py: Rework and compress second part of bytecode prelude.

This patch compresses the second part of the bytecode prelude which
contains the source file name, function name, source-line-number mapping
and cell closure information.  This part of the prelude now begins with a
single varible length unsigned integer which encodes 2 numbers, being the
byte-size of the following 2 sections in the header: the "source info
section" and the "closure section".  After decoding this variable unsigned
integer it's possible to skip over one or both of these sections very
easily.

This scheme saves about 2 bytes for most functions compared to the original
format: one in the case that there are no closure cells, and one because
padding was eliminated.

1 files changed, 11 insertions, 4 deletions

diff --git a/py/emitnative.c b/py/emitnative.c
index d0e758f56..2c976606c 100644
--- a/py/emitnative.c
+++ b/py/emitnative.c
@@ -208,6 +208,7 @@ struct _emit_t {
     uint16_t code_state_start;
     uint16_t stack_start;
     int stack_size;
+    uint16_t n_cell;
 
     uint16_t const_table_cur_obj;
     uint16_t const_table_num_obj;
@@ -587,9 +588,14 @@ STATIC void emit_native_end_pass(emit_t *emit) {
         size_t n_exc_stack = 0; // exc-stack not needed for native code
         MP_BC_PRELUDE_SIG_ENCODE(n_state, n_exc_stack, emit->scope, emit_native_write_code_info_byte, emit);
 
-        // write code info
         #if MICROPY_PERSISTENT_CODE
-        mp_asm_base_data(&emit->as->base, 1, 5);
+        size_t n_info = 4;
+        #else
+        size_t n_info = 1;
+        #endif
+        MP_BC_PRELUDE_SIZE_ENCODE(n_info, emit->n_cell, emit_native_write_code_info_byte, emit);
+
+        #if MICROPY_PERSISTENT_CODE
         mp_asm_base_data(&emit->as->base, 1, emit->scope->simple_name);
         mp_asm_base_data(&emit->as->base, 1, emit->scope->simple_name >> 8);
         mp_asm_base_data(&emit->as->base, 1, emit->scope->source_file);
@@ -599,14 +605,15 @@ STATIC void emit_native_end_pass(emit_t *emit) {
         #endif
 
         // bytecode prelude: initialise closed over variables
+        size_t cell_start = mp_asm_base_get_code_pos(&emit->as->base);
         for (int i = 0; i < emit->scope->id_info_len; i++) {
             id_info_t *id = &emit->scope->id_info[i];
             if (id->kind == ID_INFO_KIND_CELL) {
-                assert(id->local_num < 255);
+                assert(id->local_num <= 255);
                 mp_asm_base_data(&emit->as->base, 1, id->local_num); // write the local which should be converted to a cell
             }
         }
-        mp_asm_base_data(&emit->as->base, 1, 255); // end of list sentinel
+        emit->n_cell = mp_asm_base_get_code_pos(&emit->as->base) - cell_start;
     }
 
     ASM_END_PASS(emit->as);