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Replace snprintf-based hex float formatter with internal implementation
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Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>
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@phprus
phprus committed Nov 13, 2022
1 parent 3246a91 commit 707f9a1
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Showing 4 changed files with 183 additions and 74 deletions.
14 changes: 1 addition & 13 deletions include/fmt/format-inl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1040,8 +1040,7 @@ template <> struct cache_accessor<double> {
{0xfcf62c1dee382c42, 0x46729e03dd9ed7b6},
{0x9e19db92b4e31ba9, 0x6c07a2c26a8346d2},
{0xc5a05277621be293, 0xc7098b7305241886},
{ 0xf70867153aa2db38,
0xb8cbee4fc66d1ea8 }
{0xf70867153aa2db38, 0xb8cbee4fc66d1ea8}
#else
{0xff77b1fcbebcdc4f, 0x25e8e89c13bb0f7b},
{0xce5d73ff402d98e3, 0xfb0a3d212dc81290},
Expand Down Expand Up @@ -1425,17 +1424,6 @@ template <typename T> decimal_fp<T> to_decimal(T x) noexcept {
return ret_value;
}
} // namespace dragonbox

#ifdef _MSC_VER
FMT_FUNC auto fmt_snprintf(char* buf, size_t size, const char* fmt, ...)
-> int {
auto args = va_list();
va_start(args, fmt);
int result = vsnprintf_s(buf, size, _TRUNCATE, fmt, args);
va_end(args);
return result;
}
#endif
} // namespace detail

template <> struct formatter<detail::bigint> {
Expand Down
158 changes: 101 additions & 57 deletions include/fmt/format.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1618,62 +1618,6 @@ FMT_CONSTEXPR inline fp get_cached_power(int min_exponent,
*(data::pow10_exponents + index)};
}

#ifndef _MSC_VER
# define FMT_SNPRINTF snprintf
#else
FMT_API auto fmt_snprintf(char* buf, size_t size, const char* fmt, ...) -> int;
# define FMT_SNPRINTF fmt_snprintf
#endif // _MSC_VER

// Formats a floating-point number with snprintf using the hexfloat format.
template <typename T>
auto snprintf_float(T value, int precision, float_specs specs,
buffer<char>& buf) -> int {
// Buffer capacity must be non-zero, otherwise MSVC's vsnprintf_s will fail.
FMT_ASSERT(buf.capacity() > buf.size(), "empty buffer");
FMT_ASSERT(specs.format == float_format::hex, "");
static_assert(!std::is_same<T, float>::value, "");

// Build the format string.
char format[7]; // The longest format is "%#.*Le".
char* format_ptr = format;
*format_ptr++ = '%';
if (specs.showpoint) *format_ptr++ = '#';
if (precision >= 0) {
*format_ptr++ = '.';
*format_ptr++ = '*';
}
if (std::is_same<T, long double>()) *format_ptr++ = 'L';
*format_ptr++ = specs.upper ? 'A' : 'a';
*format_ptr = '\0';

// Format using snprintf.
auto offset = buf.size();
for (;;) {
auto begin = buf.data() + offset;
auto capacity = buf.capacity() - offset;
abort_fuzzing_if(precision > 100000);
// Suppress the warning about a nonliteral format string.
// Cannot use auto because of a bug in MinGW (#1532).
int (*snprintf_ptr)(char*, size_t, const char*, ...) = FMT_SNPRINTF;
int result = precision >= 0
? snprintf_ptr(begin, capacity, format, precision, value)
: snprintf_ptr(begin, capacity, format, value);
if (result < 0) {
// The buffer will grow exponentially.
buf.try_reserve(buf.capacity() + 1);
continue;
}
auto size = to_unsigned(result);
// Size equal to capacity means that the last character was truncated.
if (size < capacity) {
buf.try_resize(size + offset);
return 0;
}
buf.try_reserve(size + offset + 1); // Add 1 for the terminating '\0'.
}
}

template <typename T>
using convert_float_result =
conditional_t<std::is_same<T, float>::value ||
Expand Down Expand Up @@ -3171,6 +3115,106 @@ FMT_CONSTEXPR20 inline void format_dragon(basic_fp<uint128_t> value,
buf[num_digits - 1] = static_cast<char>('0' + digit);
}

// Formats a floating-point number with snprintf using the hexfloat format.
template <typename Float, FMT_ENABLE_IF(!is_double_double<Float>::value)>
FMT_CONSTEXPR20 void format_hexfloat(Float value, int precision,
float_specs specs, buffer<char>& buf) {
// float is passed as double to reduce the number of instantiations and to
// simplify implementation.
static_assert(!std::is_same<Float, float>::value, "");
static_assert(std::numeric_limits<Float>::digits <= 113, "unsupported FP");

using info = dragonbox::float_info<Float>;

