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-rw-r--r--absl/strings/internal/str_format/float_conversion.cc497
1 files changed, 486 insertions, 11 deletions
diff --git a/absl/strings/internal/str_format/float_conversion.cc b/absl/strings/internal/str_format/float_conversion.cc
index 6176db9c..20012b58 100644
--- a/absl/strings/internal/str_format/float_conversion.cc
+++ b/absl/strings/internal/str_format/float_conversion.cc
@@ -2,15 +2,476 @@
#include <string.h>
#include <algorithm>
+#include <array>
#include <cassert>
#include <cmath>
+#include <limits>
#include <string>
+#include "absl/base/attributes.h"
+#include "absl/base/internal/bits.h"
+#include "absl/base/optimization.h"
+#include "absl/meta/type_traits.h"
+#include "absl/numeric/int128.h"
+#include "absl/types/span.h"
+
namespace absl {
namespace str_format_internal {
namespace {
+// Calculates `10 * (*v) + carry` and stores the result in `*v` and returns
+// the carry.
+template <typename Int>
+inline Int MultiplyBy10WithCarry(Int *v, Int carry) {
+ using NextInt = absl::conditional_t<sizeof(Int) == 4, uint64_t, uint128>;
+ static_assert(sizeof(void *) >= sizeof(Int),
+ "Don't want to use uint128 in 32-bit mode. It is too slow.");
+ NextInt tmp = 10 * static_cast<NextInt>(*v) + carry;
+ *v = static_cast<Int>(tmp);
+ return static_cast<Int>(tmp >> (sizeof(Int) * 8));
+}
+
+// Calculates `(2^64 * carry + *v) / 10`.
+// Stores the quotient in `*v` and returns the remainder.
+// Requires: `0 <= carry <= 9`
+inline uint64_t DivideBy10WithCarry(uint64_t *v, uint64_t carry) {
+ constexpr uint64_t divisor = 10;
+ // 2^64 / divisor = word_quotient + word_remainder / divisor
+ constexpr uint64_t word_quotient = (uint64_t{1} << 63) / (divisor / 2);
+ constexpr uint64_t word_remainder = uint64_t{} - word_quotient * divisor;
+
+ const uint64_t mod = *v % divisor;
+ const uint64_t next_carry = word_remainder * carry + mod;
+ *v = *v / divisor + carry * word_quotient + next_carry / divisor;
+ return next_carry % divisor;
+}
+
+int LeadingZeros(uint64_t v) { return base_internal::CountLeadingZeros64(v); }
+int LeadingZeros(uint128 v) {
+ auto high = static_cast<uint64_t>(v >> 64);
+ auto low = static_cast<uint64_t>(v);
+ return high != 0 ? base_internal::CountLeadingZeros64(high)
+ : 64 + base_internal::CountLeadingZeros64(low);
+}
+
+int TrailingZeros(uint64_t v) {
+ return base_internal::CountTrailingZerosNonZero64(v);
+}
+int TrailingZeros(uint128 v) {
+ auto high = static_cast<uint64_t>(v >> 64);
+ auto low = static_cast<uint64_t>(v);
+ return low == 0 ? 64 + base_internal::CountTrailingZerosNonZero64(high)
+ : base_internal::CountTrailingZerosNonZero64(low);
+}
+
+// The buffer must have an extra digit that is known to not need rounding.
+// This is done below by having an extra '0' digit on the left.
+void RoundUp(char *last_digit) {
+ char *p = last_digit;
+ while (*p == '9' || *p == '.') {
+ if (*p == '9') *p = '0';
+ --p;
+ }
+ ++*p;
+}
+
+void RoundToEven(char *last_digit) {
+ char *p = last_digit;
+ if (*p == '.') --p;
+ if (*p % 2 == 1) RoundUp(p);
+}
+
+char *PrintIntegralDigitsFromRightDynamic(uint128 v, Span<uint32_t> array,
+ int exp, char *p) {
+ if (v == 0) {
+ *--p = '0';
+ return p;
+ }
+
+ int w = exp / 32;
+ const int offset = exp % 32;
+ // Left shift v by exp bits.
