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, 7 insertions, 4 deletions

diff --git a/py/vm.c b/py/vm.c
index 82218fd97..7487ff61b 100644
--- a/py/vm.c
+++ b/py/vm.c
@@ -1441,10 +1441,13 @@ unwind_loop:
                 && *code_state->ip != MP_BC_RAISE_LAST) {
                 const byte *ip = code_state->fun_bc->bytecode;
                 MP_BC_PRELUDE_SIG_DECODE(ip);
-                size_t bc = code_state->ip - ip;
-                size_t code_info_size = mp_decode_uint_value(ip);
-                ip = mp_decode_uint_skip(ip); // skip code_info_size
-                bc -= code_info_size;
+                MP_BC_PRELUDE_SIZE_DECODE(ip);
+                const byte *bytecode_start = ip + n_info + n_cell;
+                #if !MICROPY_PERSISTENT_CODE
+                // so bytecode is aligned
+                bytecode_start = MP_ALIGN(bytecode_start, sizeof(mp_uint_t));
+                #endif
+                size_t bc = code_state->ip - bytecode_start;
                 #if MICROPY_PERSISTENT_CODE
                 qstr block_name = ip[0] | (ip[1] << 8);
                 qstr source_file = ip[2] | (ip[3] << 8);