py: Change jump opcodes to emit 1-byte jump offset when possible.

This commit introduces changes:

- All jump opcodes are changed to have variable length arguments, of either
  1 or 2 bytes (previously they were fixed at 2 bytes).  In most cases only
  1 byte is needed to encode the short jump offset, saving bytecode size.

- The bytecode emitter now selects 1 byte jump arguments when the jump
  offset is guaranteed to fit in 1 byte.  This is achieved by checking if
  the code size changed during the last pass and, if it did (if it shrank),
  then requesting that the compiler make another pass to get the correct
  offsets of the now-smaller code.  This can continue multiple times until
  the code stabilises.  The code can only ever shrink so this iteration is
  guaranteed to complete.  In most cases no extra passes are needed, the
  original 4 passes are enough to get it right by the 4th pass (because the
  2nd pass computes roughly the correct labels and the 3rd pass computes
  the correct size for the jump argument).

This change to the jump opcode encoding reduces .mpy files and RAM usage
(when bytecode is in RAM) by about 2% on average.

The performance of the VM is not impacted, at least within measurment of
the performance benchmark suite.

Code size is reduced for builds that include a decent amount of frozen
bytecode.  ARM Cortex-M builds without any frozen code increase by about
350 bytes.

Signed-off-by: Damien George <damien@micropython.org>

1 files changed, 8 insertions, 4 deletions

diff --git a/py/compile.c b/py/compile.c
index eb7389ec5..d61dabb9a 100644
--- a/py/compile.c
+++ b/py/compile.c
@@ -219,7 +219,7 @@ STATIC void mp_emit_common_start_pass(mp_emit_common_t *emit, pass_kind_t pass)
     } else if (pass > MP_PASS_STACK_SIZE) {
         emit->ct_cur_obj = emit->ct_cur_obj_base;
     }
-    if (pass == MP_PASS_EMIT) {
+    if (pass == MP_PASS_CODE_SIZE) {
         if (emit->ct_cur_child == 0) {
             emit->children = NULL;
         } else {
@@ -3020,7 +3020,7 @@ STATIC void check_for_doc_string(compiler_t *comp, mp_parse_node_t pn) {
     #endif
 }
 
-STATIC void compile_scope(compiler_t *comp, scope_t *scope, pass_kind_t pass) {
+STATIC bool compile_scope(compiler_t *comp, scope_t *scope, pass_kind_t pass) {
     comp->pass = pass;
     comp->scope_cur = scope;
     comp->next_label = 0;
@@ -3187,10 +3187,12 @@ STATIC void compile_scope(compiler_t *comp, scope_t *scope, pass_kind_t pass) {
         EMIT(return_value);
     }
 
-    EMIT(end_pass);
+    bool pass_complete = EMIT(end_pass);
 
     // make sure we match all the exception levels
     assert(comp->cur_except_level == 0);
+
+    return pass_complete;
 }
 
 #if MICROPY_EMIT_INLINE_ASM
@@ -3600,8 +3602,10 @@ mp_compiled_module_t mp_compile_to_raw_code(mp_parse_tree_t *parse_tree, qstr so
             }
 
             // final pass: emit code
+            // the emitter can request multiple of these passes
             if (comp->compile_error == MP_OBJ_NULL) {
-                compile_scope(comp, s, MP_PASS_EMIT);
+                while (!compile_scope(comp, s, MP_PASS_EMIT)) {
+                }
             }
         }
     }