+ array[w] = static_cast<uint32_t>(v << offset);
+ for (v >>= (32 - offset); v; v >>= 32) array[++w] = static_cast<uint32_t>(v);
+
+ // While we have more than one word available, go in chunks of 1e9.
+ // We are guaranteed to have at least those many digits.
+ // `w` holds the largest populated word, so keep it updated.
+ while (w > 0) {
+ uint32_t carry = 0;
+ for (int i = w; i >= 0; --i) {
+ uint64_t tmp = uint64_t{array[i]} + (uint64_t{carry} << 32);
+ array[i] = tmp / uint64_t{1000000000};
+ carry = tmp % uint64_t{1000000000};
+ }
+ // If the highest word is now empty, remove it from view.
+ if (array[w] == 0) --w;
+
+ for (int i = 0; i < 9; ++i, carry /= 10) {
+ *--p = carry % 10 + '0';
+ }
+ }
+
+ // Print the leftover of the last word.
+ for (auto last = array[0]; last != 0; last /= 10) {
+ *--p = last % 10 + '0';
+ }
+
+ return p;
+}
+
+struct FractionalResult {
+ const char *end;
+ int precision;
+};
+
+FractionalResult PrintFractionalDigitsDynamic(uint128 v, Span<uint32_t> array,
+ char *p, int exp, int precision) {
+ int w = exp / 32;
+ const int offset = exp % 32;
+
+ // Right shift `v` by `exp` bits.
+ array[w] = static_cast<uint32_t>(v << (32 - offset));
+ v >>= offset;
+ // Make sure we don't overflow the array. We already calculated that non-zero
+ // bits fit, so we might not have space for leading zero bits.
+ for (int pos = w; v; v >>= 32) array[--pos] = static_cast<uint32_t>(v);
+
+ // Multiply the whole sequence by 10.
+ // On each iteration, the leftover carry word is the next digit.
+ // `w` holds the largest populated word, so keep it updated.
+ for (; w >= 0 && precision > 0; --precision) {
+ uint32_t carry = 0;
+ for (int i = w; i >= 0; --i) {
+ carry = MultiplyBy10WithCarry(&array[i], carry);
+ }
+ // If the lowest word is now empty, remove it from view.
+ if (array[w] == 0) --w;
+ *p++ = carry + '0';
+ }
+
+ constexpr uint32_t threshold = 0x80000000;
+ if (array[0] < threshold) {
+ // We round down, so nothing to do.
+ } else if (array[0] > threshold ||
+ std::any_of(&array[1], &array[w + 1],
+ [](uint32_t word) { return word != 0; })) {
+ RoundUp(p - 1);
+ } else {
+ RoundToEven(p - 1);
+ }
+ return {p, precision};
+}
+
+// Generic digit printer.
+// `bits` determines how many bits of termporary space it needs for the
+// calcualtions.
+template <int bits, typename = void>
+class DigitPrinter {
+ static constexpr int kInts = (bits + 31) / 32;
+
+ public:
+ // Quick upper bound for the number of decimal digits we need.
+ // This would be std::ceil(std::log10(std::pow(2, bits))), but that is not
+ // constexpr.
+ static constexpr int kDigits10 = 1 + (bits + 9) / 10 * 3 + bits / 900;
+ using InputType = uint128;
+
+ static char *PrintIntegralDigitsFromRight(InputType v, int exp, char *end) {
+ std::array<uint32_t, kInts> array{};
+ return PrintIntegralDigitsFromRightDynamic(v, absl::MakeSpan(array), exp,
+ end);
+ }
+
+ static FractionalResult PrintFractionalDigits(InputType v, char *p, int exp,
+ int precision) {
+ std::array<uint32_t, kInts> array{};
+ return PrintFractionalDigitsDynamic(v, absl::MakeSpan(array), p, exp,
+ precision);
+ }
+};
+
+// Specialiation for 64-bit working space.
