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Improve performance of add/sub when exponent of two bigdecimals have huge difference #478

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Merged
tompng merged 2 commits into from
Dec 28, 2025
Merged

Improve performance of add/sub when exponent of two bigdecimals have huge difference #478

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9df6e59
Improve performance of add/sub when exponent of two bigdecimals have …
tompng Dec 12, 2025
0649946
Remove workaround of add/sub performance bug
tompng Dec 25, 2025
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27 changes: 23 additions & 4 deletions ext/bigdecimal/bigdecimal.c
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Original file line number Diff line number Diff line change
Expand Up @@ -986,7 +986,7 @@ BigDecimal_mode(int argc, VALUE *argv, VALUE self)
static size_t
GetAddSubPrec(Real *a, Real *b)
{
if (!VpIsDef(a) || !VpIsDef(b)) return (size_t)-1L;
if (VpIsZero(a) || VpIsZero(b)) return Max(a->Prec, b->Prec);
ssize_t min_a = a->exponent - a->Prec;
ssize_t min_b = b->exponent - b->Prec;
return Max(a->exponent, b->exponent) - Min(min_a, min_b);
Expand Down Expand Up @@ -1290,13 +1290,32 @@ BigDecimal_addsub_with_coerce(VALUE self, VALUE r, size_t prec, int operation)
if (VpIsNaN(a.real)) return CheckGetValue(a);
if (VpIsNaN(b.real)) return CheckGetValue(b);

mx = GetAddSubPrec(a.real, b.real);
if (mx == (size_t)-1L) {
/* a or b is inf */
if (VpIsInf(a.real) || VpIsInf(b.real)) {
c = NewZeroWrap(1, BASE_FIG);
VpAddSub(c.real, a.real, b.real, operation);
}
else {

// Optimization when exponent difference is large
// (1.234e+1000).add(5.678e-1000, 10) == (1.234e+1000).add(0.1e+990, 10) in every rounding mode
if (prec && !VpIsZero(a.real) && !VpIsZero(b.real)) {
size_t precRoom = roomof(prec, BASE_FIG);
if (a.real->exponent - (ssize_t)Max(a.real->Prec, precRoom) - 1 > b.real->exponent) {
BDVALUE b2 = NewZeroWrap(1, BASE_FIG);
VpSetOne(b2.real)
VpSetSign(b2.real, b.real->sign);
b2.real->exponent = a.real->exponent - (ssize_t)Max(a.real->Prec, precRoom) - 1;
b = b2;
} else if (b.real->exponent - (ssize_t)Max(b.real->Prec, precRoom) - 1 > a.real->exponent) {
BDVALUE a2 = NewZeroWrap(1, BASE_FIG);
VpSetOne(a2.real)
VpSetSign(a2.real, a.real->sign);
a2.real->exponent = b.real->exponent - (ssize_t)Max(b.real->Prec, precRoom) - 1;
a = a2;
}
}

mx = GetAddSubPrec(a.real, b.real);
c = NewZeroWrap(1, (mx + 1) * BASE_FIG);
size_t pl = VpGetPrecLimit();
if (prec) VpSetPrecLimit(prec);
Expand Down
16 changes: 2 additions & 14 deletions lib/bigdecimal/math.rb
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Expand Up @@ -574,14 +574,7 @@ def expm1(x, prec)

return BigDecimal(-1) if exp_prec <= 0

exp = exp(x, exp_prec)

if exp.exponent > prec + BigDecimal.double_fig
# Workaroudn for https://github.com/ruby/bigdecimal/issues/464
exp
else
exp.sub(1, prec)
end
exp(x, exp_prec).sub(1, prec)
end

# erf(decimal, numeric) -> BigDecimal
Expand All @@ -608,12 +601,7 @@ def erf(x, prec)
log10_erfc = -xf ** 2 / Math.log(10) - Math.log10(xf * Math::PI ** 0.5)
erfc_prec = [prec + log10_erfc.ceil, 1].max
erfc = _erfc_asymptotic(x, erfc_prec)
if erfc
# Workaround for https://github.com/ruby/bigdecimal/issues/464
return BigDecimal(1) if erfc.exponent < -prec - BigDecimal.double_fig

return BigDecimal(1).sub(erfc, prec)
end
return BigDecimal(1).sub(erfc, prec) if erfc
end

prec2 = prec + BigDecimal.double_fig
Expand Down
46 changes: 46 additions & 0 deletions test/bigdecimal/test_bigdecimal.rb
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Expand Up @@ -523,6 +523,52 @@ def test_add_sub_underflow
assert_negative_zero((-x) - (-y))
end

def test_add_sub_huge_with_zero
huge = BigDecimal("0.1e#{EXPONENT_MAX}")
assert_equal(huge, huge + 0)
assert_equal(huge, huge - 0)
assert_equal(huge, huge.add(BigDecimal(0), 10))
assert_equal(huge, huge.sub(BigDecimal(0), 10))
end

def test_add_sub_huge_exponent_difference
huge = BigDecimal('1e+1000000000000')
zero = BigDecimal(0)
small = BigDecimal('1e-1000000000000')
assert_equal(huge, huge.add(small, 10))
assert_equal(huge, huge.sub(small, 10))
assert_equal(huge, small.add(huge, 10))
assert_equal(-huge, small.sub(huge, 10))
assert_equal(huge, zero.add(huge, 10))
assert_equal(huge, huge.add(zero, 10))
assert_equal(small, zero.add(small, 10))
assert_equal(small, small.add(zero, 10))

BigDecimal.mode(BigDecimal::ROUND_MODE, BigDecimal::ROUND_DOWN)
assert_equal(huge, huge.add(small, 10))
assert_equal(huge * BigDecimal('0.9999999999'), huge.sub(small, 10))
assert_equal(huge, huge.add(zero, 10))
assert_equal(huge, huge.sub(zero, 10))

BigDecimal.mode(BigDecimal::ROUND_MODE, BigDecimal::ROUND_UP)
assert_equal(huge * BigDecimal('1.000000001'), huge.add(small, 10))
assert_equal(huge, huge.sub(small, 10))
assert_equal(huge, huge.add(zero, 10))
assert_equal(huge, huge.sub(zero, 10))

[BigDecimal::ROUND_UP, BigDecimal::ROUND_DOWN, BigDecimal::ROUND_HALF_UP].each do |mode|
BigDecimal.mode(BigDecimal::ROUND_MODE, mode)
xs = [BigDecimal(1), BigDecimal(2)]
ys = [BigDecimal('0.5e-89'), BigDecimal('0.5e-90'), BigDecimal('0.5e-91')]
xs.product(ys).each do |x, y|
assert_equal((x + y).mult(1, 91), x.add(y, 91))
assert_equal((y + x).mult(1, 91), y.add(x, 91))
assert_equal((x - y).mult(1, 91), x.sub(y, 91))
assert_equal((y - x).mult(1, 91), y.sub(x, 91))
end
end
end

def test_mult_div_overflow_underflow_sign
BigDecimal.mode(BigDecimal::EXCEPTION_OVERFLOW, false)
BigDecimal.mode(BigDecimal::EXCEPTION_UNDERFLOW, false)
Expand Down
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