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, 24 insertions, 2 deletions

diff --git a/py/vm.c b/py/vm.c
index 497a56962..990009c00 100644
--- a/py/vm.c
+++ b/py/vm.c
@@ -61,8 +61,30 @@
     do { \
         unum = (unum << 7) + (*ip & 0x7f); \
     } while ((*ip++ & 0x80) != 0)
-#define DECODE_ULABEL size_t ulab = (ip[0] | (ip[1] << 8)); ip += 2
-#define DECODE_SLABEL size_t slab = (ip[0] | (ip[1] << 8)) - 0x8000; ip += 2
+
+#define DECODE_ULABEL \
+    size_t ulab; \
+    do { \
+        if (ip[0] & 0x80) { \
+            ulab = ((ip[0] & 0x7f) | (ip[1] << 7)); \
+            ip += 2; \
+        } else { \
+            ulab = ip[0]; \
+            ip += 1; \
+        } \
+    } while (0)
+
+#define DECODE_SLABEL \
+    size_t slab; \
+    do { \
+        if (ip[0] & 0x80) { \
+            slab = ((ip[0] & 0x7f) | (ip[1] << 7)) - 0x4000; \
+            ip += 2; \
+        } else { \
+            slab = ip[0] - 0x40; \
+            ip += 1; \
+        } \
+    } while (0)
 
 #if MICROPY_EMIT_BYTECODE_USES_QSTR_TABLE