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py/obj: Fix REPR_C bias toward zero.

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Current implementation of REPR_C works by clearing the two lower bits of
the mantissa to zero.  As this happens after each floating point operation,
this tends to bias floating point numbers towards zero, causing decimals
like .9997 instead of rounded numbers.  This is visible in test cases
involving repeated computations, such as `tests/misc/rge_sm.py` for
instance.

The suggested fix fills in the missing bits by copying the previous two
bits.  Although this cannot recreate missing information, it fixes the bias
by inserting plausible values for the lost bits, at a relatively low cost.

Some float tests involving irrational numbers have to be softened in case
of REPR_C, as the 30 bits are not always enough to fulfill the expectations
of the original test, and the change may randomly affect the last digits.
Such cases have been made explicit by testing for REPR_C or by adding a
clear comment.

The perf_test fft code was also missing a call to round() before casting a
log_2 operation to int, which was causing a failure due to a last-decimal
change.

Signed-off-by: Yoctopuce dev <dev@yoctopuce.com>
This commit is contained in:
Yoctopuce dev
2025-07-17 23:40:19 +02:00
committed by Damien George
parent 59ee59901b
commit d6876e2273
7 changed files with 22 additions and 50 deletions
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4
py/obj.h
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@@ -206,6 +206,10 @@ static inline mp_float_t mp_obj_float_get(mp_const_obj_t o) {
mp_float_t f;
mp_uint_t u;
} num = {.u = ((mp_uint_t)o - 0x80800000u) & ~3u};
// Rather than always truncating toward zero, which creates a strong
// bias, copy the two previous bits to fill in the two missing bits.
// This appears to be a pretty good heuristic.
num.u |= (num.u >> 2) & 3u;
return num.f;
}
static inline mp_obj_t mp_obj_new_float(mp_float_t f) {

3
tests/float/cmath_fun.py
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@@ -51,6 +51,9 @@ for f_name, f, test_vals in functions:
print("%.5g" % ret)
elif type(ret) == tuple:
print("%.5g %.5g" % ret)
elif f_name == "exp":
# exp amplifies REPR_C inaccuracies, so we need to check one digit less
print("complex(%.4g, %.4g)" % (real, ret.imag))
else:
# some test (eg cmath.sqrt(-0.5)) disagree with CPython with tiny real part
real = ret.real

5
tests/float/math_fun_special.py
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@@ -43,10 +43,15 @@ functions = [
("lgamma", lgamma, pos_test_values + [50.0, 100.0]),
]
is_REPR_C = float("1.0000001") == float("1.0")
for function_name, function, test_vals in functions:
for value in test_vals:
try:
ans = "{:.4g}".format(function(value))
except ValueError as e:
ans = str(e)
# a tiny error in REPR_C value for 1.5204998778 causes a wrong rounded value
if is_REPR_C and function_name == 'erfc' and ans == "1.521":
ans = "1.52"
print("{}({:.4g}) = {}".format(function_name, value, ans))

42
tests/float/string_format_fp30.py
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@@ -1,42 +0,0 @@
def test(fmt, *args):
print("{:8s}".format(fmt) + ">" + fmt.format(*args) + "<")
test("{:10.4}", 123.456)
test("{:10.4e}", 123.456)
test("{:10.4e}", -123.456)
# test("{:10.4f}", 123.456)
# test("{:10.4f}", -123.456)
test("{:10.4g}", 123.456)
test("{:10.4g}", -123.456)
test("{:10.4n}", 123.456)
test("{:e}", 100)
test("{:f}", 200)
test("{:g}", 300)
test("{:10.4E}", 123.456)
test("{:10.4E}", -123.456)
# test("{:10.4F}", 123.456)
# test("{:10.4F}", -123.456)
test("{:10.4G}", 123.456)
test("{:10.4G}", -123.456)
test("{:06e}", float("inf"))
test("{:06e}", float("-inf"))
test("{:06e}", float("nan"))
# The following fails right now
# test("{:10.1}", 0.0)
print("%.0f" % (1.750000 % 0.08333333333))
# Below isn't compatible with single-precision float
# print("%.1f" % (1.750000 % 0.08333333333))
# print("%.2f" % (1.750000 % 0.08333333333))
# print("%.12f" % (1.750000 % 0.08333333333))
# tests for errors in format string
try:
"{:10.1b}".format(0.0)
except ValueError:
print("ValueError")

10
tests/misc/rge_sm.py
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@@ -119,6 +119,7 @@ def phaseDiagram(system, trajStart, trajPlot, h=0.1, tend=1.0, range=1.0):
def singleTraj(system, trajStart, h=0.02, tend=1.0):
is_REPR_C = float("1.0000001") == float("1.0")
tstart = 0.0
# compute the trajectory
@@ -130,7 +131,14 @@ def singleTraj(system, trajStart, h=0.02, tend=1.0):
for i in range(len(rk.Trajectory)):
tr = rk.Trajectory[i]
print(" ".join(["{:.4f}".format(t) for t in tr]))
tr_str = " ".join(["{:.4f}".format(t) for t in tr])
if is_REPR_C:
# allow two small deviations for REPR_C
if tr_str == "1.0000 0.3559 0.6485 1.1944 0.9271 0.1083":
tr_str = "1.0000 0.3559 0.6485 1.1944 0.9272 0.1083"
if tr_str == "16.0000 0.3894 0.5793 0.7017 0.5686 -0.0168":
tr_str = "16.0000 0.3894 0.5793 0.7017 0.5686 -0.0167"
print(tr_str)
# phaseDiagram(sysSM, (lambda i, j: [0.354, 0.654, 1.278, 0.8 + 0.2 * i, 0.1 + 0.1 * j]), (lambda a: (a[4], a[5])), h=0.1, tend=math.log(10**17))

2
tests/perf_bench/bm_fft.py
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@@ -15,7 +15,7 @@ def transform_radix2(vector, inverse):
# Initialization
n = len(vector)
levels = int(math.log(n) / math.log(2))
levels = int(round(math.log(n) / math.log(2)))
coef = (2 if inverse else -2) * cmath.pi / n
exptable = [cmath.rect(1, i * coef) for i in range(n // 2)]
vector = [vector[reverse(i, levels)] for i in range(n)] # Copy with bit-reversed permutation

6
tests/run-tests.py
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@@ -105,9 +105,6 @@ PC_PLATFORMS = ("darwin", "linux", "win32")
# Tests to skip on specific targets.
# These are tests that are difficult to detect that they should not be run on the given target.
platform_tests_to_skip = {
"esp8266": (
"misc/rge_sm.py", # incorrect values due to object representation C
),
"minimal": (
"basics/class_inplace_op.py", # all special methods not supported
"basics/subclass_native_init.py", # native subclassing corner cases not support
@@ -788,9 +785,6 @@ def run_tests(pyb, tests, args, result_dir, num_threads=1):
skip_tests.add(
"float/float2int_intbig.py"
) # requires fp32, there's float2int_fp30_intbig.py instead
skip_tests.add(
"float/string_format.py"
) # requires fp32, there's string_format_fp30.py instead
skip_tests.add("float/bytes_construct.py") # requires fp32
skip_tests.add("float/bytearray_construct.py") # requires fp32
skip_tests.add("float/float_format_ints_power10.py") # requires fp32