PDD 3: Calling Conventions

Abstract

Parrot's inter-routine calling conventions.

Description

This document describes how to pass arguments from registers to subroutines, and how subroutines can extract their parameters into registers for use.

Since Parrot's calling conventions are continuation-based, there is arguably very little difference between a call and a return. Because of this, the conversion rules are the same regardless of whether code is invoking a subroutine or a return continuation.

Common Features of Argument/Return Opcodes

There are four opcodes involved in parameter and return value propagation:

set_args, for passing arguments;
set_returns, for returning values;
get_params, for accepting parameters; and
get_results, for accepting return values.

FAQ: Given Parrot's internal use of continuation-passing style ["CPS"], it would be possible to use one pair of opcodes for both call and return, since under CPS returns are calls. And perhaps someday we will have only two opcodes. But for now, certain efficiency hacks are easier with four opcodes.

The common syntax of these opcodes is:

 <set_opcode> "flags0, flags1, ..., flagsN", VAL0, VAL1, ... VALN
 <get_opcode> "flags0, flags1, ..., flagsN", REG0, REG1, ... REGN
 <get_opcode> "..., 0x200, flags0, ...", ..., "name", REG0, ...

The flags string is a literal quoted string denoting a list of zero or more comma-separated integers. Integers may be specified either in decimal, or if prefixed with "0b"/"0x", in binary/hexadecimal. There must be exactly one integer for each value or register given.

For documentation purposes we'll number the bits 0 (low) through 30 (high). Bit 31 (and higher, where available) will not be used.

Some values and registers do not correspond directly to values passed or received. (See the descriptions of the OPT_FLAG and NAMED bits, below.)

Flag Words; Common Flag Word Bits

Each integer in the flag string controls the processing of the corresponding value or register.

These bits of each flag word have common meanings for all argument/ return-value opcodes:

Don't set these bits yourself; the assembler will do it.

Passing Arguments, Returning Values

Just before calling a subroutine with invokecc or calling a method with call_methodcc, use the set_args opcode to tell Parrot where the subroutine's or method's arguments will come from and how they should be expanded by the target.

Similarly, just before returning from such a subroutine or method, use the set_returns opcode to tell Parrot where the return values will come from and how to expand them for the caller's use.

Flag Word Bits For 'Setting'

These bits of each flag word have these meanings specific to set_args and set_returns:

4 CONSTANT
5 FLAT (P only)
6 (unused)
7 (unused)
8 (unused)
9 NAMED (FLAT or string constant only)

Accepting Parameters, Accepting Return Values

As the first opcode in a subroutine that will be called with invokecc or a method that will be called with call_methodcc, use the get_params opcode to tell Parrot where the subroutine's or method's arguments should be stored and how they should be expanded.

Similarly, just before (yes, before) calling such a subroutine or method, use the get_results opcode to tell Parrot where the return values should be stored and how to expand them for your use.

NOTE: It should be obvious, but in case it's not: You must name only registers as targets of these opcodes, not constants. (You can't store anything into a constant. That would make it a variable.)

Flag Word Bits For 'Getting'

These bits of each flag word have these meanings specific to get_params and get_results:

4 (unused)
5 SLURPY (P only)
6 (unused)
7 OPTIONAL
8 OPT_FLAG (I only)
8 READONLY (P only)
8 (unused for S and N)
9 NAMED (SLURPY or string constant only)

Overflow and underflow

If too many or too few values are provided for the given target registers, Parrot by default will throw an exception for get_params, but not for get_results. This error behavior can be controlled via the errorson and errorsoff opcodes using PARROT_ERRORS_PARAM_COUNT_FLAG for get_params and PARROT_ERRORS_RESULT_COUNT_FLAG for get_results. (It is not possible to control underflow behavior separately from overflow.)

Note that if the final target is a P register with FLAT set, then overflow can never occur. Similarly, if all target registers are marked OPTIONAL, then underflow is impossible.

Note also that when these errors are off, any excess destination registers in the case of underflow are left untouched, rather than being reset to zero or null. Excess registers explicitly marked OPTIONAL are always reset, regardless of the error flag settings.

Ordering of named values (outgoing)

Named values (arguments, or values to return) must be listed textually after all the positional values. FLAT and non-FLAT values may be mixed in any order.

Ordering of named targets (incoming)

Named targets (parameters, or returned values) must appear after all the positional targets. A SLURPY positional target, if present, must be the last positional target; a SLURPY named target, if present, must be the last named target.

So the acceptable ordering of targets is:

positional non-SLURPY (any number)
positional SLURPY array (optional)
NAMED non-SLURPY (any number)
NAMED SLURPY hash (optional)

Mixing named and positional values

Positional targets can only be filled with positional values.

Named targets can be filled with either positional or named values. However, if a named target was already filled by a positional value, and then a named value is also given, this is an overflow error.

Type Conversions

Unlike the set_* opcodes, the get_* opcodes must perform conversion from one register type to another. Here are the conversion rules:

When the target is an I, N, or S register, storage will behave like an assign (standard conversion).
When the target and source are both P registers, storage will behave like a set (pass by reference).
When the target is a P register and the source is an integer, the P will be set to a new Integer[1] which has been assigned the given integer.
When the target is a P register and the source is a number, the P will be set to a new Float[1] which has been assigned the given number.
When the target is a P register and the source is a string, the P will be set to a new String[1] which has been assigned the given string.

[1] or some other type specified by the current HLL type map, which may substitute an alternative type for each default low-level Parrot type (array, hash, string, number, etc.).

Implementation

Not applicable.

Bugs

Required features are missing:

Specific exceptions to throw for specific errors.

PIR Syntax Examples

Function Calls

  .local pmc foo, i, ar, y, p, value, kw, a, b, c, z
  # ...
  foo(1, i)                       # 2 positional arguments
  foo(x, ar :flat, y)             # flattening array
  foo(p, 'key' => value)          # named argument
  foo(p, value :named('key'))     # the same
  foo(kw :named :flat)            # a flattening hash

  # all together now: three positional (one flat) with two named (one flat)
  foo(a, b, c :flat, 'x' => 3, 'y' => 4, z :flat :named('z'))

Parameters

  .sub foo 
    .param int i                    # positional parameter
    .param pmc argv :slurpy         # slurpy array
    .param pmc value :named('key')  # named parameter
    .param int x :optional          # optional parameter
    .param int has_x :opt_flag      # flag 0/1 x was passed
    .param pmc kw :slurpy :named    # slurpy hash
    # ...
  .end

Return Values

  .sub foo
    .local pmc i, ar, value
    .return (i, ar: flat, value :named('key') )
  .end

Call Results

  .local pmc x, foo, i, j, ar, value
  x = foo()                       # single result
  (i, j :optional, ar :slurpy, value :named('key') ) = foo()

References

pdd23_exceptions.pod