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[libc] Fix printf long double bugs #166474

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Merged
merged 1 commit into from
Nov 5, 2025
+18 −2
Merged

[libc] Fix printf long double bugs #166474

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8 changes: 6 additions & 2 deletions libc/src/__support/float_to_string.h
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Original file line number Diff line number Diff line change
Expand Up @@ -700,7 +700,11 @@ template <> class FloatToString<long double> {

const int SHIFT_AMOUNT = FLOAT_AS_INT_WIDTH + exponent;
static_assert(EXTRA_INT_WIDTH >= sizeof(long double) * 8);
float_as_fixed <<= SHIFT_AMOUNT;
if (SHIFT_AMOUNT > 0) {
float_as_fixed <<= SHIFT_AMOUNT;
} else {
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 Expand Up @@ -769,7 +773,7 @@ template <> class FloatToString<long double> {
// The decimal representation of 2**(-i) will have exactly i digits after
// the decimal point.
const int num_requested_digits =
static_cast<int>((negative_block_index + 1) * BLOCK_SIZE);
static_cast<int>(negative_block_index * BLOCK_SIZE);

return num_requested_digits > -exponent;
}
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12 changes: 12 additions & 0 deletions libc/test/src/stdio/sprintf_test.cpp
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Expand Up @@ -1537,6 +1537,14 @@ TEST(LlvmLibcSPrintfTest, FloatDecimalLongDoubleConv) {
#if defined(LIBC_TYPES_LONG_DOUBLE_IS_X86_FLOAT80)

#ifndef LIBC_COPT_FLOAT_TO_STR_REDUCED_PRECISION
written = LIBC_NAMESPACE::sprintf(
buff, "%.75Lf",
0.0833333333333333333355920878593448009041821933351457118988037109375L);
ASSERT_STREQ_LEN(written, buff,
"0."
"08333333333333333333559208785934480090418219333514571189880"
"3710937500000000");

written = LIBC_NAMESPACE::sprintf(buff, "%Lf", 1e100L);
ASSERT_STREQ_LEN(written, buff,
"99999999999999999996693535322073426194986990198284960792713"
Expand Down Expand Up @@ -2976,6 +2984,10 @@ TEST(LlvmLibcSPrintfTest, FloatAutoLongDoubleConv) {
written = LIBC_NAMESPACE::sprintf(buff, "%Lg", 0xf.fffffffffffffffp+16380L);
ASSERT_STREQ_LEN(written, buff, "1.18973e+4932");

// Minimum normal
written = LIBC_NAMESPACE::sprintf(buff, "%Lg", 3.36210314311209350626E-4932L);
ASSERT_STREQ_LEN(written, buff, "3.3621e-4932");

written = LIBC_NAMESPACE::sprintf(buff, "%Lg", 0xa.aaaaaaaaaaaaaabp-7L);
ASSERT_STREQ_LEN(written, buff, "0.0833333");

Expand Down
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