The substraction contrained the register allocator
to allocate a different register for the result and
the second operand, now, we use a neg trick to compile
it down. The machinery that was setup is, regardless,
interesting and will have to be used for floating
point computations (division).
The first bug in rega made broke the explicited loop
invariant: we were using register allocation unavailable
information from other blocks. It's still unclear
how we got wrong results from that considering mappings
are all 0-initialized.
The second bug is a stupid one, one sizeof operator was
missing from a memcpy...
e5983ba1a2