1 // Copyright 2012 the V8 project authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
10 #include "src/bignum.h"
11 #include "src/cached-powers.h"
12 #include "src/double.h"
13 #include "src/globals.h"
14 #include "src/strtod.h"
15 #include "src/utils.h"
20 // 2^53 = 9007199254740992.
21 // Any integer with at most 15 decimal digits will hence fit into a double
22 // (which has a 53bit significand) without loss of precision.
23 static const int kMaxExactDoubleIntegerDecimalDigits = 15;
24 // 2^64 = 18446744073709551616 > 10^19
25 static const int kMaxUint64DecimalDigits = 19;
27 // Max double: 1.7976931348623157 x 10^308
28 // Min non-zero double: 4.9406564584124654 x 10^-324
29 // Any x >= 10^309 is interpreted as +infinity.
30 // Any x <= 10^-324 is interpreted as 0.
31 // Note that 2.5e-324 (despite being smaller than the min double) will be read
32 // as non-zero (equal to the min non-zero double).
33 static const int kMaxDecimalPower = 309;
34 static const int kMinDecimalPower = -324;
36 // 2^64 = 18446744073709551616
37 static const uint64_t kMaxUint64 = V8_2PART_UINT64_C(0xFFFFFFFF, FFFFFFFF);
40 static const double exact_powers_of_ten[] = {
51 10000000000.0, // 10^10
59 1000000000000000000.0,
60 10000000000000000000.0,
61 100000000000000000000.0, // 10^20
62 1000000000000000000000.0,
63 // 10^22 = 0x21e19e0c9bab2400000 = 0x878678326eac9 * 2^22
64 10000000000000000000000.0
66 static const int kExactPowersOfTenSize = arraysize(exact_powers_of_ten);
68 // Maximum number of significant digits in the decimal representation.
69 // In fact the value is 772 (see conversions.cc), but to give us some margin
70 // we round up to 780.
71 static const int kMaxSignificantDecimalDigits = 780;
73 static Vector<const char> TrimLeadingZeros(Vector<const char> buffer) {
74 for (int i = 0; i < buffer.length(); i++) {
75 if (buffer[i] != '0') {
76 return buffer.SubVector(i, buffer.length());
79 return Vector<const char>(buffer.start(), 0);
83 static Vector<const char> TrimTrailingZeros(Vector<const char> buffer) {
84 for (int i = buffer.length() - 1; i >= 0; --i) {
85 if (buffer[i] != '0') {
86 return buffer.SubVector(0, i + 1);
89 return Vector<const char>(buffer.start(), 0);
93 static void TrimToMaxSignificantDigits(Vector<const char> buffer,
95 char* significant_buffer,
96 int* significant_exponent) {
97 for (int i = 0; i < kMaxSignificantDecimalDigits - 1; ++i) {
98 significant_buffer[i] = buffer[i];
100 // The input buffer has been trimmed. Therefore the last digit must be
101 // different from '0'.
102 DCHECK(buffer[buffer.length() - 1] != '0');
103 // Set the last digit to be non-zero. This is sufficient to guarantee
105 significant_buffer[kMaxSignificantDecimalDigits - 1] = '1';
106 *significant_exponent =
107 exponent + (buffer.length() - kMaxSignificantDecimalDigits);
111 // Reads digits from the buffer and converts them to a uint64.
112 // Reads in as many digits as fit into a uint64.
113 // When the string starts with "1844674407370955161" no further digit is read.
114 // Since 2^64 = 18446744073709551616 it would still be possible read another
115 // digit if it was less or equal than 6, but this would complicate the code.
116 static uint64_t ReadUint64(Vector<const char> buffer,
117 int* number_of_read_digits) {
120 while (i < buffer.length() && result <= (kMaxUint64 / 10 - 1)) {
121 int digit = buffer[i++] - '0';
122 DCHECK(0 <= digit && digit <= 9);
123 result = 10 * result + digit;
125 *number_of_read_digits = i;
130 // Reads a DiyFp from the buffer.