+// This is a performance optimization over the generic primary template.
+// Only enabled in 64-bit platforms. The generic one is faster in 32-bit
+// platforms.
+template <int bits>
+class DigitPrinter<bits, absl::enable_if_t<bits == 64 && (sizeof(void *) >=
+ sizeof(uint64_t))>> {
+ public:
+ static constexpr size_t kDigits10 = 20;
+ using InputType = uint64_t;
+
+ static char *PrintIntegralDigitsFromRight(uint64_t v, int exp, char *p) {
+ v <<= exp;
+ do {
+ *--p = DivideBy10WithCarry(&v, 0) + '0';
+ } while (v != 0);
+ return p;
+ }
+
+ static FractionalResult PrintFractionalDigits(uint64_t v, char *p, int exp,
+ int precision) {
+ v <<= (64 - exp);
+ while (precision > 0) {
+ if (!v) return {p, precision};
+ *p++ = MultiplyBy10WithCarry(&v, uint64_t{}) + '0';
+ --precision;
+ }
+
+ // We need to round.
+ if (v < 0x8000000000000000) {
+ // We round down, so nothing to do.
+ } else if (v > 0x8000000000000000) {
+ // We round up.
+ RoundUp(p - 1);
+ } else {
+ RoundToEven(p - 1);
+ }
+
+ assert(precision == 0);
+ // Precision can only be zero here. Return a constant instead.
+ return {p, 0};
+ }
+};
+
+// Specialiation for 128-bit working space.
+// This is a performance optimization over the generic primary template.
+template <int bits>
+class DigitPrinter<bits, absl::enable_if_t<bits == 128 && (sizeof(void *) >=
+ sizeof(uint64_t))>> {
+ public:
+ static constexpr size_t kDigits10 = 40;
+ using InputType = uint128;
+
+ static char *PrintIntegralDigitsFromRight(uint128 v, int exp, char *p) {
+ v <<= exp;
+ auto high = static_cast<uint64_t>(v >> 64);
+ auto low = static_cast<uint64_t>(v);
+
+ do {
+ uint64_t carry = DivideBy10WithCarry(&high, 0);
+ carry = DivideBy10WithCarry(&low, carry);
+ *--p = carry + '0';
+ } while (high != 0u);
+
+ while (low != 0u) {
+ *--p = DivideBy10WithCarry(&low, 0) + '0';
+ }
+ return p;
+ }
+
+ static FractionalResult PrintFractionalDigits(uint128 v, char *p, int exp,
+ int precision) {
+ v <<= (128 - exp);
+ auto high = static_cast<uint64_t>(v >> 64);
+ auto low = static_cast<uint64_t>(v);
+
+ // While we have digits to print and `low` is not empty, do the long
+ // multiplication.
+ while (precision > 0 && low != 0) {
+ uint64_t carry = MultiplyBy10WithCarry(&low, uint64_t{});
+ carry = MultiplyBy10WithCarry(&high, carry);
+
+ *p++ = carry + '0';
+ --precision;
+ }
+
+ // Now `low` is empty, so use a faster approach for the rest of the digits.
+ // This block is pretty much the same as the main loop for the 64-bit case
+ // above.
+ while (precision > 0) {
+ if (!high) return {p, precision};
+ *p++ = MultiplyBy10WithCarry(&high, uint64_t{}) + '0';
+ --precision;
+ }
+
+ // We need to round.
+ if (high < 0x8000000000000000) {
+ // We round down, so nothing to do.
+ } else if (high > 0x8000000000000000 || low != 0) {
+ // We round up.
+ RoundUp(p - 1);
+ } else {
+ RoundToEven(p - 1);
+ }
+
+ assert(precision == 0);
+ // Precision can only be zero here. Return a constant instead.
