9. Server Protocol

A Futhark program can be compiled to a server executable. Such a server maintains a Futhark context and presents a line-oriented interface (over stdin/stdout) for loading and dumping values, as well as calling the entry points in the program. The main advantage over the plain executable interface is that program initialisation is done only once, and we can work with opaque values.

The server interface is not intended for human consumption, but is useful for writing tools on top of Futhark programs, without having to use the C API. Futhark’s built-in benchmarking and testing tools use server executables.

A server executable is started like any other executable, and supports most of the same command line options (Executable Options).

9.1. Basics

Each command is sent as a single line on standard input. A command consists of space-separated words. A word is either a sequence of non-space characters (foo), or double quotes surrounding a sequence of non-newline and non-quote characters ("foo bar").

The response is sent on standard output. The server will print %%% OK on a line by itself to indicate that a command has finished. It will also print %%% OK at startup once initialisation has finished. If initialisation fails, the process will terminate. If a command fails, the server will print %%% FAILURE followed by the error message, and then %%% OK when it is ready for more input. Some output may also precede %%% FAILURE, e.g. logging statements that occured before failure was detected. Fatal errors that lead to server shutdown may be printed to stderr.

9.2. Variables

Some commands produce or read variables. A variable is a mapping from a name to a Futhark value. Values can be both transparent (arrays and primitives), but they can also be opaque values. These can be produced by entry points and passed to other entry points, but cannot be directly inspected.

9.3. Types

All variables have types, and all entry points accept inputs and produce an output of defined types. The notion of transparent and opaque types are the same as in the C API: primitives and array of primitives are directly supported, and everything else is treated as opaque. See also Value Mapping. When printed, types follow basic Futhark type syntax without sizes (e.g. [][]i32). Uniqueness is not part of the types, but is indicated with an asterisk in the inputs and output commands (see below).

9.4. Consumption and aliasing

Since the server protocol closely models the C API, the same rules apply to entry points that consume their arguments (see Consumption and Aliasing). In particular, consumed variables must still be freed with the free command - but this is the only operation that may be used on consumed variables.

9.5. Commands

The following commands are supported.

9.5.1. General Commands

9.5.1.1. types

Print the names of available types, one per line.

9.5.1.2. entry_points

Print the names of available entry points.

9.5.1.3. call entry o i1 … iM

Call the given entry point with input from the variables i1 to iM. The results are stored in o, which must not already exist.

9.5.1.4. restore file v1 t1 … vN tN

Load N values from file and store them in the variables v1 to vN of types t1 to tN, which must not already exist.

9.5.1.5. store file v1 … vN

Store the N values in variables v1 to vN in file.

9.5.1.6. free v1 … vN

Delete the given variables.

9.5.1.7. rename oldname newname

Rename the variable oldname to newname, which must not already exist.

9.5.1.8. inputs entry

Print the types of inputs accepted by the given entry point, one per line. If the given input is consumed, the type is prefixed by *.

9.5.1.9. output entry

Print the type of the output produced by the given entry point, on a single line. If the output is guaranteed to be unique (does not alias any inputs), the type is prefixed by *.

9.5.1.10. clear

Clear all internal caches and counters maintained by the Futhark context. Corresponds to futhark_context_clear_caches().

9.5.1.11. kind type

Print the kind of type type. Always one of:

• primitive

• array

• record

• sum

• opaque

9.5.1.12. type v

Print type name of variable v.

9.5.1.13. pause_profiling

Corresponds to futhark_context_pause_profiling().

9.5.1.14. unpause_profiling

Corresponds to futhark_context_unpause_profiling().

9.5.1.15. report

Corresponds to futhark_context_report().

9.5.1.16. set_tuning_param param value

Corresponds to futhark_context_config_set_tuning_param().

9.5.1.17. tuning_params entry

For each tuning parameters relevant to the given entry point, print its name, then a space, then its class.

This is similar to on futhark_tuning_params_for_sum(), but note that this command prints names and not integers.

9.5.1.18. tuning_param_class param

Corresponds to futhark_get_tuning_param_class().

9.5.1.19. attributes entry

Print the attributes of the provided entry point in no particular order. Does not include the attribute brackets used in the Futhark syntax, meaning that an attribute written as #[foo] becomes simply "foo" in the output of this command.

9.5.2. Array Commands

9.5.2.1. rank t

Print the rank of array type t.

9.5.2.2. elemtype t

Print the typename of the elements of array type t.

9.5.2.3. new_array v0 t s0 … sN-1 v1 … vM

Create a new variable v0 of type t, which must be an array type of rank N. The size of each dimension of the array is given by s0 … sN-1, and the values by v1 … vM in row-major order, where M is the product of the dimension sizes.

9.5.2.4. set v0 v1 i0 … iN-1

Perform an in-place replacement on array-typed variable v0 of rank N at indices i0 … iN-1 with the value of variable v1.

9.5.2.5. shape v

Print the shape of array-typed variable v, with one integer per line.

9.5.2.6. index v0 v1 i0 … iN-1

Create a new variable v0 whose value is the result of indexing the variable v1, which must be an array of rank N, at position [i0]…[iN-1], where each i is an integer. Fails if the index is out of bounds.

9.5.2.7. zip v0 type v1 … vN

Create a new variable v0 of type type, which must be an array of records where the elements have N fields, where v1 to vN are variables that are arrays of the corresponding field types. The order in which the arrays must be passed are given by the fields command on type.

9.5.2.8. unzip v0 v1 … vN

Unzip an array of records into new variables. The variable v0 must be an array whose element type is a record with N fields. The order of constructed arrays corresponds to the field order given by the fields command on the type of v0.

9.5.3. Record Commands

9.5.3.1. fields type

If the given type is a record, print a line for each field of the record. The line will contain the name of the field, followed by a space, followed by the type of the field. Note that the type name can contain spaces. The order of fields is significant, as it is the one expected by the new_record command.

9.5.3.2. new v0 type v1 … vN

Create a new variable v0 of type type, which must be a record type with N fields, where v1 to vN are variables with the corresponding field types (the expected order is given by the fields command).

9.5.3.3. project to from field

Create a new variable to whose value is the field field of record-typed variable from.

9.5.4. Sum Commands

9.5.4.1. variants type

Print the names of each variant of type, which must be a sum type. Each variant is followed by a line for each payload value, giving its type. The lines of payload types are prefixed with a dash and a space (- ``). The order of payload types is significant, as it is the one expected by the ``construct and destruct commands.

9.5.4.2. construct v0 type variant v1 … vN

Create a new variable v0 of type type, which must be a sum type including a variant named variant with a payload of N values. v1 to vN are variables of the same types as the values held by the variant. The expected order is given by the variants command.

9.5.4.3. destruct v0 v1 .. vN

Copy the values held by an instance of a sum type, given in variable v0, to variables v1 to vN, where N is the number of values stored in the variant of v0. The expected order is given by the variants command.

9.5.4.4. variant v

Print the variant name of sum-typed variable v.

9.6. Environment Variables

9.6.1. FUTHARK_COMPILER_DEBUGGING

Turns on debugging output for the server when set to 1.