/ 
parrot-0.3.1
/ PIR

NAME

PIR - calling conventions

VERSION

0.5

NOTE ABOUT CHANGES IN PROGRESS

The syntax here is going to stay, but the calling convention is in the middle of a transition. Instead of fixed registers, the new convention allows the user to use any registers in any order, and converts types as required. Additional syntax features, like the :flat adverb for arguments and :slurpy for parameters, will make sense to explain here once the document as a whole is updated.

One thing that's handy: If you have a list in a PMC and you want it to be flattened out into a list, append :flat to the argument name in the .arg directive or in the shortcut parameter list.

OVERVIEW

This document describes subroutine and method calling conventions.

DESCRIPTION

As imcc does register allocation, it has to track the life span of variables. This includes the (possible) data flow in and out of subroutines.

Parrot calling conventions - CPS

Explicitely Calling PASM Subroutines

newsub $P0, .Sub, _sub_label
newsub $P1, .Continuation, ret_addr
...
.local int x
.local num y
.local str z
.pcc_begin
.arg x
.arg y
.arg z
.pcc_call $P0, $P1	# r = _sub_label(x, y, z)
ret_addr:
.local int r  # optional - new result var
.result r
.pcc_end

The Short Way

...  # variable decls
r = _sub_label(x, y, z)
(r1[, r2 ...]) = _sub_label(x, y, z)
_sub_label(x, y, z)

Instead of the label a Subroutine object can be used too:

find_global $P0, "_sub_label"
$P0(args)

Subroutines

.sub _sub_label [Subpragma, ...]
 .param int a # I5
 .param int b # I6
 .param int c # I7
...
.pcc_begin_return
 .return xy   # e.g. I5
.pcc_end_return
...
.end

An alternative syntaxs allow to express a return in one line. The surrounded parentheses are mandatory. Besides making sequence break more conspiscuous, this is necessary to distinguish this syntax from other uses of the .return directive that will be probably deprecated.

.return ( a, b )      # return the values of a and b

.return ()            # return no value

.return func_call()   # tail call function

.return o."meth"()    # tail method call

Similarly, one can yield using the .yield directive

.yield ( a, b )      # yield with the values of a and b

.yield ()            # yield with no value

Subpragma

This is a comma separated list of zero or more items with the following meaning:

  • :main

    Define "main" entry point to start execution. If multiple subroutines are marked as :main, the last marked subroutine is entered.

  • :load

    Run this subroutine during the load_library opcode. :load is ignored, if another subroutine in that file is marked with :main.

  • :anon

    Do not install this subroutine in the namespace. Allows the subroutine name to be reused.

  • :multi(Type1, Type2...)

    Engage in multiple dispatch with the listed types.

  • method

    Declare subroutine being a method.

Notes:

  • newsub

    Currently needs the .Class syntax, i.e. a dot in front of the class name.

  • pcc_call

    Takes either 2 arguments: the sub and the return continuation, or the sub only. For the latter case an invokecc gets emitted. Providing an explicit return continuation is more efficient, if its created outside of a loop and the call is done inside a loop.

  • Saved Regs:

    Only the top half of registers are preserved currently.

  • .args, .param, .result, and .return are optional.

  • .param

    The .param declarations must be the first statements in the sub if any. No other statements are allowed between .param - not even comments currently.

  • pcc_begin_return, pcc_end_return

    If there is no return value and the return should be the last instruction of the subroutine, this declaration pair can be omitted. Parrot provides an invoke P1 as last instruction automatically.

Calling Methods

The syntax is very similar to subroutine calls. The call is done with meth_call which must immediately be preceded by the .invocant:

 .local pmc class
 .local pmc obj
 newclass class, "Foo"
 find_type $I0, "Foo"
 new obj, $I0
.pcc_begin
.arg x
.arg y
.arg z
.invocant obj
.meth_call "_method" [, $P1 ]	# r = obj."_method"(x, y, z)
.local int r  # optional - new result var
.result r
.pcc_end

The return continuation is optional. The method can be a string constant or a string variable.

Shortcuts

r = obj."_method"(args)
(r1, r2) = obj."_method"(args)
obj."_method"(args)

Methods

.namespace [ "Foo" ]

.sub _sub_label method [,Subpragma, ...]
 .param int a # I5
 .param int b # I6
 .param int c # I7
 ...
 self."_other_meth"()
...
.pcc_begin_return
 .return xy   # e.g. I5
.pcc_end_return
...
.end

The variable "self" automatically refers to the invocating object, if the subroutine declaration contains "method".

Restore namespace to the global namespace:

.namespace [ "" ]

NCI

Proposed syntax:

load_lib $P0, "libname"
dlfunc $P1, $P0, "funcname", "signature"
...
.pcc_begin
.arg x
.arg y
.arg z
.nci_call $P1	# r = funcname(x, y, z)
.local int r  # optional - new result var
.result r
.pcc_end

This prepares parameters as described in pdd03_calling_conventions.pod, saves the registers and invokes the function. The .arg pseudo ops put the given argument into increasing registers of the appropriate type.

Exception handlers

TBD.

Stack calling conventions

Arguments are saved in reverse order onto the user stack:

.arg y	# save args in reversed order
.arg x
call _foo	#(r, s) = _foo(x,y)
.local int r
.local int s
.result r	# restore results in order
.result s	#

and return values are restored in argument order from there.

The subroutine is responsible for preserving registers.

.sub _foo		# sub foo(int a, int b)
  saveall
  .param int a         # receive arguments from left to right
  .param int b
  ...

  .return mi		# return (pl, mi), push results
  .return pl		# in reverse order
  restoreall
  ret
.end

Rational

Pushing arguments in reversed order on the user stack makes the left most argument the top of stack entry. This allows for a variable number of function arguments (and return values), where the left most argument before a variable number of following arguments is the argument count.

Status

Implemented. When the subroutine is in the same compilation unit, the callee can saveall registers; when the subroutine is in a different compilation unit, the callee must preserve all used registers.

Invoking subroutines

IMCC tries to keep track of the address where the invoke will branch to, but can only succeed to do so when the set_addr and the invoke opcodes are located together.

$P10 = new Sub
$I1 = addr _the_sub
$P10 = $I1
invoke $P10   # ok

But not:

  bsr get_addr
  invoke $P10 # error
  ...
get_addr:
  $P10 = new Sub
  $I1 = addr _the_sub
  $P10 = $I1
  ret

The latter example will very likely lead to an incorrect CFG and thus to incorrect register allocation.

Status

Implemented. When the subroutine does saveall/restoreall, the branch from the ret statement back is ignored in the CFG.

Namespaces and lexicals

- Should imcc keep track of pad opcodes?
- Should imcc even emit such opcodes from e.g. .local directives?

FILES

imcc/imcc.y, imcc/t/syn/bsr.t, imcc/t/syn/pcc.t, imcc/t/syn/objects.t, docs/pdds/pdd03_calling_conventions.pod

AUTHOR

Leopold Toetsch <lt@toetsch.at>

CHANGES

0.5 eliminate {non_,}prototyped distinction
0.4 methods and shortcuts documented
0.3 updated parrot calling conventions and invoke
0.2 initial, checked in
0.1 initial proposal