131 // The returned DiyFp is not necessarily normalized.
132 // If remaining_decimals is zero then the returned DiyFp is accurate.
133 // Otherwise it has been rounded and has error of at most 1/2 ulp.
134 static void ReadDiyFp(Vector<const char> buffer,
136 int* remaining_decimals) {
138 uint64_t significand = ReadUint64(buffer, &read_digits);
139 if (buffer.length() == read_digits) {
140 *result = DiyFp(significand, 0);
141 *remaining_decimals = 0;
143 // Round the significand.
144 if (buffer[read_digits] >= '5') {
147 // Compute the binary exponent.
149 *result = DiyFp(significand, exponent);
150 *remaining_decimals = buffer.length() - read_digits;
155 static bool DoubleStrtod(Vector<const char> trimmed,
158 #if (V8_TARGET_ARCH_IA32 || V8_TARGET_ARCH_X87 || defined(USE_SIMULATOR)) && \
160 // On x86 the floating-point stack can be 64 or 80 bits wide. If it is
161 // 80 bits wide (as is the case on Linux) then double-rounding occurs and the
162 // result is not accurate.
163 // We know that Windows32 with MSVC, unlike with MinGW32, uses 64 bits and is
164 // therefore accurate.
165 // Note that the ARM and MIPS simulators are compiled for 32bits. They
166 // therefore exhibit the same problem.
169 if (trimmed.length() <= kMaxExactDoubleIntegerDecimalDigits) {
171 // The trimmed input fits into a double.
172 // If the 10^exponent (resp. 10^-exponent) fits into a double too then we
173 // can compute the result-double simply by multiplying (resp. dividing) the
175 // This is possible because IEEE guarantees that floating-point operations
176 // return the best possible approximation.
177 if (exponent < 0 && -exponent < kExactPowersOfTenSize) {
178 // 10^-exponent fits into a double.
179 *result = static_cast<double>(ReadUint64(trimmed, &read_digits));
180 DCHECK(read_digits == trimmed.length());
181 *result /= exact_powers_of_ten[-exponent];
184 if (0 <= exponent && exponent < kExactPowersOfTenSize) {
185 // 10^exponent fits into a double.
186 *result = static_cast<double>(ReadUint64(trimmed, &read_digits));
187 DCHECK(read_digits == trimmed.length());
188 *result *= exact_powers_of_ten[exponent];
191 int remaining_digits =
192 kMaxExactDoubleIntegerDecimalDigits - trimmed.length();
193 if ((0 <= exponent) &&
194 (exponent - remaining_digits < kExactPowersOfTenSize)) {
195 // The trimmed string was short and we can multiply it with
196 // 10^remaining_digits. As a result the remaining exponent now fits
197 // into a double too.
198 *result = static_cast<double>(ReadUint64(trimmed, &read_digits));
199 DCHECK(read_digits == trimmed.length());
200 *result *= exact_powers_of_ten[remaining_digits];
201 *result *= exact_powers_of_ten[exponent - remaining_digits];
209 // Returns 10^exponent as an exact DiyFp.
210 // The given exponent must be in the range [1; kDecimalExponentDistance[.
211 static DiyFp AdjustmentPowerOfTen(int exponent) {
212 DCHECK(0 < exponent);
213 DCHECK(exponent < PowersOfTenCache::kDecimalExponentDistance);
214 // Simply hardcode the remaining powers for the given decimal exponent
216 DCHECK(PowersOfTenCache::kDecimalExponentDistance == 8);
218 case 1: return DiyFp(V8_2PART_UINT64_C(0xa0000000, 00000000), -60);
219 case 2: return DiyFp(V8_2PART_UINT64_C(0xc8000000, 00000000), -57);
220 case 3: return DiyFp(V8_2PART_UINT64_C(0xfa000000, 00000000), -54);
221 case 4: return DiyFp(V8_2PART_UINT64_C(0x9c400000, 00000000), -50);
222 case 5: return DiyFp(V8_2PART_UINT64_C(0xc3500000, 00000000), -47);
223 case 6: return DiyFp(V8_2PART_UINT64_C(0xf4240000, 00000000), -44);
224 case 7: return DiyFp(V8_2PART_UINT64_C(0x98968000, 00000000), -40);
232 // If the function returns true then the result is the correct double.
