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[libc] Properly fix printf long double subnormals #172103

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Merged
michaelrj-google merged 2 commits into from
Dec 12, 2025
Merged

[libc] Properly fix printf long double subnormals #172103

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0db462d
[libc] Properly fix printf long double subnormals
michaelrj-google Dec 12, 2025
b684321
fix formatting
michaelrj-google Dec 12, 2025
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19 changes: 10 additions & 9 deletions libc/src/__support/float_to_string.h
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Original file line number Diff line number Diff line change
Expand Up @@ -618,16 +618,18 @@ template <> class FloatToString<long double> {
fputil::FPBits<long double> float_bits;
bool is_negative = 0;
int exponent = 0;
FPBits::StorageType mantissa = 0;
fputil::FPBits<long double>::StorageType mantissa = 0;

static constexpr int FRACTION_LEN = fputil::FPBits<long double>::FRACTION_LEN;
static constexpr int EXP_BIAS = fputil::FPBits<long double>::EXP_BIAS;
static constexpr size_t UINT_WORD_SIZE = 64;

static constexpr size_t FLOAT_AS_INT_WIDTH =
internal::div_ceil(fputil::FPBits<long double>::MAX_BIASED_EXPONENT -
FPBits::EXP_BIAS,
UINT_WORD_SIZE) *
internal::div_ceil(
fputil::FPBits<long double>::MAX_BIASED_EXPONENT -
fputil::FPBits<long double>::EXP_BIAS +
FRACTION_LEN, // Add fraction len to provide space for subnormals.
UINT_WORD_SIZE) *
UINT_WORD_SIZE;
static constexpr size_t EXTRA_INT_WIDTH =
internal::div_ceil(sizeof(long double) * CHAR_BIT, UINT_WORD_SIZE) *
Expand Down Expand Up @@ -699,12 +701,11 @@ template <> class FloatToString<long double> {
float_as_fixed = mantissa;

const int SHIFT_AMOUNT = FLOAT_AS_INT_WIDTH + exponent;
// if the shift amount would be negative, then the shift would cause a
// loss of precision.
LIBC_ASSERT(SHIFT_AMOUNT >= 0);
static_assert(EXTRA_INT_WIDTH >= sizeof(long double) * 8);
if (SHIFT_AMOUNT > 0) {
float_as_fixed <<= SHIFT_AMOUNT;
} else {
float_as_fixed >>= -SHIFT_AMOUNT;
}
float_as_fixed <<= SHIFT_AMOUNT;

// If there are still digits above the decimal point, handle those.
if (cpp::countl_zero(float_as_fixed) <
Expand Down
12 changes: 12 additions & 0 deletions libc/test/src/stdio/sprintf_test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2484,6 +2484,18 @@ TEST(LlvmLibcSPrintfTest, FloatExponentLongDoubleConv) {

written = LIBC_NAMESPACE::sprintf(buff, "%Le", 1.2345678e4900L);
ASSERT_STREQ_LEN(written, buff, "1.234568e+4900");

#ifndef LIBC_COPT_FLOAT_TO_STR_REDUCED_PRECISION
// Minimum normal + epsilon
written =
LIBC_NAMESPACE::sprintf(buff, "%.20Le", 3.36210314311209350663E-4932L);
ASSERT_STREQ_LEN(written, buff, "3.36210314311209350663e-4932");

// Minimum subnormal
written =
LIBC_NAMESPACE::sprintf(buff, "%.20Le", 3.64519953188247460253E-4951L);
ASSERT_STREQ_LEN(written, buff, "3.64519953188247460253e-4951");
#endif // LIBC_COPT_FLOAT_TO_STR_REDUCED_PRECISION
#endif
}

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