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[libc] Fix printf long double truncation bound (#70705)

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The calculation for if a number being printed is truncated and should be
rounded up assumed a double for one of its constants, causing
occassional misrounding. This fixes that by making the constant based on
the mantissa width.
This commit is contained in:
michaelrj-google
2023-10-30 14:04:00 -07:00
committed by GitHub
parent 55c9f24344
commit 8ca565cd3b
2 changed files with 10 additions and 5 deletions
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12
libc/src/stdio/printf_core/float_dec_converter.h
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@@ -85,11 +85,13 @@ LIBC_INLINE RoundDirection get_round_direction(int last_digit, bool truncated,
template <typename T>
LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_integral_v<T>, bool>
zero_after_digits(int32_t base_2_exp, int32_t digits_after_point, T mantissa) {
zero_after_digits(int32_t base_2_exp, int32_t digits_after_point, T mantissa,
const int32_t mant_width) {
const int32_t required_twos = -base_2_exp - digits_after_point - 1;
// Add 8 to mant width since this is a loose bound.
const bool has_trailing_zeros =
required_twos <= 0 ||
(required_twos < 60 &&
(required_twos < (mant_width + 8) &&
multiple_of_power_of_2(mantissa, static_cast<uint32_t>(required_twos)));
return has_trailing_zeros;
}
@@ -568,7 +570,7 @@ LIBC_INLINE int convert_float_decimal_typed(Writer *writer,
RoundDirection round;
const bool truncated =
!zero_after_digits(exponent - MANT_WIDTH, precision,
float_bits.get_explicit_mantissa());
float_bits.get_explicit_mantissa(), MANT_WIDTH);
round = get_round_direction(last_digit, truncated, is_negative);
RET_IF_RESULT_NEGATIVE(
@@ -733,7 +735,7 @@ LIBC_INLINE int convert_float_dec_exp_typed(Writer *writer,
// Use the formula from %f.
truncated =
!zero_after_digits(exponent - MANT_WIDTH, precision - final_exponent,
float_bits.get_explicit_mantissa());
float_bits.get_explicit_mantissa(), MANT_WIDTH);
}
}
round = get_round_direction(last_digit, truncated, is_negative);
@@ -979,7 +981,7 @@ LIBC_INLINE int convert_float_dec_auto_typed(Writer *writer,
// Use the formula from %f.
truncated =
!zero_after_digits(exponent - MANT_WIDTH, exp_precision - base_10_exp,
float_bits.get_explicit_mantissa());
float_bits.get_explicit_mantissa(), MANT_WIDTH);
}
}

3
libc/test/src/stdio/sprintf_test.cpp
View File

@@ -1041,6 +1041,9 @@ TEST_F(LlvmLibcSPrintfTest, FloatDecimalConv) {
written = LIBC_NAMESPACE::sprintf(buff, "%Lf", 1.0L);
ASSERT_STREQ_LEN(written, buff, "1.000000");
written = LIBC_NAMESPACE::sprintf(buff, "%.Lf", -2.5L);
ASSERT_STREQ_LEN(written, buff, "-2");
#if defined(SPECIAL_X86_LONG_DOUBLE)
written = LIBC_NAMESPACE::sprintf(buff, "%Lf", 1e100L);