Skip to content

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

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

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

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
5 commits
Select commit Hold shift + click to select a range
a3b6177
[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
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
29 changes: 16 additions & 13 deletions libc/src/math/generic/log1p.cpp
View file
Open in desktop
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 @@ -882,7 +882,6 @@ LLVM_LIBC_FUNCTION(double, log1p, (double x)) {

constexpr int EXP_BIAS = FPBits_t::EXP_BIAS;
constexpr int FRACTION_LEN = FPBits_t::FRACTION_LEN;
constexpr uint64_t FRACTION_MASK = FPBits_t::FRACTION_MASK;
FPBits_t xbits(x);
uint64_t x_u = xbits.uintval();

Expand Down Expand Up @@ -954,12 +953,12 @@ LLVM_LIBC_FUNCTION(double, log1p, (double x)) {
// |x_dd.lo| < ulp(x_dd.hi)

FPBits_t xhi_bits(x_dd.hi);
uint64_t xhi_frac = xhi_bits.get_mantissa();
x_u = xhi_bits.uintval();
// Range reduction:
// Find k such that |x_hi - k * 2^-7| <= 2^-8.
int idx =
static_cast<int>(((x_u & FRACTION_MASK) + (1ULL << (FRACTION_LEN - 8))) >>
(FRACTION_LEN - 7));
int idx = static_cast<int>((xhi_frac + (1ULL << (FRACTION_LEN - 8))) >>
(FRACTION_LEN - 7));
int x_e = xhi_bits.get_exponent() + (idx >> 7);
double e_x = static_cast<double>(x_e);

Expand All @@ -974,17 +973,21 @@ LLVM_LIBC_FUNCTION(double, log1p, (double x)) {
constexpr double ERR_HI[2] = {0x1.0p-85, 0.0};
double err_hi = ERR_HI[hi == 0.0];

// Scaling factior = 2^(-xh_bits.get_exponent())
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];
// Scale x_dd by 2^(-xh_bits.get_exponent()).
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 = FPBits_t::one().uintval() | xhi_frac;

uint64_t m_lo =
FPBits_t(x_dd.lo).abs().get_val() > x_dd.hi * 0x1.0p-127
? static_cast<uint64_t>(cpp::bit_cast<int64_t>(x_dd.lo) - s_u)
: 0;

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
View file
Open in desktop
Original file line number Diff line number Diff line change
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
Loading