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[libc][math] Fix log1p SEGV with large inputs when FTZ/DAZ flags are set. #115541

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Merged
merged 5 commits into from
Nov 9, 2024
Merged

[libc][math] Fix log1p SEGV with large inputs when FTZ/DAZ flags are set. #115541

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[libc][math] Fix log1p SEGV with large inputs when FTZ/DAZ flags are …
lntue Nov 8, 2024
e34408d
format.
lntue Nov 8, 2024
ba33556
Fix typo.
lntue Nov 8, 2024
ab057bd
Update comment.
lntue Nov 8, 2024
4bf657f
Fix when m_lo is underflowed.
lntue Nov 9, 2024
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18 changes: 10 additions & 8 deletions libc/src/math/generic/log1p.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -822,8 +822,8 @@ constexpr Float128 BIG_COEFFS[4]{
{Sign::NEG, -128, 0x80000000'00000000'00000000'00000000_u128},
};

LIBC_INLINE double log1p_accurate(int e_x, int index,
fputil::DoubleDouble m_x) {
[[maybe_unused]] LIBC_INLINE double log1p_accurate(int e_x, int index,
fputil::DoubleDouble m_x) {
Float128 e_x_f128(static_cast<float>(e_x));
Float128 sum = fputil::quick_mul(LOG_2, e_x_f128);
sum = fputil::quick_add(sum, LOG_R1[index]);
Expand Down Expand Up @@ -975,16 +975,18 @@ LLVM_LIBC_FUNCTION(double, log1p, (double x)) {
double err_hi = ERR_HI[hi == 0.0];

// Scaling factior = 2^(-xh_bits.get_exponent())
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@michaelrj-google michaelrj-google Nov 8, 2024

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since you flipped the subtraction in the calculation of s_u is this comment still accurate?

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@lntue lntue Nov 8, 2024 •
edited
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yes, it's still the same scaling factor, but we do subtract exponents (ldexp style) instead of multiply by 2^(-exp). I updated the comment to make it a bit clearer.

uint64_t s_u = (static_cast<uint64_t>(EXP_BIAS) << (FRACTION_LEN + 1)) -
(x_u & FPBits_t::EXP_MASK);
// When the exponent of x is 2^1023, its inverse, 2^(-1023), is subnormal.
const double EXPONENT_CORRECTION[2] = {0.0, 0x1.0p-1023};
double scaling = FPBits_t(s_u).get_val() + EXPONENT_CORRECTION[s_u == 0];
int64_t s_u = static_cast<int64_t>(x_u & FPBits_t::EXP_MASK) -
(static_cast<int64_t>(EXP_BIAS) << FRACTION_LEN);
// Normalize arguments:
// 1 <= m_dd.hi < 2
// |m_dd.lo| < 2^-52.
// This is exact.
fputil::DoubleDouble m_dd{scaling * x_dd.lo, scaling * x_dd.hi};
uint64_t m_hi = static_cast<uint64_t>(static_cast<int64_t>(x_u) - s_u);
uint64_t m_lo =
(x_dd.lo != 0.0)
? FPBits_t(x_dd.lo).uintval()
: static_cast<uint64_t>(cpp::bit_cast<int64_t>(x_dd.lo) - s_u);
fputil::DoubleDouble m_dd{FPBits_t(m_lo).get_val(), FPBits_t(m_hi).get_val()};

// Perform range reduction:
// r * m - 1 = r * (m_dd.hi + m_dd.lo) - 1
Expand Down
11 changes: 9 additions & 2 deletions libc/test/src/math/smoke/log1p_test.cpp
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Expand Up @@ -13,8 +13,6 @@
#include "test/UnitTest/FPMatcher.h"
#include "test/UnitTest/Test.h"

#include <stdint.h>

using LlvmLibcLog1pTest = LIBC_NAMESPACE::testing::FPTest<double>;

TEST_F(LlvmLibcLog1pTest, SpecialNumbers) {
Expand All @@ -26,6 +24,9 @@ TEST_F(LlvmLibcLog1pTest, SpecialNumbers) {
EXPECT_FP_EQ(neg_zero, LIBC_NAMESPACE::log1p(-0.0));
EXPECT_FP_EQ_WITH_EXCEPTION(neg_inf, LIBC_NAMESPACE::log1p(-1.0),
FE_DIVBYZERO);

EXPECT_FP_EQ(0x1.62c829bf8fd9dp9,
LIBC_NAMESPACE::log1p(0x1.9b536cac3a09dp1023));
}

#ifdef LIBC_TEST_FTZ_DAZ
Expand All @@ -36,18 +37,24 @@ TEST_F(LlvmLibcLog1pTest, FTZMode) {
ModifyMXCSR mxcsr(FTZ);

EXPECT_FP_EQ(0.0, LIBC_NAMESPACE::log1p(min_denormal));
EXPECT_FP_EQ(0x1.62c829bf8fd9dp9,
LIBC_NAMESPACE::log1p(0x1.9b536cac3a09dp1023));
}

TEST_F(LlvmLibcLog1pTest, DAZMode) {
ModifyMXCSR mxcsr(DAZ);

EXPECT_FP_EQ(0.0, LIBC_NAMESPACE::log1p(min_denormal));
EXPECT_FP_EQ(0x1.62c829bf8fd9dp9,
LIBC_NAMESPACE::log1p(0x1.9b536cac3a09dp1023));
}

TEST_F(LlvmLibcLog1pTest, FTZDAZMode) {
ModifyMXCSR mxcsr(FTZ | DAZ);

EXPECT_FP_EQ(0.0, LIBC_NAMESPACE::log1p(min_denormal));
EXPECT_FP_EQ(0x1.62c829bf8fd9dp9,
LIBC_NAMESPACE::log1p(0x1.9b536cac3a09dp1023));
}

#endif
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