diff options
Diffstat (limited to 'py/parsenum.c')
| -rw-r--r-- | py/parsenum.c | 18 |
1 files changed, 17 insertions, 1 deletions
diff --git a/py/parsenum.c b/py/parsenum.c index b7e5a3c83..ae9b83419 100644 --- a/py/parsenum.c +++ b/py/parsenum.c @@ -175,14 +175,20 @@ mp_obj_t mp_parse_num_decimal(const char *str, size_t len, bool allow_imag, bool // DEC_VAL_MAX only needs to be rough and is used to retain precision while not overflowing // SMALL_NORMAL_VAL is the smallest power of 10 that is still a normal float +// EXACT_POWER_OF_10 is the largest value of x so that 10^x can be stored exactly in a float +// Note: EXACT_POWER_OF_10 is at least floor(log_5(2^mantissa_length)). Indeed, 10^n = 2^n * 5^n +// so we only have to store the 5^n part in the mantissa (the 2^n part will go into the float's +// exponent). #if MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_FLOAT #define DEC_VAL_MAX 1e20F #define SMALL_NORMAL_VAL (1e-37F) #define SMALL_NORMAL_EXP (-37) +#define EXACT_POWER_OF_10 (9) #elif MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_DOUBLE #define DEC_VAL_MAX 1e200 #define SMALL_NORMAL_VAL (1e-307) #define SMALL_NORMAL_EXP (-307) +#define EXACT_POWER_OF_10 (22) #endif const char *top = str + len; @@ -295,7 +301,17 @@ mp_obj_t mp_parse_num_decimal(const char *str, size_t len, bool allow_imag, bool exp_val -= SMALL_NORMAL_EXP; dec_val *= SMALL_NORMAL_VAL; } - dec_val *= MICROPY_FLOAT_C_FUN(pow)(10, exp_val); + + // At this point, we need to multiply the mantissa by its base 10 exponent. If possible, + // we would rather manipulate numbers that have an exact representation in IEEE754. It + // turns out small positive powers of 10 do, whereas small negative powers of 10 don't. + // So in that case, we'll yield a division of exact values rather than a multiplication + // of slightly erroneous values. + if (exp_val < 0 && exp_val >= -EXACT_POWER_OF_10) { + dec_val /= MICROPY_FLOAT_C_FUN(pow)(10, -exp_val); + } else { + dec_val *= MICROPY_FLOAT_C_FUN(pow)(10, exp_val); + } } // negate value if needed |