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boxing_word.h: lossless float encoding using rotation

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Replace lossy 2-bit truncation with rotation-based encoding for
64-bit word boxing with float64. The new scheme uses
rotl64(float_bits - ADDEND, 3) to embed floats inline with full
52-bit mantissa precision. Floats with exponents outside [-255,+256]
(0.0, NaN, Inf, very small/large values) fall back to heap-allocated
RFloat.

Co-authored-by: Claude <noreply@anthropic.com>
This commit is contained in:
Yukihiro "Matz" Matsumoto
2026-02-18 09:40:13 +09:00
parent 4e63489d1a
commit b6148c893f
3 changed files with 96 additions and 32 deletions
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include/mruby/boxing_word.h
+15 -18
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@@ -11,7 +11,7 @@
# define MRB_WORDBOX_NO_FLOAT_TRUNCATE
#endif
#if !defined(MRB_NO_FLOAT) && defined(MRB_WORDBOX_NO_FLOAT_TRUNCATE)
#ifndef MRB_NO_FLOAT
struct RFloat {
MRB_OBJECT_HEADER;
mrb_float f;
@@ -83,27 +83,22 @@ enum mrb_special_consts {
/*
* mrb_value representation:
*
* 64-bit word with inline float:
* 64-bit word with inline float (rotation encoding, lossless):
* nil : ...0000 0000 (all bits are 0)
* false : ...0000 0100 (mrb_fixnum(v) != 0)
* true : ...0000 1100
* undef : ...0001 0100
* symbol: ...0001 1100 (use only upper 32-bit as symbol value with MRB_64BIT)
* fixnum: ...IIII III1
* float : ...FFFF FF10 (51 bit significands; require MRB_64BIT)
* float : ...FFFF FF10 (rotl64(float64-ADDEND, 3); exponent [-255,+256])
* object: ...PPPP P000
* (floats outside inline range are heap-allocated as RFloat)
*
* 32-bit word with inline float:
* nil : ...0000 0000 (all bits are 0)
* false : ...0000 0100 (mrb_fixnum(v) != 0)
* true : ...0000 1100
* undef : ...0001 0100
* symbol: ...SSS1 0100 (symbol occupies 20bits)
* fixnum: ...IIII III1
* float : ...FFFF FF10 (22 bit significands; require MRB_64BIT)
* object: ...PPPP P000
* 64-bit word with inline float32 (MRB_USE_FLOAT32):
* float : ...FFFF FF10 (float32 shifted left by 2)
* (other values same as above)
*
* and word boxing without inline float (MRB_WORDBOX_NO_FLOAT_TRUNCATE):
* word boxing without inline float (MRB_WORDBOX_NO_FLOAT_TRUNCATE):
* nil : ...0000 0000 (all bits are 0)
* false : ...0000 0100 (mrb_fixnum(v) != 0)
* true : ...0000 1100
@@ -120,10 +115,9 @@ union mrb_value_ {
void *p;
struct RBasic *bp;
#ifndef MRB_NO_FLOAT
#ifndef MRB_WORDBOX_NO_FLOAT_TRUNCATE
mrb_float f;
#else
struct RFloat *fp;
#if !defined(MRB_WORDBOX_NO_FLOAT_TRUNCATE) && defined(MRB_USE_FLOAT32)
mrb_float f;
#endif
#endif
struct RInteger *ip;
@@ -182,10 +176,13 @@ mrb_integer_func(mrb_value o) {
#define mrb_false_p(o) ((o).w == MRB_Qfalse)
#define mrb_true_p(o) ((o).w == MRB_Qtrue)
#ifndef MRB_NO_FLOAT
#ifndef MRB_WORDBOX_NO_FLOAT_TRUNCATE
#ifdef MRB_WORDBOX_NO_FLOAT_TRUNCATE
#define mrb_float_p(o) WORDBOX_OBJ_TYPE_P(o, FLOAT)
#elif defined(MRB_USE_FLOAT32)
#define mrb_float_p(o) WORDBOX_SHIFT_VALUE_P(o, FLOAT)
#else
#define mrb_float_p(o) WORDBOX_OBJ_TYPE_P(o, FLOAT)
/* rotation encoding: most floats inline, edge cases on heap */
#define mrb_float_p(o) (WORDBOX_SHIFT_VALUE_P(o, FLOAT) || WORDBOX_OBJ_TYPE_P(o, FLOAT))
#endif
#else
#define mrb_float_p(o) FALSE