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, 14 deletions

diff --git a/py/showbc.c b/py/showbc.c
index 216f35266..d154511dc 100644
--- a/py/showbc.c
+++ b/py/showbc.c
@@ -85,10 +85,8 @@ void mp_bytecode_print(const void *descr, const byte *ip, mp_uint_t len, const m
 
     // Decode prelude
     MP_BC_PRELUDE_SIG_DECODE(ip);
-
+    MP_BC_PRELUDE_SIZE_DECODE(ip);
     const byte *code_info = ip;
-    mp_uint_t code_info_size = mp_decode_uint(&code_info);
-    ip += code_info_size;
 
     #if MICROPY_PERSISTENT_CODE
     qstr block_name = code_info[0] | (code_info[1] << 8);
@@ -102,7 +100,9 @@ void mp_bytecode_print(const void *descr, const byte *ip, mp_uint_t len, const m
         qstr_str(source_file), qstr_str(block_name), descr, mp_showbc_code_start, len);
 
     // raw bytecode dump
-    printf("Raw bytecode (code_info_size=" UINT_FMT ", bytecode_size=" UINT_FMT "):\n", code_info_size, len - code_info_size);
+    size_t prelude_size = ip - mp_showbc_code_start + n_info + n_cell;
+    printf("Raw bytecode (code_info_size=" UINT_FMT ", bytecode_size=" UINT_FMT "):\n",
+        prelude_size, len - prelude_size);
     for (mp_uint_t i = 0; i < len; i++) {
         if (i > 0 && i % 16 == 0) {
             printf("\n");
@@ -121,21 +121,18 @@ void mp_bytecode_print(const void *descr, const byte *ip, mp_uint_t len, const m
     printf("(N_STATE %u)\n", (unsigned)n_state);
     printf("(N_EXC_STACK %u)\n", (unsigned)n_exc_stack);
 
-    // for printing line number info
-    const byte *bytecode_start = ip;
+    // skip over code_info
+    ip += n_info;
 
     // bytecode prelude: initialise closed over variables
-    {
-        uint local_num;
-        while ((local_num = *ip++) != 255) {
-            printf("(INIT_CELL %u)\n", local_num);
-        }
-        len -= ip - mp_showbc_code_start;
+    for (size_t i = 0; i < n_cell; ++i) {
+        uint local_num = *ip++;
+        printf("(INIT_CELL %u)\n", local_num);
     }
 
     // print out line number info
     {
-        mp_int_t bc = bytecode_start - ip;
+        mp_int_t bc = 0;
         mp_uint_t source_line = 1;
         printf("  bc=" INT_FMT " line=" UINT_FMT "\n", bc, source_line);
         for (const byte* ci = code_info; *ci;) {
@@ -153,7 +150,7 @@ void mp_bytecode_print(const void *descr, const byte *ip, mp_uint_t len, const m
             printf("  bc=" INT_FMT " line=" UINT_FMT "\n", bc, source_line);
         }
     }
-    mp_bytecode_print2(ip, len - 0, const_table);
+    mp_bytecode_print2(ip, len - prelude_size, const_table);
 }
 
 const byte *mp_bytecode_print_str(const byte *ip) {