How can I write self-modifying code that runs efficiently on modern x64 processors?

Question

I\'m trying to speed up a variable-bitwidth integer compression scheme and I\'m interested in generating and executing assembly code on-the-fly. Currently a lot of time is spe

Answer 1

This is less in the scope of SMC and more into Dynamic Binary Optimization, i.e. - you don't really manipulate the code you're running (as in writing new instructions), you can just generate a different piece of code, and reroute the appropriate call in your code to jump there instead. The only modification is at the entry point, and it's only done once, so you don't need to worry too much about the overhead (it usually means flushing all the pipelines to make sure the old instruction isn't still alive anywhere in the machine, i'd guess the penalty is a few hundreds of clock cycles, depending on how loaded the CPU is. Only relevant if it's occurring repeatedly).

In the same sense, you shouldn't worry too much about doing this ahead enough of time. By the way, regarding your question - the CPU would only be able to start executing ahead as far its ROB size, which in haswell is 192 uop (not instructions, but close enough), according to this - http://www.realworldtech.com/haswell-cpu/3/ , and would be able to see slightly further ahead thanks to the predictor and fetch units, so we're talking about overall of let's say a few hundreds).

Having that said, let me reiterate what was said here before - experiment, experiment experiment :)