233 // Otherwise it is either the correct double or the double that is just below
234 // the correct double.
235 static bool DiyFpStrtod(Vector<const char> buffer,
239 int remaining_decimals;
240 ReadDiyFp(buffer, &input, &remaining_decimals);
241 // Since we may have dropped some digits the input is not accurate.
242 // If remaining_decimals is different than 0 than the error is at most
243 // .5 ulp (unit in the last place).
244 // We don't want to deal with fractions and therefore keep a common
246 const int kDenominatorLog = 3;
247 const int kDenominator = 1 << kDenominatorLog;
248 // Move the remaining decimals into the exponent.
249 exponent += remaining_decimals;
250 int64_t error = (remaining_decimals == 0 ? 0 : kDenominator / 2);
252 int old_e = input.e();
254 error <<= old_e - input.e();
256 DCHECK(exponent <= PowersOfTenCache::kMaxDecimalExponent);
257 if (exponent < PowersOfTenCache::kMinDecimalExponent) {
262 int cached_decimal_exponent;
263 PowersOfTenCache::GetCachedPowerForDecimalExponent(exponent,
265 &cached_decimal_exponent);
267 if (cached_decimal_exponent != exponent) {
268 int adjustment_exponent = exponent - cached_decimal_exponent;
269 DiyFp adjustment_power = AdjustmentPowerOfTen(adjustment_exponent);
270 input.Multiply(adjustment_power);
271 if (kMaxUint64DecimalDigits - buffer.length() >= adjustment_exponent) {
272 // The product of input with the adjustment power fits into a 64 bit
274 DCHECK(DiyFp::kSignificandSize == 64);
276 // The adjustment power is exact. There is hence only an error of 0.5.
277 error += kDenominator / 2;
281 input.Multiply(cached_power);
282 // The error introduced by a multiplication of a*b equals
283 // error_a + error_b + error_a*error_b/2^64 + 0.5
284 // Substituting a with 'input' and b with 'cached_power' we have
285 // error_b = 0.5 (all cached powers have an error of less than 0.5 ulp),
286 // error_ab = 0 or 1 / kDenominator > error_a*error_b/ 2^64
287 int error_b = kDenominator / 2;
288 int error_ab = (error == 0 ? 0 : 1); // We round up to 1.
289 int fixed_error = kDenominator / 2;
290 error += error_b + error_ab + fixed_error;
294 error <<= old_e - input.e();
296 // See if the double's significand changes if we add/subtract the error.
297 int order_of_magnitude = DiyFp::kSignificandSize + input.e();
298 int effective_significand_size =
299 Double::SignificandSizeForOrderOfMagnitude(order_of_magnitude);
300 int precision_digits_count =
301 DiyFp::kSignificandSize - effective_significand_size;
302 if (precision_digits_count + kDenominatorLog >= DiyFp::kSignificandSize) {
303 // This can only happen for very small denormals. In this case the
304 // half-way multiplied by the denominator exceeds the range of an uint64.
305 // Simply shift everything to the right.
306 int shift_amount = (precision_digits_count + kDenominatorLog) -
307 DiyFp::kSignificandSize + 1;
308 input.set_f(input.f() >> shift_amount);
309 input.set_e(input.e() + shift_amount);
310 // We add 1 for the lost precision of error, and kDenominator for
311 // the lost precision of input.f().
312 error = (error >> shift_amount) + 1 + kDenominator;
313 precision_digits_count -= shift_amount;
315 // We use uint64_ts now. This only works if the DiyFp uses uint64_ts too.