+ return {p, 0};
+ }
+};
+
+struct FormatState {
+ char sign_char;
+ int precision;
+ const ConversionSpec &conv;
+ FormatSinkImpl *sink;
+};
+
+void FinalPrint(string_view data, int trailing_zeros,
+ const FormatState &state) {
+ if (state.conv.width() < 0) {
+ // No width specified. Fast-path.
+ if (state.sign_char != '\0') state.sink->Append(1, state.sign_char);
+ state.sink->Append(data);
+ state.sink->Append(trailing_zeros, '0');
+ return;
+ }
+
+ int left_spaces = 0, zeros = 0, right_spaces = 0;
+ int total_size = (state.sign_char != 0 ? 1 : 0) +
+ static_cast<int>(data.size()) + trailing_zeros;
+ int missing_chars = std::max(state.conv.width() - total_size, 0);
+ if (state.conv.flags().left) {
+ right_spaces = missing_chars;
+ } else if (state.conv.flags().zero) {
+ zeros = missing_chars;
+ } else {
+ left_spaces = missing_chars;
+ }
+
+ state.sink->Append(left_spaces, ' ');
+ if (state.sign_char != '\0') state.sink->Append(1, state.sign_char);
+ state.sink->Append(zeros, '0');
+ state.sink->Append(data);
+ state.sink->Append(trailing_zeros, '0');
+ state.sink->Append(right_spaces, ' ');
+}
+
+template <int num_bits, typename Int>
+void FormatFPositiveExp(Int v, int exp, const FormatState &state) {
+ using IntegralPrinter = DigitPrinter<num_bits>;
+ char buffer[IntegralPrinter::kDigits10 + /* . */ 1];
+ buffer[IntegralPrinter::kDigits10] = '.';
+
+ const char *digits = IntegralPrinter::PrintIntegralDigitsFromRight(
+ static_cast<typename IntegralPrinter::InputType>(v), exp,
+ buffer + sizeof(buffer) - 1);
+ size_t size = buffer + sizeof(buffer) - digits;
+
+ // In `alt` mode (flag #) we keep the `.` even if there are no fractional
+ // digits. In non-alt mode, we strip it.
+ if (ABSL_PREDICT_FALSE(state.precision == 0 && !state.conv.flags().alt)) {
+ --size;
+ }
+
+ FinalPrint(string_view(digits, size), state.precision, state);
+}
+
+template <int num_bits, typename Int>
+void FormatFNegativeExp(Int v, int exp, const FormatState &state) {
+ constexpr int input_bits = sizeof(Int) * 8;
+
+ using IntegralPrinter = DigitPrinter<input_bits>;
+ using FractionalPrinter = DigitPrinter<num_bits>;
+
+ static constexpr size_t integral_size =
+ 1 + /* in case we need to round up an extra digit */
+ IntegralPrinter::kDigits10 + 1;
+ char buffer[integral_size + /* . */ 1 + num_bits];
+ buffer[integral_size] = '.';
+ char *const integral_digits_end = buffer + integral_size;
+ char *integral_digits_start;
+ char *const fractional_digits_start = buffer + integral_size + 1;
+
+ if (exp < input_bits) {
+ integral_digits_start = IntegralPrinter::PrintIntegralDigitsFromRight(
+ v >> exp, 0, integral_digits_end);
+ } else {
+ integral_digits_start = integral_digits_end - 1;
+ *integral_digits_start = '0';
+ }
+
+ // PrintFractionalDigits may pull a carried 1 all the way up through the
+ // integral portion.
+ integral_digits_start[-1] = '0';
+ auto fractional_result = FractionalPrinter::PrintFractionalDigits(
+ static_cast<typename FractionalPrinter::InputType>(v),
+ fractional_digits_start, exp, state.precision);
+ if (integral_digits_start[-1] != '0') --integral_digits_start;
+
+ size_t size = fractional_result.end - integral_digits_start;
+
+ // In `alt` mode (flag #) we keep the `.` even if there are no fractional
+ // digits. In non-alt mode, we strip it.
