[Python-Dev] Why is Bytecode the way it is?

[Chris King]

While recently goofing around with the bytecode, I thought of doing 
something like this:

  case LOAD_CONST:
      x = GETITEM(consts, oparg);
      Py_INCREF(x);
+     if (*next_instr == RETURN_VAL) {
+         retval = x;
+         why = WHY_RETURN;
+         goto fast_block_end;
+     }
      PUSH(x);
      goto fast_next_opcode;

This would skip the stack and a trip through the loop without changing 
the parser or the bytecode,  and with a minimal amount of added code or 
overhead. This could (of course) be applied to other opcodes, too.

Perhaps instead of littering the function with that block, a macro 
"PUSH_MAYBE_RET" could be added that would replace the final PUSH in the 
opcode's case block:

#define PUSH_MAYBE_RET { if (*next_instr == RETURN_VAL) { \
                           retval = x; \
                           why = WHY_RETURN; \
                           goto fast_block_end; } \
                       PUSH(x); }

Not sure how much this would help speed, if any.