I explained very badly. The issue is not spilling (at the parrot level) The problem is: if you only pick the highest priority parrot registers and put them in real registers you are losing oportunities where copying the date once will save you from copying it many times. You are, in some sense, underspilling.
Let's see an example. Imagine you are compilling this imc, to be run in a machine which has 3 registers free (after temporaries): set $I1, 1 add $I1, $I1, 1 print $I1 set $I2, 1 add $I2, $I2, 1 print $I2 set $I3, 1 add $I3, $I3, 1 print $I3 set $I4, 1 add $I4, $I4, 1 print $I4 set $I5, 1 add $I5, $I5, 1 print $I5 print $I1 print $I2 print $I3 print $I4 print $I5 Very silly code indeed, but you get the idea. Since we have only 5 vars, imcc would turn this into: set I1, 1 add I1, I1, 1 print I1 set I2, 1 add I2, I2, 1 print I2 set I3, 1 add I3, I3, 1 print I3 set I4, 1 add I4, I4, 1 print I4 set I5, 1 add I5, I5, 1 print I5 print I1 print I2 print I3 print I4 print I5 Now, assuming you put registers I1-I3 in real registers, what would it take to execute this code in JIT? It would have to move the values of I4 and I5 from memory to registers a total of 10 times (4 saves and 6 restores if you assume the JIT is smart) [This particular example could be improved by making the jit look if the same parrot register is going to be used in the next op, but that's not the point] But, if IMCC knew that there were really only 3 registers in the machine, it would generate: set I1, 1 add I1, I1, 1 print I1 set I2, 1 add I2, I2, 1 print I2 set I3, 1 add I3, I3, 1 print I3 fast_save I3, 1 set I3, 1 add I3, I3, 1 print I3 fast_save I3, 2 set I3, 1 add I3, I3, 1 print I3 fast_save I3, 3 print I1 print I2 fast_restore I3, 3 print I3 fast_restore I3, 2 print I3 fast_restore I3, 1 print I3 When running this code in the JIT, it would only require 6 moves (3 saves, 3 restores): exactly the ones generated by imcc. In reality this would be even better, because as you have the garantee of having the data already in real registers you need less temporaries and so have more machine registers free. > So the final goal could be, to emit these load/stores too, which > then could be optimized to avoid duplicate loading/storing. Or imcc > could emit a register move, if in the next instruction the parrot > register is used again. Yes, that's the idea: making imcc generate the loads/stores, using the info about how many registers are actually available in the real machine _and_ its own knowledge about the program flow. An even better goal would be to have imcc know how many temporaries every JITed op requires, and use this information during register allocation. All this is obviously machine dependent: the code generated should only run in the machine it was compiled for. So we should always keep the original imc code in case we copy the pbc file to another machine. -angel
