py: Adjust object repr C (30-bit stuffed float) to reduce code size.

This patch adds/subtracts a constant from the 30-bit float representation
so that str/qstr representations are favoured: they now have all the high
bits set to zero.  This makes encoding/decoding qstr strings more
efficient (and they are used more often than floats, which are now
slightly less efficient to encode/decode).

Saves about 300 bytes of code space on Thumb 2 arch.

1 files changed, 5 insertions, 2 deletions

diff --git a/py/mpconfig.h b/py/mpconfig.h
index 98e9406c6..4f023934e 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -63,14 +63,17 @@
 //  - xxxx...xxx0 : a pointer to an mp_obj_base_t (unless a fake object)
 #define MICROPY_OBJ_REPR_B (1)
 
-// A MicroPython object is a machine word having the following form:
+// A MicroPython object is a machine word having the following form (called R):
 //  - iiiiiiii iiiiiiii iiiiiiii iiiiiii1 small int with 31-bit signed value
-//  - x1111111 1qqqqqqq qqqqqqqq qqqqq110 str with 20-bit qstr value
+//  - 01111111 1qqqqqqq qqqqqqqq qqqqq110 str with 20-bit qstr value
 //  - s1111111 10000000 00000000 00000010 +/- inf
 //  - s1111111 1xxxxxxx xxxxxxxx xxxxx010 nan, x != 0
 //  - seeeeeee efffffff ffffffff ffffff10 30-bit fp, e != 0xff
 //  - pppppppp pppppppp pppppppp pppppp00 ptr (4 byte alignment)
+// Str and float stored as O = R + 0x80800000, retrieved as R = O - 0x80800000.
+// This makes strs easier to encode/decode as they have zeros in the top 9 bits.
 // This scheme only works with 32-bit word size and float enabled.
+
 #define MICROPY_OBJ_REPR_C (2)
 
 #ifndef MICROPY_OBJ_REPR