Mark H Weaver <m...@netris.org> writes: > I certainly agree that we should have a generous number of registers, > but I suspect that the sweet spot for a VM is 256, because it enables > more compact dispatching code in the VM, and yet is more than enough to > allow a decent register allocator to generate good code. > > That's my educated guess anyway. Feel free to prove me wrong :)
The counterproof will usually be done by benchmarking, and will even differ between different processors sharing the same instruction set. I see two ways out: a) pick the register size individually for each function, as small as possible without spillage. Which makes the whole indistinguishable from a stack-based VM. b) don't generate the final bytecode until the code is actually being run. That means that _if_ code is precompiled, it will be precompiled into either stack-based VM or some other representation better suited to compile into code for a certain amount of registers. Of course, the threshold to picking actual registers of the available processor and compiling native code is then not all too large. -- David Kastrup