316 DCHECK(DiyFp::kSignificandSize == 64);
317 DCHECK(precision_digits_count < 64);
319 uint64_t precision_bits_mask = (one64 << precision_digits_count) - 1;
320 uint64_t precision_bits = input.f() & precision_bits_mask;
321 uint64_t half_way = one64 << (precision_digits_count - 1);
322 precision_bits *= kDenominator;
323 half_way *= kDenominator;
324 DiyFp rounded_input(input.f() >> precision_digits_count,
325 input.e() + precision_digits_count);
326 if (precision_bits >= half_way + error) {
327 rounded_input.set_f(rounded_input.f() + 1);
329 // If the last_bits are too close to the half-way case than we are too
330 // inaccurate and round down. In this case we return false so that we can
331 // fall back to a more precise algorithm.
333 *result = Double(rounded_input).value();
334 if (half_way - error < precision_bits && precision_bits < half_way + error) {
335 // Too imprecise. The caller will have to fall back to a slower version.
336 // However the returned number is guaranteed to be either the correct
337 // double, or the next-lower double.
345 // Returns the correct double for the buffer*10^exponent.
346 // The variable guess should be a close guess that is either the correct double
347 // or its lower neighbor (the nearest double less than the correct one).
349 // buffer.length() + exponent <= kMaxDecimalPower + 1
350 // buffer.length() + exponent > kMinDecimalPower
351 // buffer.length() <= kMaxDecimalSignificantDigits
352 static double BignumStrtod(Vector<const char> buffer,
355 if (guess == V8_INFINITY) {
359 DiyFp upper_boundary = Double(guess).UpperBoundary();
361 DCHECK(buffer.length() + exponent <= kMaxDecimalPower + 1);
362 DCHECK(buffer.length() + exponent > kMinDecimalPower);
363 DCHECK(buffer.length() <= kMaxSignificantDecimalDigits);
364 // Make sure that the Bignum will be able to hold all our numbers.
365 // Our Bignum implementation has a separate field for exponents. Shifts will
366 // consume at most one bigit (< 64 bits).
367 // ln(10) == 3.3219...
368 DCHECK(((kMaxDecimalPower + 1) * 333 / 100) < Bignum::kMaxSignificantBits);
371 input.AssignDecimalString(buffer);
372 boundary.AssignUInt64(upper_boundary.f());
374 input.MultiplyByPowerOfTen(exponent);
376 boundary.MultiplyByPowerOfTen(-exponent);
378 if (upper_boundary.e() > 0) {
379 boundary.ShiftLeft(upper_boundary.e());
381 input.ShiftLeft(-upper_boundary.e());
383 int comparison = Bignum::Compare(input, boundary);
384 if (comparison < 0) {
386 } else if (comparison > 0) {
387 return Double(guess).NextDouble();
388 } else if ((Double(guess).Significand() & 1) == 0) {
389 // Round towards even.
392 return Double(guess).NextDouble();
397 double Strtod(Vector<const char> buffer, int exponent) {
398 Vector<const char> left_trimmed = TrimLeadingZeros(buffer);
399 Vector<const char> trimmed = TrimTrailingZeros(left_trimmed);
400 exponent += left_trimmed.length() - trimmed.length();
401 if (trimmed.length() == 0) return 0.0;
402 if (trimmed.length() > kMaxSignificantDecimalDigits) {
403 char significant_buffer[kMaxSignificantDecimalDigits];
404 int significant_exponent;
405 TrimToMaxSignificantDigits(trimmed, exponent,
406 significant_buffer, &significant_exponent);
407 return Strtod(Vector<const char>(significant_buffer,
408 kMaxSignificantDecimalDigits),
409 significant_exponent);
411 if (exponent + trimmed.length() - 1 >= kMaxDecimalPower) return V8_INFINITY;
412 if (exponent + trimmed.length() <= kMinDecimalPower) return 0.0;
415 if (DoubleStrtod(trimmed, exponent, &guess) ||
416 DiyFpStrtod(trimmed, exponent, &guess)) {
419 return BignumStrtod(trimmed, exponent, guess);
422 } } // namespace v8::internal