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py/binary: Support half-float 'e' format in struct pack/unpack.
This commit implements the 'e' half-float format: 10-bit mantissa, 5-bit exponent. It uses native _Float16 if supported by the compiler, otherwise uses custom bitshifting encoding/decoding routines. Signed-off-by: Matthias Urlichs <matthias@urlichs.de> Signed-off-by: Damien George <damien@micropython.org>
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4 files changed

Lines changed: 119 additions & 3 deletions

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‎docs/library/struct.rst‎

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -45,6 +45,8 @@ The following data types are supported:
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+--------+--------------------+-------------------+---------------+
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| Q | unsigned long long | integer (`1<fn>`) | 8 |
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+--------+--------------------+-------------------+---------------+
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| e | n/a (half-float) | float (`2<fn>`) | 2 |
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+--------+--------------------+-------------------+---------------+
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| f | float | float (`2<fn>`) | 4 |
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+--------+--------------------+-------------------+---------------+
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| d | double | float (`2<fn>`) | 8 |

‎py/binary.c‎

Lines changed: 107 additions & 2 deletions
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@@ -74,11 +74,14 @@ size_t mp_binary_get_size(char struct_type, char val_type, size_t *palign) {
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case 'S':
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size = sizeof(void *);
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break;
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case 'e':
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size = 2;
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break;
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case 'f':
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size = sizeof(float);
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size = 4;
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break;
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case 'd':
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size = sizeof(double);
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size = 8;
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break;
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}
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break;
@@ -122,6 +125,10 @@ size_t mp_binary_get_size(char struct_type, char val_type, size_t *palign) {
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align = alignof(void *);
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size = sizeof(void *);
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break;
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case 'e':
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align = 2;
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size = 2;
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break;
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case 'f':
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align = alignof(float);
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size = sizeof(float);
@@ -144,6 +151,99 @@ size_t mp_binary_get_size(char struct_type, char val_type, size_t *palign) {
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return size;
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}
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#if MICROPY_PY_BUILTINS_FLOAT && MICROPY_FLOAT_USE_NATIVE_FLT16
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static inline float mp_decode_half_float(uint16_t hf) {
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union {
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uint16_t i;
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_Float16 f;
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} fpu = { .i = hf };
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return fpu.f;
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}
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static inline uint16_t mp_encode_half_float(float x) {
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union {
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uint16_t i;
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_Float16 f;
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} fp_sp = { .f = (_Float16)x };
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return fp_sp.i;
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}
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#elif MICROPY_PY_BUILTINS_FLOAT
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static float mp_decode_half_float(uint16_t hf) {
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union {
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uint32_t i;
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float f;
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} fpu;
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uint16_t m = hf & 0x3ff;
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int e = (hf >> 10) & 0x1f;
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if (e == 0x1f) {
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// Half-float is infinity.
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e = 0xff;
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} else if (e) {
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// Half-float is normal.
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e += 127 - 15;
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} else if (m) {
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// Half-float is subnormal, make it normal.
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e = 127 - 15;
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while (!(m & 0x400)) {
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m <<= 1;
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--e;
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}
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m -= 0x400;
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++e;
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}
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fpu.i = ((hf & 0x8000) << 16) | (e << 23) | (m << 13);
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return fpu.f;
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}
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static uint16_t mp_encode_half_float(float x) {
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union {
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uint32_t i;
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float f;
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} fpu = { .f = x };
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uint16_t m = (fpu.i >> 13) & 0x3ff;
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if (fpu.i & (1 << 12)) {
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// Round up.
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++m;
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}
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int e = (fpu.i >> 23) & 0xff;
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if (e == 0xff) {
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// Infinity.
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e = 0x1f;
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} else if (e != 0) {
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e -= 127 - 15;
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if (e < 0) {
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// Underflow: denormalized, or zero.
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if (e >= -11) {
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m = (m | 0x400) >> -e;
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if (m & 1) {
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m = (m >> 1) + 1;
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} else {
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m >>= 1;
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}
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} else {
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m = 0;
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}
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e = 0;
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} else if (e > 0x3f) {
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// Overflow: infinity.
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e = 0x1f;
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m = 0;
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}
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}
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uint16_t bits = ((fpu.i >> 16) & 0x8000) | (e << 10) | m;
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return bits;
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}
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#endif
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mp_obj_t mp_binary_get_val_array(char typecode, void *p, size_t index) {
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mp_int_t val = 0;
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switch (typecode) {
@@ -240,6 +340,8 @@ mp_obj_t mp_binary_get_val(char struct_type, char val_type, byte *p_base, byte *
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const char *s_val = (const char *)(uintptr_t)(mp_uint_t)val;
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return mp_obj_new_str(s_val, strlen(s_val));
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#if MICROPY_PY_BUILTINS_FLOAT
343+
} else if (val_type == 'e') {
344+
return mp_obj_new_float_from_f(mp_decode_half_float(val));
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} else if (val_type == 'f') {
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union {
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uint32_t i;
@@ -309,6 +411,9 @@ void mp_binary_set_val(char struct_type, char val_type, mp_obj_t val_in, byte *p
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val = (mp_uint_t)val_in;
310412
break;
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#if MICROPY_PY_BUILTINS_FLOAT
414+
case 'e':
415+
val = mp_encode_half_float(mp_obj_get_float_to_f(val_in));
416+
break;
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case 'f': {
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union {
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uint32_t i;

‎py/mpconfig.h‎

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Original file line numberDiff line numberDiff line change
@@ -830,6 +830,15 @@ typedef double mp_float_t;
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#define MICROPY_PY_BUILTINS_COMPLEX (MICROPY_PY_BUILTINS_FLOAT)
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#endif
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833+
// Whether to use the native _Float16 for 16-bit float support
834+
#ifndef MICROPY_FLOAT_USE_NATIVE_FLT16
835+
#ifdef __FLT16_MAX__
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#define MICROPY_FLOAT_USE_NATIVE_FLT16 (1)
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#else
838+
#define MICROPY_FLOAT_USE_NATIVE_FLT16 (0)
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#endif
840+
#endif
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// Whether to provide a high-quality hash for float and complex numbers.
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// Otherwise the default is a very simple but correct hashing function.
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#ifndef MICROPY_FLOAT_HIGH_QUALITY_HASH

‎tests/float/float_struct.py‎

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@@ -8,7 +8,7 @@
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i = 1.0 + 1 / 2
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# TODO: it looks like '=' format modifier is not yet supported
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# for fmt in ('f', 'd', '>f', '>d', '<f', '<d', '=f', '=d'):
11-
for fmt in ("f", "d", ">f", ">d", "<f", "<d"):
11+
for fmt in ("e", "f", "d", ">e", ">f", ">d", "<e", "<f", "<d"):
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x = struct.pack(fmt, i)
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v = struct.unpack(fmt, x)[0]
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print("%2s: %.17f - %s" % (fmt, v, (i == v) and "passed" or "failed"))

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