=head1 NAME

Imager::regmach - documents the register virtual machine used by
Imager::transform2().

=head1 SYNOPSIS

The register machine is a complete rewrite of the stack machine
originally used by Imager::transform(), written for use by
Imager::transform2().

=head1 DESCRIPTION

(This document might be a little incoherent.)

The register machine is a fast implementation of a small instruction
set designed for evaluating an arithmetic expression to produce a
color for an image.

The machine takes as input:

=over 4

=item instructions

An array of instructions

=item numeric registers

An array of numeric registers.  Some registers are initialized as
literals.

=item color registers

An array of color registers.  Currently these registers aren't
initialized.

=item input images

An array of Imager i_img pointers.  The C<getpn> operators read pixels
from these images.

=back

The instructions supplied each take up to 4 input numeric or color
registers with a single output numeric or color register.  The
machine attempts to execute instructions as safely as possible,
assuming that correct instructions have been provided, eg. the machine
protects against divide by zero, but doesn't check register numbers
for validity.

The final instruction must be a C<ret> instruction, which returns the
result ;)

=head2 Adding new instructions

To add a new instruction:

=over 4

=item 1

Add a new opcode to the enumeration in F<regmach.h> - make sure to add
comment after the enum name giving the input registers (C<rX> for
numeric, C<pX> for color) that the instruction takes.  These must be in
the order that the instruction expects to take the.  Put a letter (r
or p) after -> to indicate the result type.

=item 2

Add a case to F<regmach.c> that executes the instruction.

=item 3

make

=back

The F<Makefile> should rebuild the F<Regops.pm> file, and your new
instruction will be added as a function.

If you want to add a single alternative instruction that might take
different argument types (it must take the same number of parameters),
create another instruction with that name followed by a p.  The
current expression parsers explicitly look for such instruction names.

=head2 Future directions

Conditional and non-conditional jumps to implement iteration.  This
will break the current optimizer in L<Imager::Expr> (and the compilers
for both expression compilers, for that matter.)

Complex arithmetic (Addi suggested this one).  This would most likely
be a separate machine.  Otherwise we'll have a very significant
performance loss.

=head1 WARNINGS

If you feed bad 'machine code' to the register machine, you have a
good chance of a C<SIGSEGV>.

=cut

