Implement fast case for strtod.

src/strtod.cc

Index: src/strtod.cc
diff --git a/src/strtod.cc b/src/strtod.cc
index 5f7a69d9c5a46ae8722f8363129e45b5348ef9aa..bfbb5db7b9cd1e29d48b088a5e99c02c54ba7f01 100644
--- a/src/strtod.cc
+++ b/src/strtod.cc
@@ -36,9 +36,47 @@
 namespace v8 {
 namespace internal {
 
+// 2^53 = 9007199254740992.
+// Any integer with at most 15 decimal digits will hence fit into a double
+// (which has a 53bit significand) without loss of precision.
+static const int kMaxExactDoubleIntegerDecimalDigits = 15;
+// 2^64 = 18446744073709551616
+// Any integer with at most 19 digits will hence fit into a 64bit datatype.
+static const int kMaxUint64DecimalDigits = 19;
+
+static const double exact_powers_of_ten[] = {
+  1.0,  // 10^0
+  10.0,
+  100.0,
+  1000.0,
+  10000.0,
+  100000.0,
+  1000000.0,
+  10000000.0,
+  100000000.0,
+  1000000000.0,
+  10000000000.0,  // 10^10
+  100000000000.0,
+  1000000000000.0,
+  10000000000000.0,
+  100000000000000.0,
+  1000000000000000.0,
+  10000000000000000.0,
+  100000000000000000.0,
+  1000000000000000000.0,
+  10000000000000000000.0,
+  100000000000000000000.0,  // 10^20
+  1000000000000000000000.0,
+  // 10^22 = 0x21e19e0c9bab2400000 = 0x878678326eac9 * 2^22
+  10000000000000000000000.0
+};
+
+static const int kExactPowersOfTenSize = ARRAY_SIZE(exact_powers_of_ten);
+
+
 extern "C" double gay_strtod(const char* s00, const char** se);
 
-double strtod(Vector<char> buffer, int exponent) {
+static double old_strtod(Vector<char> buffer, int exponent) {
   char gay_buffer[1024];
   Vector<char> gay_buffer_vector(gay_buffer, sizeof(gay_buffer));
   buffer.start()[buffer.length()] = '\0';
@@ -46,4 +84,63 @@ double strtod(Vector<char> buffer, int exponent) {
   return gay_strtod(gay_buffer, NULL);
 }
 
+
+static Vector<char> TrimTrailingZeros(Vector<char> buffer) {
+  for (int i = buffer.length() - 1; i >= 0; --i) {
+    if (buffer[i] != '0') {
+      return Vector<char>(buffer.start(), i + 1);
+    }
+  }
+  return Vector<char>(buffer.start(), 0);
+}
+
+
+uint64_t ReadUint64(Vector<char> buffer) {
+  ASSERT(buffer.length() <= kMaxUint64DecimalDigits);
+  uint64_t result = 0;
+  for (int i = 0; i < buffer.length(); ++i) {
+    int digit = buffer[i] - '0';
+    ASSERT(0 <= digit && digit <= 9);
+    result = 10 * result + digit;
+  }
+  return result;
+}
+
+
+double Strtod(Vector<char> buffer, int exponent) {
+  Vector<char> trimmed = TrimTrailingZeros(buffer);
+  if (trimmed.length() == 0) return 0.0;
+  exponent += buffer.length() - trimmed.length();
+  if (trimmed.length() <= kMaxExactDoubleIntegerDecimalDigits) {
+    // The trimmed input fits into a double.
+    // If the 10^exponent (resp. 10^-exponent) fits into a double too then we
+    // can compute the result-double simply by multiplying (resp. dividing) the
+    // two numbers.
+    // This is possible because IEEE guarantees that floating-point operations
+    // return the best possible approximation.
+    if (exponent < 0 && -exponent < kExactPowersOfTenSize) {
+      // 10^-exponent fits into a double.
+      double buffer_d = static_cast<double>(ReadUint64(trimmed));
+      return buffer_d / exact_powers_of_ten[-exponent];
+    }
+    if (0 <= exponent && exponent < kExactPowersOfTenSize) {
+      // 10^exponent fits into a double.
+      double buffer_d = static_cast<double>(ReadUint64(trimmed));
+      return buffer_d * exact_powers_of_ten[exponent];
+    }
+    int remaining_digits =
+        kMaxExactDoubleIntegerDecimalDigits - trimmed.length();
+    if ((0 <= exponent) &&
+        (exponent - remaining_digits < kExactPowersOfTenSize)) {
+      // The trimmed string was short and we can multiply it with
+      // 10^remaining_digits. As a result the remaining exponent now fits
+      // into a double too.
+      double buffer_d = static_cast<double>(ReadUint64(trimmed));
+      buffer_d *= exact_powers_of_ten[remaining_digits];
+      return buffer_d * exact_powers_of_ten[exponent - remaining_digits];
+    }
+  }
+  return old_strtod(trimmed, exponent);
+}
+
 } }  // namespace v8::internal