+ if (ABSL_PREDICT_FALSE(state.precision == 0 && !state.conv.flags().alt)) {
+ --size;
+ }
+ FinalPrint(string_view(integral_digits_start, size),
+ fractional_result.precision, state);
+}
+
+template <typename Int>
+void FormatF(Int mantissa, int exp, const FormatState &state) {
+ // Remove trailing zeros as they are not useful.
+ // This helps use faster implementations/less stack space in some cases.
+ if (mantissa != 0) {
+ int trailing = TrailingZeros(mantissa);
+ mantissa >>= trailing;
+ exp += trailing;
+ }
+
+ // The table driven dispatch gives us two benefits: fast distpatch and
+ // prevent inlining.
+ // We must not inline any of the functions below (other than the ones for
+ // 64-bit) to avoid blowing up this stack frame.
+
+ if (exp >= 0) {
+ // We will left shift the mantissa. Calculate how many bits we need.
+ // Special case 64-bit as we will use a uint64_t for it. Use a table for the
+ // rest and unconditionally use uint128.
+ const int total_bits = sizeof(Int) * 8 - LeadingZeros(mantissa) + exp;
+
+ if (total_bits <= 64) {
+ return FormatFPositiveExp<64>(mantissa, exp, state);
+ } else {
+ using Formatter = void (*)(uint128, int, const FormatState &);
+ static constexpr Formatter kFormatters[] = {
+ FormatFPositiveExp<1 << 7>, FormatFPositiveExp<1 << 8>,
+ FormatFPositiveExp<1 << 9>, FormatFPositiveExp<1 << 10>,
+ FormatFPositiveExp<1 << 11>, FormatFPositiveExp<1 << 12>,
+ FormatFPositiveExp<1 << 13>, FormatFPositiveExp<1 << 14>,
+ FormatFPositiveExp<1 << 15>,
+ };
+ static constexpr int max_total_bits =
+ sizeof(Int) * 8 + std::numeric_limits<long double>::max_exponent;
+ assert(total_bits <= max_total_bits);
+ static_assert(max_total_bits <= (1 << 15), "");
+ const int log2 =
+ 64 - LeadingZeros((static_cast<uint64_t>(total_bits) - 1) / 128);
+ assert(log2 < std::end(kFormatters) - std::begin(kFormatters));
+ kFormatters[log2](mantissa, exp, state);
+ }
+ } else {
+ exp = -exp;
+
+ // We know we don't need more than Int itself for the integral part.
+ // We need `precision` fractional digits, but there are at most `exp`
+ // non-zero digits after the decimal point. The rest will be zeros.
+ // Special case 64-bit as we will use a uint64_t for it. Use a table for the
+ // rest and unconditionally use uint128.
