed integers (i8, i16, i32, i64)
unsigned integers (u8, u16, u32, u64)
floating point numbers (f16, f32)

Arrays types are spelled [num_elements] base_type, for example: [3] i16. Array literals are specified by an array type followed by a list of values, for example: [3]i16 {1, 2, 3}

For a detailed introduction to the type system of CSL, see Type System in CSL.

Variables¶

Variable declarations are composed of a mutability specifier, a name, a type and an initializer:

const ten_i : i16 = 10;
var   ten_f : f16 = 10.0;
param ten_d : f32 = 10.0;

A const or param variable cannot have its value changed after it has been initialized, whereas a var variable has no such restriction.

The initializer expression is:

Mandatory for const variables.
Optional for var variables.
Optional for param variables. If one is not provided, the param must be initialized through the module import system. See Modules.

The type expression is optional. If one is not provided, the initializer expression is mandatory and it is used to deduce the type of the variable:

const ten_a : i16 = 10;
const ten_b       = ten_a;  // ten_b is also i16.
param my_param1; // ok, the initializer is provided later.

Variable declarations may optionally have an alignment requirement:

const aligned_var1 : i16 align(32) = 10;
const aligned_var2       align(64) = ten_a;

The memory address of the corresponding variable is guaranteed to have at least the specified alignment. Alignment is specified as a number of bytes and must always be a power of two.

Global variables can be used before their declaration. For example, the following is legal:

fn my_fn(x: f16) void {
  my_global = x;
}

var my_global : f16;

Global variable declarations may also optionally specify the name of the link section:

var global_var1 : i16 linksection(".mySection") = 10;

By specifying the link section name .mySection, the global variable gets placed into a separate object file section named .mySection, instead of being placed into the object file section with the rest of the global variables.

The linksection attribute can be used together with the compiler flag --link-section-start-address-bytes to place global variables at particular memory addresses:

var section1: u16 linksection(".mySection1") = 0xabcd;
var section2: u16 linksection(".mySection2") = 0x1234;

// $ cslc-driver ... \
//   --link-section-start-address-bytes=".mySection1:40960,.mySection2:40980"

In the example above, the variable section1 is placed at the memory address 40960 (bytes), and section2 is placed at 40980.

Global variable declarations may also optionally specify the name of the ELF symbol corresponding to the variable:

var global_var : i16 linkname("different_name") = 10;

In this example, the global variable known as global_var within CSL gets assigned the name different_name in the compiled object file. This can be useful to control the name of symbols that are intended to be referenced by other object files as external data. Any comptime expression evaluating to a value of type comptime_string may be used for linkname.

Global variable declarations may optionally specify a storage class (either export or extern). If a variable is declared export, it is made accessible to other separately-compiled objects, and is guaranteed not to be eliminated from the compiled object. If a variable is declared extern, it is assumed that its definition will be supplied by another object that will later be linked with the object we are compiling. An extern declaration must _not_ initialize the variable.

Variables with the export or extern storage classes must have an export-compatible type. See Storage Classes for details.

// Variable 'x' will be available to other objects that are linked with
// this program.
export var x: i16 = 12;

// We expect that variable 'y' will be provided by another object that is
// to be linked with this program.
extern var y: i16;

// Variable 'foo' will be available under the name 'alias_for_foo' to other
// objects that are linked with this program.
export var foo: i16 linkname("alias_for_foo") = 42;

// Variable 'alias_for_bar' will be aliased to the a variable 'bar' provided
// by another object that is to be linked with this program.
extern var alias_for_bar: i16 linkname("bar");

Pointers¶

To obtain a pointer to a variable, the address-of operator & is used:

var x = [2]i16 {0, 1};
var ptr = &x; // ptr is a *[2]i16

const y = [2]i16 {0, 1};
const const_ptr = &y; // const_ptr is a *const[2]i16

Only variables are addressable, as such it is illegal to obtain the address of a temporary:

const x = [2]i16 {0,0};
const ok_ptr = &x[1];

const bad_ptr = &(([2]i16 {0,0})[1]); // compile-time error.

