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

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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.
This commit is contained in:
Damien George
2015-11-06 23:25:10 +00:00
parent 8f7ff854b0
commit 8b8d189bc0
2 changed files with 13 additions and 10 deletions
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7
py/mpconfig.h
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@@ -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