# Copyright (C) 2001-2005, The Perl Foundation.
# $Id: operation.pod 12839 2006-05-30 14:34:31Z coke $

=head1 NAME

IMCC - operation

=head1 VERSION

=over 4

=item 0.1 initial

=item 0.2 uninitialized warning; optimizations

=back

=head1 OVERVIEW

This document describes the principles of IMCC operation.

=head1 DESCRIPTION

The main features of imcc are:

=over 4

=item Source file parsing

=item Register allocation

=item Optimization

=item Code generation

=item Running the code

=back

=head1 Source file parsing

See F<parsing.pod>, F<macros.pod> and F<syntax.pod> for more.

=head1 Register allocation

Register allocation is done per I<compilation unit>.

IMCC I<identifiers> and I<temporary variables> e.g. $I0 are assigned a
physical parrot register depending on the life range of these
variables. If the life range of one variable doesn't overlap the range
of another variable, they might get the same parrot register. For 
instance:

   $I0 = 10
   $I1 = 20

will translate to

   set I0, 10
   set I0, 20

provided that $I0 is not used after these lines. In this case, the 
assignment to $I0 is redundant and will be optimized away if imcc is
run with optimization level B<-O2>.

I<PASM registers> keep their register. During the usage of a I<PASM
register> this register will be not get assigned to. Therefore, they
should be used only when absolutely necessary, and you should try to
avoid using them within long pieces of code.

=head2 Basic blocks

To determine the life range of variables, the code gets separated into
pieces, called B<basic block>s. A B<basic block> starts at a label,
which can get jumped to, and ends at a branch instruction.

=head2 Call graph

All connections between the B<basic block>s are calculated. This allows
for:

=head2 Loop detection

where the range and depth of loops is calculated.

=head2 Life analysis

Whenever an operand is marked as an B<OUT> argument, this
operand starts with a new value. This means that at this point the
life range of the symbol ends and a new life range is started, which
allows the allocation of a different register to the same variable or
the same register to a different variable.

Variables used as B<IN> parameters must keep their parrot register
over their usage range.

When C<imcc> detects a register usage, where the first operation is
using (reading) a register (and warnings are enabled), C<imcc> emits
an appropriate message.

Consider these two code snippets (block numbers are attached):

  .sub main :main
 0      $I0 = 0		# initialized
 0      if $I0 goto l1
 1      $I1 = 1		# init in block 1
 1      goto l2
 2  l1:
 2      $I1 = 2		# init in block 2
 3  l2:
 3      print $I0
 3      print $I1	# all paths leading here do init
 3      print "\n"
 3      end
  .end

and:

  .sub main :main
 0      $I0 = 0		# initialized
 0      if $I0 goto l1	# branch to bb 1 or 2
 1      $I1 = 1		# init only in block 1
 2  l1:
 2      print $I0
 2      print $I1	# no init in code path from block 0
 2      print "\n"
 2      end
  .end

The latter of these emits the warning:

  warning:imcc:propagate_need: '$I1' might be used \
  uninitialized in _main:7

=head2 Interference graph

Once the above information is calculated, the next step is to look at
which variables interfere with which others. Non-interfering variables 
can be given the same parrot register.

=head2 Register allocation

C<imcc> then starts allocating registers according to a variable's
score. Variables deeply nested inside loops have the highest score and
get a parrot register first. Variables with a long life range (i.e.
with many interferences) get allocated last.

=head1 Optimization

Optimizations are only done when enabled with the B<-O> switch. Please
consult F<t/imcpasm/*.t> for examples. They occur between various
stages and may be repeatedly done: e.g. after converting a conditional
branch to an absolute one, unreachable code will be removed then,
which might cause unused labels ...

=head1 Optimizations with -O1

=head2 Constant substitution

Constant arguments to many ops are evaluated. Conditional branches
with constant conditions are converted to unconditional branches.
Integer arguments to float operations are converted to float operands.

=head2 If-branch optimization

A sequence of code:

    if cond, L1
    branch L2
 L1:
 ...
 L2:

will be converted to

    unless cond, L2
    ...
 L2:

The same is done for other conditional branches B<gt>, B<ge>, B<eq>
and their reverse meanings.

=head2 Branches to branches

Unconditional branch sequences get optimized to jumps to the final label.

=head2 Unused labels

Unreferenced labels are deleted.

=head2 Dead code removal

Code not reachable after an unconditional branch instruction and basic
blocks that are not entered from somewhere get removed.

=head1 Optimizations with -O2

Note: These are currently experimental and might not do the Right
Thing.

=head2 Used LHS once

For a sequence of code

   $I0 = 10
   $I1 = 20

where B<$I0> is not used again, the first assignment will be tossed,
resulting in code like:

   set I0, 20

=head2 Loop optimization

Instructions which are invariant to a loop are pulled out of the loop
and inserted in front of the loop entry.

=head1 Code generation

C<imcc> either generates PASM or else directly generates a PBC file for
running with parrot.

=head1 Running code

Additionally the generated code can be run immediately inside imcc.
All parrot runtime options like B<-j> or B<-t> are available.

=head1 FILES

F<imc.c>, F<cfg.c>, F<optimizer.c>, F<pbc.c>

=head1 AUTHOR

Leopold Toetsch <lt@toetsch.at>