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, 22 insertions, 21 deletions

diff --git a/py/emitbc.c b/py/emitbc.c
index 1aa219ca8..34f6362ff 100644
--- a/py/emitbc.c
+++ b/py/emitbc.c
@@ -64,6 +64,9 @@ struct _emit_t {
     size_t bytecode_size;
     byte *code_base; // stores both byte code and code info
 
+    size_t n_info;
+    size_t n_cell;
+
     #if MICROPY_PERSISTENT_CODE
     uint16_t ct_cur_obj;
     uint16_t ct_num_obj;
@@ -123,10 +126,6 @@ STATIC void emit_write_code_info_byte(emit_t* emit, byte val) {
     *emit_get_cur_to_write_code_info(emit, 1) = val;
 }
 
-STATIC void emit_write_code_info_uint(emit_t* emit, mp_uint_t val) {
-    emit_write_uint(emit, emit_get_cur_to_write_code_info, val);
-}
-
 STATIC void emit_write_code_info_qstr(emit_t *emit, qstr qst) {
     #if MICROPY_PERSISTENT_CODE
     assert((qst >> 16) == 0);
@@ -346,29 +345,17 @@ void mp_emit_bc_start_pass(emit_t *emit, pass_kind_t pass, scope_t *scope) {
         MP_BC_PRELUDE_SIG_ENCODE(n_state, n_exc_stack, scope, emit_write_code_info_byte, emit);
     }
 
-    // Write size of the rest of the code info.  We don't know how big this
-    // variable uint will be on the MP_PASS_CODE_SIZE pass so we reserve 2 bytes
-    // for it and hope that is enough!  TODO assert this or something.
-    if (pass == MP_PASS_EMIT) {
-        emit_write_code_info_uint(emit, emit->code_info_size - emit->code_info_offset);
-    } else  {
-        emit_get_cur_to_write_code_info(emit, 2);
+    // Write number of cells and size of the source code info
+    if (pass >= MP_PASS_CODE_SIZE) {
+        MP_BC_PRELUDE_SIZE_ENCODE(emit->n_info, emit->n_cell, emit_write_code_info_byte, emit);
     }
 
+    emit->n_info = emit->code_info_offset;
+
     // Write the name and source file of this function.
     emit_write_code_info_qstr(emit, scope->simple_name);
     emit_write_code_info_qstr(emit, scope->source_file);
 
-    // bytecode prelude: initialise closed over variables
-    for (int i = 0; i < scope->id_info_len; i++) {
-        id_info_t *id = &scope->id_info[i];
-        if (id->kind == ID_INFO_KIND_CELL) {
-            assert(id->local_num < 255);
-            emit_write_bytecode_raw_byte(emit, id->local_num); // write the local which should be converted to a cell
-        }
-    }
-    emit_write_bytecode_raw_byte(emit, 255); // end of list sentinel
-
     #if MICROPY_PERSISTENT_CODE
     emit->ct_cur_obj = 0;
     emit->ct_cur_raw_code = 0;
@@ -414,6 +401,20 @@ void mp_emit_bc_end_pass(emit_t *emit) {
 
     emit_write_code_info_byte(emit, 0); // end of line number info
 
+    // Calculate size of source code info section
+    emit->n_info = emit->code_info_offset - emit->n_info;
+
+    // Emit closure section of prelude
+    emit->n_cell = 0;
+    for (size_t 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);
+            emit_write_code_info_byte(emit, id->local_num); // write the local which should be converted to a cell
+            ++emit->n_cell;
+        }
+    }
+
     #if MICROPY_PERSISTENT_CODE
     assert(emit->pass <= MP_PASS_STACK_SIZE || (emit->ct_num_obj == emit->ct_cur_obj));
     emit->ct_num_obj = emit->ct_cur_obj;