The old coalescing algorithm had some algorithmic complexity issues when
dealing with large virtual registers. Reimplement to use a proper
union-find algorithm so we only need one pass through the dominator
forests for virtual registers that are interference free.
Virtual registers that do have interference are split and new registers
built.
This pass is about twice as fast as the old one when dealing with
complex virtual registers.
When we detect interference between the values that have already been
merged into the candidate virtual register and an EBB argument, we first
try to resolve the conflict by splitting. We also check if the existing
interfering value is fundamentally incompatible with the branch
instruction so it needs to be removed from the virtual register,
restarting the merge operation.
However, this existing interfering value is not necessarily the only
interference, so the split is not guaranteed to resolve the conflict. If
it turns out that splitting didn't resolve the conflict, restart the
merge after removing this second conflicting value.
Fixes#56.
We now have complete support for value location annotations in the
textual IL format. Values defined by instructions as well as EBB
arguments are covered.
We allow ghost instructions to exist if they have no side effects.
Instructions that affect control flow or that have other side effects
must be encoded.
Teach the IL verifier to enforce this. Once any instruction has an
encoding, all instructions with side effects must have an encoding.
A function parameter in an incoming_arg stack slot should not be
coalesced into any virtual registers. We don't want to force the whole
virtual register to spill to the incoming_arg slot.
When coloring registers for a branch instruction, also make sure that
the values passed as EBB arguments are in the registers expected by the
EBB.
The first time a branch to an EBB is processed, assign the EBB arguments
to the registers where the branch arguments already reside so no
regmoves are needed.
Coalescing means creating virtual registers and transforming the code
into conventional SSA form. This means that every value used as a branch
argument will belong to the same virtual register as the corresponding
EBB argument value.
Conventional SSA form makes it easy to avoid memory-memory copies when
spilling values, and the virtual registers can be used as hints when
picking registers too. This reduces the number of register moves needed
for EBB arguments.
