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.

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

py/obj.h

@@ -130,16 +130,16 @@ static inline bool MP_OBJ_IS_SMALL_INT(mp_const_obj_t o)
 #define MP_OBJ_SMALL_INT_VALUE(o) (((mp_int_t)(o)) >> 1)
 #define MP_OBJ_NEW_SMALL_INT(small_int) ((mp_obj_t)((((mp_int_t)(small_int)) << 1) | 1))
 
-#define mp_const_float_e ((mp_obj_t)((0x402df854 & ~3) | 2))
-#define mp_const_float_pi ((mp_obj_t)((0x40490fdb & ~3) | 2))
+#define mp_const_float_e ((mp_obj_t)(((0x402df854 & ~3) | 2) + 0x80800000))
+#define mp_const_float_pi ((mp_obj_t)(((0x40490fdb & ~3) | 2) + 0x80800000))
 
 static inline bool mp_obj_is_float(mp_const_obj_t o)
-    { return (((mp_uint_t)(o)) & 3) == 2 && (((mp_uint_t)(o)) & 0x7f800004) != 0x7f800004; }
+    { return (((mp_uint_t)(o)) & 3) == 2 && (((mp_uint_t)(o)) & 0xff800007) != 0x00000006; }
 static inline mp_float_t mp_obj_float_get(mp_const_obj_t o) {
     union {
         mp_float_t f;
         mp_uint_t u;
-    } num = {.u = (mp_uint_t)o & ~3};
+    } num = {.u = ((mp_uint_t)o - 0x80800000) & ~3};
     return num.f;
 }
 static inline mp_obj_t mp_obj_new_float(mp_float_t f) {
@@ -147,13 +147,13 @@ static inline mp_obj_t mp_obj_new_float(mp_float_t f) {
         mp_float_t f;
         mp_uint_t u;
     } num = {.f = f};
-    return (mp_obj_t)((num.u & ~0x3) | 2);
+    return (mp_obj_t)(((num.u & ~0x3) | 2) + 0x80800000);
 }
 
 static inline bool MP_OBJ_IS_QSTR(mp_const_obj_t o)
-    { return (((mp_uint_t)(o)) & 0x7f800007) == 0x7f800006; }
-#define MP_OBJ_QSTR_VALUE(o) ((((mp_uint_t)(o)) >> 3) & 0xfffff)
-#define MP_OBJ_NEW_QSTR(qst) ((mp_obj_t)((((mp_uint_t)(qst)) << 3) | 0x7f800006))
+    { return (((mp_uint_t)(o)) & 0xff800007) == 0x00000006; }
+#define MP_OBJ_QSTR_VALUE(o) (((mp_uint_t)(o)) >> 3)
+#define MP_OBJ_NEW_QSTR(qst) ((mp_obj_t)((((mp_uint_t)(qst)) << 3) | 0x00000006))
 
 static inline bool MP_OBJ_IS_OBJ(mp_const_obj_t o)
     { return ((((mp_int_t)(o)) & 3) == 0); }