// Assume Float is in the format [sign][exponent][significand].
using carrier_uint = typename info::carrier_uint;

carrier_uint f;
int e;

const auto num_significand_bits = detail::num_significand_bits<Float>();
const auto implicit_bit = carrier_uint(1) << num_significand_bits;
const auto significand_mask = implicit_bit - 1;
auto u = bit_cast<carrier_uint>(value);
f = static_cast<carrier_uint>(u & significand_mask);
auto biased_e =
static_cast<int>((u & exponent_mask<Float>()) >> num_significand_bits);

if (biased_e == 0)
biased_e = 1; // Subnormals use biased exponent 1 (min exponent).
else if (has_implicit_bit<Float>())
f += static_cast<carrier_uint>(implicit_bit);

e = biased_e - (1 << (info::exponent_bits - 1)) + 1;

const auto num_fraction_bits =
num_significand_bits + (has_implicit_bit<Float>() ? 1 : 0);
const auto num_xdigits = (num_fraction_bits + 3) / 4;

const auto leading_shift = ((num_xdigits - 1) * 4);
const auto leading_mask = carrier_uint(0xF) << leading_shift;
const auto leading_v =
static_cast<uint32_t>((f & leading_mask) >> leading_shift);
if (leading_v > 1) e -= (32 - FMT_BUILTIN_CLZ(leading_v) - 1);

int print_xdigits = num_xdigits - 1;
if (precision >= 0 && print_xdigits > precision) {
const int shift = ((print_xdigits - precision - 1) * 4);
const auto mask = carrier_uint(0xF) << shift;
const auto v = static_cast<uint32_t>((f & mask) >> shift);

if (v >= 8) {
const auto inc = carrier_uint(1) << (shift + 4);
f += inc;
f &= ~(inc - 1);
}

// long double overflow
if (!has_implicit_bit<Float>() && ((f & implicit_bit) == implicit_bit)) {
f >>= 4;
e += 4;
}

print_xdigits = precision;
}

char xdigits[num_bits<carrier_uint>() / 4];
detail::fill_n(xdigits, sizeof(xdigits), '0');
format_uint<4>(xdigits, f, num_xdigits, specs.upper);

// Remove zero tail
while (print_xdigits > 0 && xdigits[print_xdigits] == '0') --print_xdigits;

buf.push_back('0');
buf.push_back(specs.upper ? 'X' : 'x');
buf.push_back(xdigits[0]);
if (specs.showpoint || print_xdigits > 0 || print_xdigits < precision)
buf.push_back('.');
buf.append(xdigits + 1, xdigits + 1 + print_xdigits);

if (print_xdigits < precision)
detail::fill_n(appender(buf), precision - print_xdigits, '0');

buf.push_back(specs.upper ? 'P' : 'p');

uint32_t abs_e;
if (e < 0) {
buf.push_back('-');
abs_e = static_cast<uint32_t>(-e);
} else {
buf.push_back('+');
abs_e = static_cast<uint32_t>(e);
}
format_decimal<char>(appender(buf), abs_e, detail::count_digits(abs_e));
}

template <typename Float, FMT_ENABLE_IF(is_double_double<Float>::value)>
FMT_CONSTEXPR20 void format_hexfloat(Float value, int precision,
float_specs specs, buffer<char>& buf) {
static_assert(std::numeric_limits<double>::is_iec559, "unsupported FP");
format_hexfloat(static_cast<double>(value), precision, specs, buf);
}

template <typename Float>
FMT_CONSTEXPR20 auto format_float(Float value, int precision, float_specs specs,
buffer<char>& buf) -> int {
Expand Down Expand Up @@ -3285,7 +3329,7 @@ FMT_CONSTEXPR20 auto write_float(OutputIt out, T value,
memory_buffer buffer;
if (fspecs.format == float_format::hex) {
if (fspecs.sign) buffer.push_back(detail::sign<char>(fspecs.sign));
snprintf_float(convert_float(value), specs.precision, fspecs, buffer);
format_hexfloat(convert_float(value), specs.precision, fspecs, buffer);
return write_bytes<align::right>(out, {buffer.data(), buffer.size()},
specs);
}
Expand Down
70 changes: 68 additions & 2 deletions test/format-test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1344,16 +1344,82 @@ TEST(format_test, format_double) {
EXPECT_EQ(fmt::format("{:f}", 392.65), "392.650000");
EXPECT_EQ(fmt::format("{:F}", 392.65), "392.650000");
EXPECT_EQ(fmt::format("{:L}", 42.0), "42");
EXPECT_EQ(fmt::format("{:24a}", 4.2f), " 0x1.0cccccp+2");
EXPECT_EQ(fmt::format("{:24a}", 4.2), " 0x1.0cccccccccccdp+2");
EXPECT_EQ(fmt::format("{:<24a}", 4.2), "0x1.0cccccccccccdp+2 ");
EXPECT_EQ(fmt::format("{0:e}", 392.65), "3.926500e+02");
EXPECT_EQ(fmt::format("{0:E}", 392.65), "3.926500E+02");
EXPECT_EQ(fmt::format("{0:+010.4g}", 392.65), "+0000392.6");
char buffer[buffer_size];
double d = 0;