To dereference a pointer, the dereference operator .* is used:

var x = [2]i16 {0, 1};
var ptr_to_x = &x; // ptr is a *[2]i16

var copy_of_x = ptr_to_x.*; // copy_of_x is a [2]i16

var element_of_x = ptr_to_x.*[1]; // element_of_x is an i16

Functions¶

Function definitions require a fn or task keyword, a name, an optional sequence of parameters, a return type and a function body:

fn foo(arg : i16) i32 { ... }
task my_task(arg : i16) void { ... }

All function parameters are implicitly const variables.

It is unspecified whether function parameters are passed by value or by reference. If it is necessary to modify a function argument, the function parameter must be declared with a pointer type:

fn foo(arg : *i16) void {
  arg.* = 42;
}

fn bar() void {
  var x : i16 = 0;
  foo(&x); // x is now 42.
}

The type of a function parameter may be specified with the keyword anytype. In this case, the compiler will create a specialized copy of the function based on the type of the corresponding argument used at the call site. This is similar to typename templates in C++.

/// Computes base ^ exp
fn pow(base : anytype, exp : @type_of(base)) @type_of(base) {
  const base_type = @type_of(base);
  if (@is_same_type(base_type, i16)) {
    // ... integer implementation ...
  }
  if (@is_same_type(base_type, f16)) {
    // ... float implementation ...
  }
  return @as(base_type, 0);
}

task t() void {
  const v1 : i16 = ...;
  pow(v1, 6); // specialized for `i16`.

  const v2 : f16 = ...;
  pow(v2, 6.0); // specialized for `f16`.
}

Function parameters can optionally be marked with the comptime keyword (see Comptime). In this case, the compiler will create a specialized copy of the function based on the value of the corresponding argument at the call site. The argument must be comptime-known. This is similar to non-type template parameters in C++.

/// This function is specialized for each value of base_type.
fn copy(size : i16, comptime base_type : type,
        dest : [*]base_type, src : [*]base_type) void {

  for (@range(i16, size)) |idx| {
    dest[idx] = src[idx];
  }
}

task t() void {
  var src = @constants([10]i16, 42);
  var dest : [10]i16;
  copy(10, i16, &src, &dest); // specialized for i16.
}

Function definitions may also optionally specify the name of the ELF symbol corresponding to the function:

fn foo () linkname("bar") void { ... }

In this example, the function known as foo within CSL gets assigned the name bar in the compiled object file. This can be useful to control the name of functions that are intended to be called by other object files as extern functions. Any comptime expression evaluating to a value of type comptime_string may be used for linkname.

Function declarations may optionally specify a storage class (either export or extern). If a function is declared export, it is made accessible to other separately-compiled objects, and its definition is guaranteed not to be eliminated from the compiled object. If a function is declared extern, it is assumed that its definition will be supplied by another object that will later be linked with the object we are compiling. An extern function declaration must _not_ contain a function body.

Functions with the export or extern storage classes must have an export-compatible type. See :ref:’language-storage-classes’ for details.

// Function 'f' will be available to other objects that are linked with
// this program.
export fn f(x: i16, y: i16) { return x+y; }

// We expect that function 'g' will be provided by another object that is
// to be linked with this program.
extern fn g(f16, f16) f16;

// Function 'foo' will be available under the name 'alias_for_foo' to other
// objects that are linked with this program.
export fn foo(x: *i16) i16 linkname("alias_for_foo") { return x.*; }

