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IMCC - operation


0.1 initial
0.2 uninitialized warning; optimizations
0.3 merged parsing.pod into this file.


This document describes the principles of IMCC operation.


The main features of imcc are:

Source file parsing
Register allocation
Code generation
Running the code



IMCC parses and generates code in terms of compilation units. These are self-contained blocks of code very similar to subroutines.

Code for a compilation unit is created as soon (or not earlier) as the end of the unit is reached.

{{ Is this true? one sub calling another not-yet compiled sub would not work in that case. }}

Symbols, constants and labels

Compilation units maintain their own symbol table containing local labels and variable symbols. This symbol table, hash, is not visible to code in different units.

If you need global variables, please use the get_{hll,root}_global opcodes.

Global labels and constants are kept in the global symbol table ghash. This allows for global constant folding beyond the scope of individual subroutines.

This also means that you currently can't use the same global symbol (e.g. subroutine name) in different namespaces. The following creates invalid code:

Local labels in different compilation units with the same name are allowed, though assembling the generated PASM doesn't work. However, running this code inside imcc is ok. This will probably change in the future so that local labels are mangled to be unique.


Register allocation is done per compilation unit.

IMCC identifiers and 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:

will translate to

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 -O2.

PASM registers keep their register. During the usage of a 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.

Basic blocks

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

Call graph

All connections between the basic blocks are calculated. This allows for:

Loop detection

where the range and depth of loops is calculated.

Life analysis

Whenever an operand is marked as an 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 IN parameters must keep their parrot register over their usage range.

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

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


The latter of these emits the warning:

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

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.

Register allocation

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.


Optimizations are only done when enabled with the -O switch.

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 ...


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.

If-branch optimization

A sequence of code:

will be converted to

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

Branches to branches

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

Unused labels

Unreferenced labels are deleted.

Dead code removal

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


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

Used LHS once

For a sequence of code

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

Loop optimization

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

Code generation

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

Running code

Additionally the generated code can be run immediately inside imcc. All parrot runtime options like -R jit or -t are available.


imc.c, cfg.c, optimizer.c, pbc.c


Leopold Toetsch <>