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PDD 3: Calling Conventions ^

Abstract

Parrot's inter-routine calling conventions.

Synopsis

Not applicable.

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:

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:

    0-3   TYPE
               0b0000 = I
               0b0001 = S
               0b0010 = P
               0b0011 = N

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

The value is a literal constant, not a register. (Don't set this bit yourself; the assembler will do it.)

5 FLAT (P only)

If this bit is set on a PMC value, then the PMC must be an aggregate. The contents of the aggregate, rather than the aggregate itself, will be passed.

If the NAMED bit is also set, the aggregate will be used as a hash; its contents, as key/value pairs, will be passed as named arguments. The PMC must implement the full hash interface. {{ GH #252: Limit the required interface. }}

If the NAMED bit is not set, the aggregate will be used as an array; its contents will be passed as positional arguments.

The meaning of this bit is undefined when applied to integer, number, and string values.

6 (unused)
7 (unused)
8 (unused)
9 NAMED (FLAT or string constant only)

When the FLAT bit is also set, behavior is as described above in the "FLAT" section. Otherwise, this bit may only be set on a unique string constant specifying the name of the next argument (or returned value).

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)

If this bit is set on a P register, then it will be populated with an aggregate that will contain all of the remaining values that have not already been stored in other registers.

All such values will be converted to PMCs according to the detailed rules below, and those PMCs will be stored into the new aggregate.

If the NAMED bit is also set, the aggregate will be the HLL-specific hash type and the contents will be all unassigned _named_ arguments.

If the NAMED bit is not set, the aggregate will be the HLL-specific array type and the contents will be all unassigned positional arguments.

6 (unused)
7 OPTIONAL

If this bit is set on a register for which no value has been passed, no exception will be raised; rather, the register will be set to a default value: a Null PMC for P, an empty string for S, or zero for N or I.

8 OPT_FLAG (I only)

An I register with this bit set is set to one if the immediately preceding OPTIONAL register received a value; otherwise, it is set to zero. If the preceding register was not marked OPTIONAL, the behavior is undefined; but we promise you won't like it.

8 READONLY (P only)

XXX - PROPOSED ONLY - XXX

If this bit is set on a P register that receives a value, Parrot will ensure that the final value in the P register is read-only (i.e. will not permit modification). If the received value was a mutable PMC, then Parrot will create and set the register to a {not yet invented} read-only PMC wrapper around the original PMC.

Future Notes: Parrot's algorithm for deciding what is writable may be simplistic. In initial implementations, it may assume that any PMC not of a known read-only-wrapper type is mutable. Later it may allow the HLL to provide the test. But we must beware overdesigning this; any HLL with a truly complex notion of read-only probably needs to do this kind of wrapping itself.

8 (unused for S and N)
9 NAMED (SLURPY or string constant only)

When the SLURPY bit is also set, behavior is as described above in the "SLURPY" section. Otherwise, this bit may only be set on a unique string constant specifying the name of the next parameter (or returned value).

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:

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:

[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:

PIR Syntax Examples

Function Calls

Parameters

Return Values

Call Results

References

pdd23_exceptions.pod

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