// Function 'alias_for_bar' will be aliased to the a function 'bar' provided
// by another object that is to be linked with this program.
extern fn alias_for_bar(*f16) f16 linkname("bar");

inline fn¶

Adding the inline keyword to a function definition makes that function become semantically inlined at the callsite. This is not a hint to be possibly observed by optimizations; rather, the body of the inline function is expanded at callsites during semantic analysis. This means that unlike normal function calls, comptime-known arguments of an inline function call become comptime-known inside the expanded body. This comptime-ness can potentially propagate all the way to the return value:

inline fn foo(a: i32, b: i32) i32 {
  return a + b;
}

task main() void {
  if (foo(1200, 34) != 1234) {
    @comptime_assert(false);
  }
}

In the code above, foo(1200, 34) evaluates to 1234 at comptime, so the if condition evaluates to false and the @comptime_assert is ignored. If inline is removed, foo(1200, 34) is no longer comptime-known, so the @comptime_assert would fail.

Since inline functions are expanded at callsites, they only exist in non-inlined form at comptime. As such, inline functions may not be used in ways that require functions to be valid at runtime; for example, inline functions cannot have a linkname and it is not allowed to take the address of an inline function.

It is generally better to let the compiler decide when to inline a function, except for these scenarios:

To cause comptime-ness of the arguments to propagate to the return value of the function, as in the above example
Real world performance measurements demand it

Note that inline actually restricts how the compiler is allowed to compile a function. This can harm binary size, compilation speed, and even runtime performance.

Direct and Indirect Function Calls¶

Functions can be called directly by name or indirectly through function pointers. For example:

fn foo(a: i16, b: f32) f32 { ... }

var foo_ptr: *const fn(i16,f32)f32 = foo;

task main() void {
  foo(42, 3.14);     // Direct function call
  foo_ptr(67, 42.0); // Indirect function call
}

The function value foo in the example above is implicitly coerced to the requested function pointer type. Note however that function values can only be coerced to const function pointers as shown in the example above.

It is also possible to take the address of a function symbol using the address-of operator & as shown in the example below:

fn foo() void { ... }

var foo_ptr: *const fn()void = &foo;

task main() void {
  foo_ptr(); // Indirect function call
}

Taking the address of a function using the & operator is semantically equivalent to the implicit coercion of a function value to a const function pointer type. This means that the resulting address will always be a const pointer as well.

Tasks cannot be called directly like regular functions, for example:

task foo() void { ... }

task invalid_foo_call() void {
  foo(); // ERROR: task cannot be called.
}

Warning

Due to a compiler limitation, it is possible to bypass this restriction by taking the address of a task and then calling the task through the respective function pointer, but this results in unspecified behavior. A future release of the compiler will disallow this.

Statements¶

If-statement¶

If-statements have the following syntax:

if (condition) {
  // ...
}
else {
  // ...
}

If condition is known at compile-time, the branch not-taken is not semantically checked by the compiler, but it must still be syntactically valid.

The else clause is optional.

It is possible to combine an else clause with another if-statement:

if (condition) {
  // ...
}
else if {
  // ...
}
else {
  // ...
}

For-statement¶

A for-statement iterates over the elements of an array or range:

for (my_array) |element| {
  // ...
}

for (@range(i32, 0, 2, 100)) |element| {
  // ...
}

Inside the loop body, the variable element acts as a const declaration whose value is the element that is currently being iterated on.

For-statements may specify a const declaration for the index of the element being iterated on:

for (my_array) |element, index| {
  // ...
}

A break statement may be used to end the loop:

for (my_array) |element, idx| {
  // ...
  if (condition) {
    break;
  }
}

A continue statement may be used to end the current iteration of the loop:

for (my_array) |element, idx| {
  // ...
  if (condition) {
    continue;
  }
}

When a for loop is labeled, it can be referenced from a break nested within its body. This makes it possible to break a loop from inner loops nested within its body:

outer: for (my_array) |element, idx| {
  for (other_array) |elem2, idx2| {
    // ...
    if (condition) {
      // Exit the outermost loop
      break :outer;
    }
  }
}

Note that, to define a label for a loop, : occurs after the name, while : occurs before the name when referring to a label in break.

