Strtod fast-case that uses DiyFps and cached powers of ten.

src/double.h

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 27 matching lines @@
 static double uint64_to_double(uint64_t d64) { return BitCast<double>(d64); }
 
 // Helper functions for doubles.
 class Double {
  public:
   static const uint64_t kSignMask = V8_2PART_UINT64_C(0x80000000, 00000000);
   static const uint64_t kExponentMask = V8_2PART_UINT64_C(0x7FF00000, 00000000);
   static const uint64_t kSignificandMask =
       V8_2PART_UINT64_C(0x000FFFFF, FFFFFFFF);
   static const uint64_t kHiddenBit = V8_2PART_UINT64_C(0x00100000, 00000000);
+  static const int kPhysicalSignificandSize = 52;  // Excludes the hidden bit.
+  static const int kSignificandSize = 53;
 
   Double() : d64_(0) {}
   explicit Double(double d) : d64_(double_to_uint64(d)) {}
   explicit Double(uint64_t d64) : d64_(d64) {}
+  Double(uint64_t significand, int exponent)
+    : d64_(SignificandExponentToUint64(significand, exponent)) {}
 
   DiyFp AsDiyFp() const {
     ASSERT(!IsSpecial());
     return DiyFp(Significand(), Exponent());
   }
 
   // this->Significand() must not be 0.
   DiyFp AsNormalizedDiyFp() const {
     uint64_t f = Significand();
     int e = Exponent();
 
     ASSERT(f != 0);
 
     // The current double could be a denormal.
     while ((f & kHiddenBit) == 0) {
       f <<= 1;
       e--;
     }
-    // Do the final shifts in one go. Don't forget the hidden bit (the '-1').
-    f <<= DiyFp::kSignificandSize - kSignificandSize - 1;
-    e -= DiyFp::kSignificandSize - kSignificandSize - 1;
+    // Do the final shifts in one go.
+    f <<= DiyFp::kSignificandSize - kSignificandSize;
+    e -= DiyFp::kSignificandSize - kSignificandSize;
     return DiyFp(f, e);
   }
 
   // Returns the double's bit as uint64.
   uint64_t AsUint64() const {
     return d64_;
   }
 
   int Exponent() const {
     if (IsDenormal()) return kDenormalExponent;
 
     uint64_t d64 = AsUint64();
-    int biased_e = static_cast<int>((d64 & kExponentMask) >> kSignificandSize);
+    int biased_e =
+        static_cast<int>((d64 & kExponentMask) >> kPhysicalSignificandSize);
     return biased_e - kExponentBias;
   }
 
   uint64_t Significand() const {
     uint64_t d64 = AsUint64();
     uint64_t significand = d64 & kSignificandMask;
     if (!IsDenormal()) {
       return significand + kHiddenBit;
     } else {
       return significand;
@@ skipping 53 matching lines @@
       m_minus = DiyFp((v.f() << 1) - 1, v.e() - 1);
     }
     m_minus.set_f(m_minus.f() << (m_minus.e() - m_plus.e()));
     m_minus.set_e(m_plus.e());
     *out_m_plus = m_plus;
     *out_m_minus = m_minus;
   }
 
   double value() const { return uint64_to_double(d64_); }
 
+  // Returns the significand size for a given order of magnitude.
+  // If v = f*2^e with 2^p-1 <= f <= 2^p then p+e is v's order of magnitude.
+  // This function returns the number of significant binary digits v will have
+  // once its encoded into a double. In almost all cases this is equal to
+  // kSignificandSize. The only exception are denormals. They start with leading
+  // zeroes and their effective significand-size is hence smaller.
+  static int SignificandSizeForOrderOfMagnitude(int order) {
+    if (order >= (kDenormalExponent + kSignificandSize)) {
+      return kSignificandSize;
+    }
+    if (order <= kDenormalExponent) return 0;
+    return order - kDenormalExponent;
+  }
+
  private:
-  static const int kSignificandSize = 52;  // Excludes the hidden bit.
-  static const int kExponentBias = 0x3FF + kSignificandSize;
+  static const int kExponentBias = 0x3FF + kPhysicalSignificandSize;
   static const int kDenormalExponent = -kExponentBias + 1;
+  static const int kMaxExponent = 0x7FF - kExponentBias;
+  static const uint64_t kInfinity = V8_2PART_UINT64_C(0x7FF00000, 00000000);
 
-  uint64_t d64_;
+  const uint64_t d64_;
+
+  static uint64_t SignificandExponentToUint64(uint64_t significand,
+                                              int exponent) {
+    ASSERT(significand <= kSignificandMask + kHiddenBit);
+    ASSERT(((significand & kHiddenBit) != 0) || exponent <= kDenormalExponent);
+    // Clamp.
+    if (exponent < kDenormalExponent) {
+      return 0;
+    }
+    if (exponent >= kMaxExponent) {
+      return kInfinity;
+    }
+    uint64_t biased_exponent;
+    if (exponent == kDenormalExponent && (significand & kHiddenBit) == 0) {
+      biased_exponent = 0;
+    } else {
+      biased_exponent = static_cast<uint64_t>(exponent + kExponentBias);
+    }
+    return (significand & kSignificandMask) |
+        (biased_exponent << kPhysicalSignificandSize);
+  }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_DOUBLE_H_