+
+ if (exp <= 64) {
+ return FormatFNegativeExp<64>(mantissa, exp, state);
+ } else {
+ using Formatter = void (*)(uint128, int, const FormatState &);
+ static constexpr Formatter kFormatters[] = {
+ FormatFNegativeExp<1 << 7>, FormatFNegativeExp<1 << 8>,
+ FormatFNegativeExp<1 << 9>, FormatFNegativeExp<1 << 10>,
+ FormatFNegativeExp<1 << 11>, FormatFNegativeExp<1 << 12>,
+ FormatFNegativeExp<1 << 13>, FormatFNegativeExp<1 << 14>};
+ static_assert(
+ -std::numeric_limits<long double>::min_exponent <= (1 << 14), "");
+ const int log2 =
+ 64 - LeadingZeros((static_cast<uint64_t>(exp) - 1) / 128);
+ assert(log2 < std::end(kFormatters) - std::begin(kFormatters));
+ kFormatters[log2](mantissa, exp, state);
+ }
+ }
+}
+
char *CopyStringTo(string_view v, char *out) {
std::memcpy(out, v.data(), v.size());
return out + v.size();
@@ -95,7 +556,7 @@ template <typename Float>
bool ConvertNonNumericFloats(char sign_char, Float v,
const ConversionSpec &conv, FormatSinkImpl *sink) {
char text[4], *ptr = text;
- if (sign_char) *ptr++ = sign_char;
+ if (sign_char != '\0') *ptr++ = sign_char;
if (std::isnan(v)) {
ptr = std::copy_n(conv.conv().upper() ? "NAN" : "nan", 3, ptr);
} else if (std::isinf(v)) {
@@ -165,7 +626,12 @@ constexpr bool CanFitMantissa() {
template <typename Float>
struct Decomposed {
- Float mantissa;
+ using MantissaType =
+ absl::conditional_t<std::is_same<long double, Float>::value, uint128,
+ uint64_t>;
+ static_assert(std::numeric_limits<Float>::digits <= sizeof(MantissaType) * 8,
+ "");
+ MantissaType mantissa;
int exponent;
};
@@ -176,7 +642,8 @@ Decomposed<Float> Decompose(Float v) {
Float m = std::frexp(v, &exp);
m = std::ldexp(m, std::numeric_limits<Float>::digits);
exp -= std::numeric_limits<Float>::digits;
- return {m, exp};
+
+ return {static_cast<typename Decomposed<Float>::MantissaType>(m), exp};
}
// Print 'digits' as decimal.
@@ -334,7 +801,7 @@ bool FloatToBuffer(Decomposed<Float> decomposed, int precision, Buffer *out,
static_cast<std::uint64_t>(decomposed.exponent), precision, out, exp))
return true;
-#if defined(__SIZEOF_INT128__)
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
// If that is not enough, try with __uint128_t.
return CanFitMantissa<Float, __uint128_t>() &&
FloatToBufferImpl<__uint128_t, Float, mode>(
@@ -362,7 +829,7 @@ void WriteBufferToSink(char sign_char, string_view str,
}
sink->Append(left_spaces, ' ');
- if (sign_char) sink->Append(1, sign_char);
+ if (sign_char != '\0') sink->Append(1, sign_char);
sink->Append(zeros, '0');
sink->Append(str);
sink->Append(right_spaces, ' ');
@@ -399,12 +866,9 @@ bool FloatToSink(const Float v, const ConversionSpec &conv,
switch (conv.conv().id()) {
case ConversionChar::f:
case ConversionChar::F:
- if (!FloatToBuffer<FormatStyle::Fixed>(decomposed, precision, &buffer,
- nullptr)) {
- return FallbackToSnprintf(v, conv, sink);
- }
- if (!conv.flags().alt && buffer.back() == '.') buffer.pop_back();
- break;
+ FormatF(decomposed.mantissa, decomposed.exponent,
+ {sign_char, precision, conv, sink});
+ return true;
case ConversionChar::e:
case ConversionChar::E:
@@ -466,11 +930,22 @@ bool FloatToSink(const Float v, const ConversionSpec &conv,
bool ConvertFloatImpl(long double v, const ConversionSpec &conv,
FormatSinkImpl *sink) {
+ if (std::numeric_limits<long double>::digits ==
+ 2 * std::numeric_limits<double>::digits) {
+ // This is the `double-double` representation of `long double`.
+ // We do not handle it natively. Fallback to snprintf.
+ return FallbackToSnprintf(v, conv, sink);
+ }
+
return FloatToSink(v, conv, sink);
}
bool ConvertFloatImpl(float v, const ConversionSpec &conv,
FormatSinkImpl *sink) {
+ // DivideBy10WithCarry is not actually used in some builds. This here silences
+ // the "unused" warning. We just need to put it in any function that is really
+ // used.
+ (void)&DivideBy10WithCarry;
return FloatToSink(v, conv, sink);
}
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