Like identifiers, redefinition of a label is not allowed. However, labels belong to a separate namespace from identifiers. In other words, it is legal for a label to have the same name as a variable, function, or task that is in scope. Also, since labels are only visible within their corresponding loop, it is possible to reuse labels for loops that are not nested within each other.

foo: for (array1) |x| {
  // error: redefinition of label 'foo'
  foo: for (array2) |y| { ... }
}

fn bar() void {
  // No error
  bar: for (array3) |z| { ... }

  baz: for (array4) |a| { ... }
  // No error: not a redefinition of baz
  baz: for (array5) |b| { ... }
}

While-statement¶

While-statements have the following syntax:

while (condition) {
  // ...
}

continue or break statements may be used inside the body of a while-statement.

When a while loop is labeled, it can be referenced from a break nested within its body, including from inner loops nested within, in the same manner as a for loop:

outer: while (cond1) {
  while (cond2) {
    break :outer;
  }
}

outer: while (cond) {
  inner: for (array) |element| {
    if (cond2) {
      break :outer;
    } else if (cond3) {
      break :inner;
    }
  }
}

A while-statement may optionally specify an assignment expression:

while (condition) : (i += 3) {
  // ...
}

The assignment expression executes at the end of each loop iteration, including iterations finished with a continue statement.

Blocks¶

Blocks are used to limit the scope of variable declarations:

{
  var x: i32 = 1;
}
x += 1;
// error: use of undeclared identifier

A block may be labeled. When labeled, it can be referenced from a break nested inside, which exits the block:

var x: i32 = 0;
outer: {
  if (cond) {
    break :outer;
  }
  x += 1;
}
// x == 0 if cond is true

Note that a label is required for break to exit a block. In other words, break without a label always acts on the closest loop and a block without a label cannot be exited with break:

while (cond1) {
  blk: {
    if (cond2) {
      break; // Exits the loop
    } else {
      break :blk; // Exits 'blk'
    }
  }

  {
    if (cond3) {
      break; // Exits the loop
    }
  }
}

Blocks are also expressions. Labeled breaks that refer to blocks can be used to return a value from the block:

var y: i32 = 123;
const x = blk: {
  y += 1;
  break :blk y;
};
@assert(x == 124);
@assert(y == 124);

If multiple labeled breaks refer to a block, all of their values must have compatible type:

const M: i32 = 42;
const N: comptime_int = 100;
const x = blk: {
  if (...) {
    break :blk N;
  }
  break :blk M;
};
@comptime_assert(@is_same_type(@type_of(x), i32));

const y = blk: {
  if (...) {
    break :blk @as(i32, 1);
  }
  break :blk 0.5; // error: expected type 'i32', got: 'comptime_float'
};

A break without a value is equivalent to a break whose value is void. If control flow may reach the end of a block without breaking a value, the block’s type is void and any values broken from the block must be void. By this reasoning, unlabeled blocks always have type void since it is not possible to break them:

blk: {
  if (...) {
    break :blk {}; // OK, '{}' evaluates to void
  }
}

blk: {
  if (...) {
    break :blk false; // error: expected type 'void', got: 'bool'
                      // note: block that is broken has type 'void' because
                      //       control flow may reach the end without a break
  }
}

blk: {
  if (...) {
    break :blk false; // OK, 'return' prevents control flow from reaching end
  }
  return;
}

A block evaluates to a comptime-known value if its type is void or if all of the following hold:

The block is referred to by exactly 1 break with a value
The block is guaranteed to terminate by executing this break
The break’s value is comptime-known

A block may evaluate to a comptime-known value even if it contains runtime code:

const x = b: {
  runtime_code();
  break :b 1;
} + 42;
@comptime_assert(@is_same_type(@type_of(x), comptime_int));
@comptime_assert(x == 43);

Switch-statement¶

Switch-statements have the following syntax:

switch (input) {
  case_values1 => branch_expr1,
  case_values2 => branch_expr2,
  ...
  else => else_expr
}

input can be an expression of a fixed-width integer type (i.e., comptime_int is not allowed) or of any enum type.

