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py/modbuiltins: For round() builtin use nearbyint instead of round. 

The C nearbyint function has exactly the semantics that Python's round()
requires, whereas C's round() requires extra steps to handle rounding of
numbers half way between integers.  So using nearbyint reduces code size
and potentially eliminates any source of errors in the handling of half-way
numbers.

Also, bare-metal implementations of nearbyint can be more efficient than
round, so further code size is saved (and efficiency improved).

nearbyint is provided in the C99 standard so it should be available on all
supported platforms.
This commit is contained in:
Damien George 2017-03-24 10:40:25 +11:00
parent fb161aa45a
commit 125eae1ba3
4 changed files with 5 additions and 12 deletions
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2
esp8266/Makefile
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@ -108,7 +108,7 @@ LIB_SRC_C = $(addprefix lib/,\
libc/string0.c \
libm/math.c \
libm/fmodf.c \
libm/roundf.c \
libm/nearbyintf.c \
libm/ef_sqrt.c \
libm/kf_rem_pio2.c \
libm/kf_sin.c \

11
py/modbuiltins.c
View File

@ -473,18 +473,11 @@ STATIC mp_obj_t mp_builtin_round(size_t n_args, const mp_obj_t *args) {
mp_float_t val = mp_obj_get_float(o_in);
mp_float_t mult = MICROPY_FLOAT_C_FUN(pow)(10, num_dig);
// TODO may lead to overflow
mp_float_t rounded = MICROPY_FLOAT_C_FUN(round)(val * mult) / mult;
mp_float_t rounded = MICROPY_FLOAT_C_FUN(nearbyint)(val * mult) / mult;
return mp_obj_new_float(rounded);
}
mp_float_t val = mp_obj_get_float(o_in);
mp_float_t rounded = MICROPY_FLOAT_C_FUN(round)(val);
mp_int_t r = rounded;
// make rounded value even if it was halfway between ints
if (val - rounded == 0.5) {
r = (r + 1) & (~1);
} else if (val - rounded == -0.5) {
r &= ~1;
}
mp_float_t rounded = MICROPY_FLOAT_C_FUN(nearbyint)(val);
#else
mp_int_t r = mp_obj_get_int(o_in);
#endif

2
qemu-arm/Makefile
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@ -47,7 +47,7 @@ SRC_TEST_C = \
LIB_SRC_C = $(addprefix lib/,\
libm/math.c \
libm/fmodf.c \
libm/roundf.c \
libm/nearbyintf.c \
libm/ef_sqrt.c \
libm/kf_rem_pio2.c \
libm/kf_sin.c \

2
stmhal/Makefile
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@ -86,7 +86,7 @@ SRC_LIB = $(addprefix lib/,\
libm/atanf.c \
libm/atan2f.c \
libm/fmodf.c \
libm/roundf.c \
libm/nearbyintf.c \
libm/log1pf.c \
libm/acoshf.c \
libm/asinhf.c \