py: Compress first part of bytecode prelude.

The start of the bytecode prelude contains 6 numbers telling the amount of
stack needed for the Python values and exceptions, and the signature of the
function.  Prior to this patch these numbers were all encoded one after the
other (2x variable unsigned integers, then 4x bytes), but using so many
bytes is unnecessary.

An entropy analysis of around 150,000 bytecode functions from the CPython
standard library showed that the optimal Shannon coding would need about
7.1 bits on average to encode these 6 numbers, compared to the existing 48
bits.

This patch attempts to get close to this optimal value by packing the 6
numbers into a single, varible-length unsigned integer via bit-wise
interleaving.  The interleaving scheme is chosen to minimise the average
number of bytes needed, and at the same time keep the scheme simple enough
so it can be implemented without too much overhead in code size or speed.
The scheme requires about 10.5 bits on average to store the 6 numbers.

As a result most functions which originally took 6 bytes to encode these 6
numbers now need only 1 byte (in 80% of cases).

1 files changed, 5 insertions, 3 deletions

diff --git a/py/objgenerator.c b/py/objgenerator.c
index 39dcbefb9..359dade88 100644
--- a/py/objgenerator.c
+++ b/py/objgenerator.c
@@ -51,8 +51,8 @@ STATIC mp_obj_t gen_wrap_call(mp_obj_t self_in, size_t n_args, size_t n_kw, cons
     mp_obj_fun_bc_t *self_fun = MP_OBJ_TO_PTR(self_in);
 
     // bytecode prelude: get state size and exception stack size
-    size_t n_state = mp_decode_uint_value(self_fun->bytecode);
-    size_t n_exc_stack = mp_decode_uint_value(mp_decode_uint_skip(self_fun->bytecode));
+    const uint8_t *ip = self_fun->bytecode;
+    MP_BC_PRELUDE_SIG_DECODE(ip);
 
     // allocate the generator object, with room for local stack and exception stack
     mp_obj_gen_instance_t *o = m_new_obj_var(mp_obj_gen_instance_t, byte,
@@ -88,7 +88,9 @@ STATIC mp_obj_t native_gen_wrap_call(mp_obj_t self_in, size_t n_args, size_t n_k
 
     // Determine start of prelude, and extract n_state from it
     uintptr_t prelude_offset = ((uintptr_t*)self_fun->bytecode)[0];
-    size_t n_state = mp_decode_uint_value(self_fun->bytecode + prelude_offset);
+    const uint8_t *ip = self_fun->bytecode + prelude_offset;
+    size_t n_state, n_exc_stack_unused, scope_flags, n_pos_args, n_kwonly_args, n_def_args;
+    MP_BC_PRELUDE_SIG_DECODE_INTO(ip, n_state, n_exc_stack_unused, scope_flags, n_pos_args, n_kwonly_args, n_def_args);
     size_t n_exc_stack = 0;
 
     // Allocate the generator object, with room for local stack and exception stack