The body of the switch statement consists of 1 or more comma-separated branches where each branch consists of 2 parts: the case_values and the corresponding branch_expr. A branch may combine multiple case_value expressions via a comma:

switch (input) {
  case_value1, case_value2 => branch_expr1n2,
  case_value3 => branch_expr3,
  ...
}

A switch statement will attempt to match input with one of the provided case_value expressions. If a match is found the corresponding branch will be selected and the respective branch_expr will be executed. If no match is possible, the else branch will be selected as the default and the corresponding else_expr will be executed.

case_value expressions must be comptime-known and coercible to the type of the input expression. They must also be unique.

All branch_expr expressions (including the else_expr expression, if present) must have compatible types.

If input is known at compile-time, the branch_exprs corresponding to the branches not-taken are not semantically checked by the compiler, but they must still be syntactically valid.

A switch can also be used as an expression. In this scenario all branch_expr expressions (including the else_expr expression, if present) must be able to be coerced to the common requested type:

fn foo(e: my_enum) i16 {
  // All branch_exprs and the else_expr are coerced to 'i16' which is the
  // type requested by the 'return' expression.
  return switch (e) {
           my_enum.A => 1,
           my_enum.B => -10,
           my_enum.C => 42,
           else => 100
         };
}

Branches do not fall through. If fall through behavior is desired, case_value expressions can be combined and if-statements can be used as follows:

switch (input) {
  0, 1 => {
    if (input == 0) {
      // Logic for case 0
    }
    // Common logic for cases 0 and 1
  },
  ...
}

A switch statement must cover all possible values for a given input expression type either explicitly by specifying a case_value for each possibility or implicitly through the else branch:

var int_input: i16 = ...;
switch (int_input) {
  -5, 0 => ...
  // ERROR: Not all possible 'i16' values are covered. An 'else' branch is
  // needed.
}

const my_enum = enum { A, B, C };
var e: my_enum = ...;
switch (e) {
  my_enum.A, my_enum.B => ...,
  my_enum.C            => ...
  // OK! No 'else' branch is needed since all possible 'my_enum' values are
  // covered.
}

Inline assembly statements¶

Warning

Support for inline assembly is still experimental, and is extremely limited. Subtle pitfalls and undefined behavior are very easy to trigger with inline assembly. We suggest reading up on how inline assembly works in C before attempting to use this feature.

Inline assembly statements have the following syntax:

asm volatile (             // "volatile" keyword is optional
  "assembly instructions",
  : output_constraints     // optional
  : input_constraint       // optional
  : clobbers               // optional
);

The item output_constraints is an optional list of comma-separated items of the form:

[identifier1] "constraint_string" (identifier2)

where:

identifier1 is a name used to refer to the register assigned to this constraint within the assembly instructions,
constraint_string is a specifier for the desired register, and
identifier2 is the name of a CSL variable to which this output will be written.

Currently, the only supported output constraints are those of the form ={R} (curly braces included), where R names a general-purpose register that is valid on the target. On all current Cerebras architectures, the general-purpose registers are the 16-bit registers r0 through r15, as well as the 32-bit double-registers d0, d2, d4, …, d14. Note that each dN register is essentially aliased with rN and rN+1.

The item input_constraints is an optional comma-separate list of the form:

[identifier] "constraint_string" (expr)

where:

identifier is a name used to refer to the register assigned to this constraint within the assembly instructions,
constraint_string is a specifier for the desired register, and
expr is an expression that will supply the initial value for this register when the inline assembly is executed.

Currently, the only supported input constraints are those of the form {R} (curly braces included). See above for notes on valid general-purpose registers.