#if FMT_CPLUSPLUS >= 201703L
d = 0x1.ffffffffffp+2;
safe_sprintf(buffer, "%.*a", 10, d);
EXPECT_EQ(fmt::format("{:.10a}", d), buffer);
safe_sprintf(buffer, "%.*a", 9, d);
EXPECT_EQ(fmt::format("{:.9a}", d), buffer);

if (std::numeric_limits<long double>::digits == 64) {
auto ld = 0xf.ffffffffffp-3l;
safe_sprintf(buffer, "%La", ld);
EXPECT_EQ(fmt::format("{:a}", ld), buffer);
safe_sprintf(buffer, "%.*La", 10, ld);
EXPECT_EQ(fmt::format("{:.10a}", ld), buffer);
safe_sprintf(buffer, "%.*La", 9, ld);
EXPECT_EQ(fmt::format("{:.9a}", ld), buffer);
}
#endif

d = (std::numeric_limits<double>::min)();
safe_sprintf(buffer, "%a", d);
std::string d_min_outputs[] = {
"0x1p-1022", // UNIX
"0x1.0000000000000p-1022" // Windows
};
EXPECT_THAT(d_min_outputs, testing::Contains(buffer));
EXPECT_EQ(fmt::format("{:a}", d), "0x1p-1022");
EXPECT_EQ(fmt::format("{:#a}", d), "0x1.p-1022");

d = (std::numeric_limits<double>::max)();
safe_sprintf(buffer, "%a", d);
EXPECT_EQ(fmt::format("{:a}", d), buffer);

d = std::numeric_limits<double>::denorm_min();
EXPECT_EQ(fmt::format("{:a}", d), "0x0.0000000000001p-1022");

if (std::numeric_limits<long double>::digits == 64) {
auto ld = (std::numeric_limits<long double>::min)();
safe_sprintf(buffer, "%La", ld);
EXPECT_EQ(fmt::format("{:a}", ld), buffer);

ld = (std::numeric_limits<long double>::max)();
safe_sprintf(buffer, "%La", ld);
EXPECT_EQ(fmt::format("{:a}", ld), buffer);

ld = std::numeric_limits<long double>::denorm_min();
EXPECT_EQ(fmt::format("{:a}", ld), "0x0.000000000000001p-16382");
}

safe_sprintf(buffer, "%.*a", 10, 4.2);
EXPECT_EQ(fmt::format("{:.10a}", 4.2), buffer);

safe_sprintf(buffer, "%a", -42.0);
EXPECT_EQ(fmt::format("{:a}", -42.0), buffer);
std::string a_outputs[] = {
"-0x1.5p+5", // UNIX
"-0x1.5000000000000p+5" // Windows
};
EXPECT_THAT(a_outputs, testing::Contains(buffer));
EXPECT_EQ(fmt::format("{:a}", -42.0), "-0x1.5p+5");

safe_sprintf(buffer, "%A", -42.0);
EXPECT_EQ(fmt::format("{:A}", -42.0), buffer);
std::string A_outputs[] = {
"-0X1.5P+5", // UNIX
"-0X1.5000000000000P+5" // Windows
};
EXPECT_THAT(A_outputs, testing::Contains(buffer));
EXPECT_EQ(fmt::format("{:A}", -42.0), "-0X1.5P+5");

EXPECT_EQ(fmt::format("{:f}", 9223372036854775807.0),
"9223372036854775808.000000");
}
Expand Down
15 changes: 13 additions & 2 deletions test/printf-test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -226,9 +226,20 @@ TEST(printf_test, hash_flag) {
EXPECT_PRINTF("-42.0000", "%#G", -42.0);

safe_sprintf(buffer, "%#a", 16.0);
EXPECT_PRINTF(buffer, "%#a", 16.0);
std::string a_outputs[] = {
"0x1.p+4", // UNIX
"0x1.0000000000000p+4" // Windows
};
EXPECT_THAT(a_outputs, testing::Contains(buffer));
EXPECT_PRINTF("0x1.p+4", "%#a", 16.0);

safe_sprintf(buffer, "%#A", 16.0);
EXPECT_PRINTF(buffer, "%#A", 16.0);
std::string A_outputs[] = {
"0X1.P+4", // UNIX
"0X1.0000000000000P+4" // Windows
};
EXPECT_THAT(A_outputs, testing::Contains(buffer));
EXPECT_PRINTF("0X1.P+4", "%#A", 16.0);

// '#' flag is ignored for non-numeric types.
EXPECT_PRINTF("x", "%#c", 'x');
Expand Down

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