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Unified Diff: src/double.h

Issue 3898007: Strtod fast-case that uses DiyFps and cached powers of ten. (Closed)
Patch Set: Strtod fast-case that uses DiyFps and cached powers of ten.... Created 10 years, 2 months ago
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Index: src/double.h
===================================================================
--- src/double.h (revision 5685)
+++ src/double.h (working copy)
@@ -45,10 +45,14 @@
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) {}
+ explicit Double(DiyFp diy_fp)
+ : d64_(DiyFpToUint64(diy_fp)) {}
DiyFp AsDiyFp() const {
ASSERT(!IsSpecial());
@@ -67,9 +71,9 @@
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);
}
@@ -82,7 +86,8 @@
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;
}
@@ -156,12 +161,54 @@
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 DiyFpToUint64(DiyFp diy_fp) {
+ uint64_t significand = diy_fp.f();
+ int exponent = diy_fp.e();
+ while (significand > kHiddenBit + kSignificandMask) {
+ significand >>= 1;
+ exponent++;
+ }
+ if (exponent >= kMaxExponent) {
+ return kInfinity;
+ }
+ if (exponent < kDenormalExponent) {
+ return 0;
+ }
+ while (exponent > kDenormalExponent && (significand & kHiddenBit) == 0) {
+ significand <<= 1;
+ exponent--;
+ }
+ 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
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