The item clobbers is an optional comma-separated list of clobbers, where each clobber is simply the name of a general-purpose register. Specifying that a register is clobbered indicates to the compiler that it may be modified by the inline assembly code as a side effect, so the compiler will need to save it before entering the inline assembly block and restore it after exit.

Warning

Note that on some targets (wse1 and wse2), r7 is used as the stack pointer. Using r7 in clobbers on these targets is banned. Using r7 for input or output constraints on these targets may produce undefined behavior, and may be banned in the future.

Writing to a register without specifying it as an output or clobber register is undefined behavior. Reading from a register that is not specified in an input constraint is undefined behavior.

The volatile keyword indicates that the assembly code has side effects not expressed in the constraints or clobbers. Statements marked volatile may not be duplicated by the compiler, and will not be eliminated even if nothing uses their output values.

Example¶

The following function uses inline assembly to return twice the value of x+y. (The code here is slightly inefficient, for the purposes of demonstrating inputs, outputs, and clobbers all in one go.)

fn doubleIt(x: i16, y: i16) i16 {
  var r: i16;

  asm (
    @strcat("mov16 r6 = %[argval1]\n",
            "add16 r6 = r6, %[argval2]\n",
            "add16 r6 = r6, r6\n",
            "mov16 %[retval] = r6")
      : [retval] "={r0}" (r)
      : [argval1] "{r2}" (x), [argval2] "{r3}" (y)
      : "r6"
  );

  return r;
}

Operations on integer, floats and booleans¶

The following expressions are supported on integer or floating-point values:

a + b (addition)
a - b (subtraction)
a * b (multiplication)
a / b (division)
a += b (addition with assignment)
a -= b (subtraction with assignment)
a *= b (multiplication with assignment)
a /= b (division with assignment)
-a (negation)

The following expressions are supported on integer values:

a % b (remainder from integer division)
a << b (shift left)
a >> b (arithmetic shift right if a is signed, otherwise logical shift right)
a & b (bitwise AND)
a | b (bitwise OR)
a ^ b (bitwise XOR)
a %= b (remainder from integer division with assignment)
a <<= b (shift left with assignment)
a >>= b (shift right with assignment)
a &= b (bitwise AND with assignment)
a |= b (bitwise OR with assignment)
a ^= b (bitwise XOR with assignment)
~a (bitwise NOT)

The following expressions are supported on boolean values:

a or b (logical OR)
a and b (logical AND)
!a (logical NOT)

Logical AND and logical OR operations implement short-circuit evaluation (see Comptime Expressions for details).

Warning

Except for logical AND and logical OR, the order in which expression operands are evaluated at runtime is undefined.

For binary operations, both operands must have exactly the same type, unless one of them is a comptime_int (see Comptime). Shift operations are an exception to this rule, where the only constraint is that the right hand side operand must be an unsigned integer.

The ternary operator¶

A ternary operator has similar syntax to an if-statement:

const x : i32 = if (cond) 0 else 1;

Ternary operators do not require {} blocks, and may be used anywhere an expression is expected.

Both the “then” expression and the “else” expression must have compatible types.

If cond is known at compile-time, the branch not taken is not semantically checked by the compiler, but it must still be syntactically valid. In this case, the two expressions don’t need to have compatible types.

Comments¶

// begins a single-line comment. There are no multi-line comments in CSL.

// This function returns the value arg + 2
fn foo(arg : i16) i16 {
  var x : i16 = arg;

  // This and the next line are commented out: x will not be incremented by 1
  // x += 1;

  x += 2; // Increment x by 2

  return x;
}

SDK Documentation (1.3.0)

Syntax of CSL

Contents

Syntax of CSL¶

Type system overview¶

Variables¶

Pointers¶

Functions¶

inline fn¶

Direct and Indirect Function Calls¶

Statements¶

If-statement¶

For-statement¶

While-statement¶

Blocks¶

Switch-statement¶

Inline assembly statements¶

Example¶

Operations on integer, floats and booleans¶

The ternary operator¶

Comments¶