Comment by dkersten
7 hours ago
I've been playing around with this and its worth noting that pre-decoding the bytecode because it means every instruction (without operands) is the width of a pointer (8 bytes on x86) which means you fit far fewer instructions into cache, eg my opcodes are a byte, so that's 8x more instructions. I haven't had time to compare it in benchmarks to see what the real world difference is, but its worth keeping in mind.
Somewhat off topic, looking at that assembly... mine compiles to (for one of the opcodes):
movzx eax,BYTE PTR [rdi]
lea r9,[rip+0x1d6fd] # 2ae30 <instructions_table>
mov rax,QWORD PTR [r9+rax*8]
inc rdi
jmp rax
(also compiled from C++ with clang's musttail annotation)
I have wondered whether it's worth storing instruction offsets (from the first instruction) rather than raw instruction pointers to increase cache efficiency, then they could be encoded in just 2 (or at worst 3) bytes. At the cost of an extra register.
That sounds like a good middle ground. Ahh I wish I had some more time, I’d love to benchmark all three and see how they compare.
In my own you VM, I do actually have a spare general purpose registers available for use. Now I just need to find the time to try it out…