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, 53 insertions, 11 deletions

diff --git a/py/bc.h b/py/bc.h
index 1ae8c9925..fd52571fd 100644
--- a/py/bc.h
+++ b/py/bc.h
@@ -42,17 +42,25 @@
 //          K = n_kwonly_args       number of keyword-only arguments this function takes
 //          D = n_def_pos_args      number of default positional arguments
 //
-//  code_info_size  : var uint |    code_info_size counts bytes in this chunk
-//  simple_name     : var qstr |
-//  source_file     : var qstr |
-//  <line number info>         |
-//  <word alignment padding>   |    only needed if bytecode contains pointers
+//  prelude size    : var uint
+//      contains two values interleaved bit-wise as: xIIIIIIC repeated
+//          x = extension           another byte follows
+//          I = n_info              number of bytes in source info section
+//          C = n_cells             number of bytes/cells in closure section
+//
+//  source info section:
+//      simple_name : var qstr
+//      source_file : var qstr
+//      <line number info>
+//
+//  closure section:
+//      local_num0  : byte
+//      ...         : byte
+//      local_numN  : byte          N = n_cells-1
+//
+//  <word alignment padding>        only needed if bytecode contains pointers
 //
-//  local_num0      : byte     |
-//  ...             : byte     |
-//  local_numN      : byte     |    N = num_cells
-//  255             : byte     |    end of list sentinel
-//  <bytecode>                 |
+//  <bytecode>
 //
 //
 // constant table layout:
@@ -122,6 +130,41 @@ do {                                                            \
     size_t n_state, n_exc_stack, scope_flags, n_pos_args, n_kwonly_args, n_def_pos_args; \
     MP_BC_PRELUDE_SIG_DECODE_INTO(ip, n_state, n_exc_stack, scope_flags, n_pos_args, n_kwonly_args, n_def_pos_args)
 
+#define MP_BC_PRELUDE_SIZE_ENCODE(I, C, out_byte, out_env)      \
+do {                                                            \
+    /* Encode bit-wise as: xIIIIIIC */                          \
+    uint8_t z = 0;                                              \
+    do {                                                        \
+        z = (I & 0x3f) << 1 | (C & 1);                          \
+        C >>= 1;                                                \
+        I >>= 6;                                                \
+        if (C | I) {                                            \
+            z |= 0x80;                                          \
+        }                                                       \
+        out_byte(out_env, z);                                   \
+    } while (C | I);                                            \
+} while (0)
+
+#define MP_BC_PRELUDE_SIZE_DECODE_INTO(ip, I, C)                \
+do {                                                            \
+    uint8_t z;                                                  \
+    C = 0;                                                      \
+    I = 0;                                                      \
+    for (unsigned n = 0;; ++n) {                                \
+        z = *(ip)++;                                            \
+        /* xIIIIIIC */                                          \
+        C |= (z & 1) << n;                                      \
+        I |= ((z & 0x7e) >> 1) << (6 * n);                      \
+        if (!(z & 0x80)) {                                      \
+            break;                                              \
+        }                                                       \
+    }                                                           \
+} while (0)
+
+#define MP_BC_PRELUDE_SIZE_DECODE(ip) \
+    size_t n_info, n_cell; \
+    MP_BC_PRELUDE_SIZE_DECODE_INTO(ip, n_info, n_cell)
+
 // Sentinel value for mp_code_state_t.exc_sp_idx
 #define MP_CODE_STATE_EXC_SP_IDX_SENTINEL ((uint16_t)-1)
 
@@ -139,7 +182,6 @@ typedef struct _mp_bytecode_prelude_t {
     qstr qstr_block_name;
     qstr qstr_source_file;
     const byte *line_info;
-    const byte *locals;
     const byte *opcodes;
 } mp_bytecode_prelude_t;