buffer: optimize readDouble and readFloat methods

benchmark/buffers/buffer-read-float.js

@@ -0,0 +1,46 @@
+'use strict';
+const common = require('../common.js');
+
+const bench = common.createBenchmark(main, {
+  noAssert: ['false', 'true'],
+  type: ['Double', 'Float'],
+  endian: ['BE', 'LE'],
+  value: ['zero', 'big', 'small', 'inf', 'nan'],
+  millions: [1]
+});
+
+function main(conf) {
+  const noAssert = conf.noAssert === 'true';
+  const len = +conf.millions * 1e6;
+  const buff = Buffer.alloc(8);
+  const type = conf.type || 'Double';
+  const endian = conf.endian;
+  const fn = `read${type}${endian}`;
+  const values = {
+    Double: {
+      zero: 0,
+      big: 2 ** 1023,
+      small: 2 ** -1074,
+      inf: Infinity,
+      nan: NaN,
+    },
+    Float: {
+      zero: 0,
+      big: 2 ** 127,
+      small: 2 ** -149,
+      inf: Infinity,
+      nan: NaN,
+    },
+  };
+  const value = values[type][conf.value];
+
+  buff[`write${type}${endian}`](value, 0, noAssert);
+  const testFunction = new Function('buff', `
+    for (var i = 0; i !== ${len}; i++) {
+      buff.${fn}(0, ${JSON.stringify(noAssert)});
+    }
+  `);
+  bench.start();
+  testFunction(buff);
+  bench.end(len / 1e6);
+}

lib/buffer.js

@@ -31,10 +31,6 @@ const {
   indexOfBuffer,
   indexOfNumber,
   indexOfString,
-  readDoubleBE: _readDoubleBE,
-  readDoubleLE: _readDoubleLE,
-  readFloatBE: _readFloatBE,
-  readFloatLE: _readFloatLE,
   swap16: _swap16,
   swap32: _swap32,
   swap64: _swap64,
@@ -1201,35 +1197,80 @@ Buffer.prototype.readInt32BE = function readInt32BE(offset, noAssert) {
 };
 
 
-Buffer.prototype.readFloatLE = function readFloatLE(offset, noAssert) {
-  offset = offset >>> 0;
-  if (!noAssert)
-    checkOffset(offset, 4, this.length);
-  return _readFloatLE(this, offset);
+// For the casual reader who has not at the current time memorized the
+// IEEE-754 standard in full detail: floating point numbers consist of
+// a fraction, an exponent and a sign bit: 23+8+1 bits for single precision
+// numbers and 52+11+1 bits for double precision numbers.
+//
+// A zero exponent is either a positive or negative zero, if the fraction
+// is zero, or a denormalized number when it is non-zero.  Multiplying the
+// fraction by the smallest possible denormal yields the denormalized number.
+//
+// An all-bits-one exponent is either a positive or negative infinity, if
+// the fraction is zero, or NaN when it is non-zero.  The standard allows
+// both quiet and signalling NaNs but since NaN is a canonical value in
+// JavaScript, we cannot (and do not) distinguish between the two.
+//
+// Other exponents are regular numbers and are computed by subtracting the bias
+// from the exponent (127 for single precision, 1023 for double precision),
+// yielding an exponent in the ranges -126-127 and -1022-1024 respectively.
+//
+// Of interest is that the fraction of a normal number has an extra bit of
+// precision that is not stored but is reconstructed by adding one after
+// multiplying the fraction with the result of 2**-bits_in_fraction.
+
+
+function toDouble(x0, x1) {
+  const frac = x0 + 0x100000000 * (x1 & 0xFFFFF);
+  const expt = (x1 >>> 20) & 2047;
+  const sign = (x1 >>> 31) ? -1 : 1;
+  if (expt === 0) {
+    if (frac === 0) return sign * 0;
+    return sign * frac * 2 ** -1074;
+  } else if (expt === 2047) {
+    if (frac === 0) return sign * Infinity;
+    return NaN;
+  }
+  return sign * 2 ** (expt - 1023) * (1 + frac * 2 ** -52);
+}
+
+
+function toFloat(x) {
+  const frac = x & 0x7FFFFF;
+  const expt = (x >>> 23) & 255;
+  const sign = (x >>> 31) ? -1 : 1;
+  if (expt === 0) {
+    if (frac === 0) return sign * 0;
+    return sign * frac * 2 ** -149;
+  } else if (expt === 255) {
+    if (frac === 0) return sign * Infinity;
+    return NaN;
+  }
+  return sign * 2 ** (expt - 127) * (1 + frac * 2 ** -23);
+}
+
+
+Buffer.prototype.readDoubleBE = function(offset, noAssert) {
+  const x1 = this.readUInt32BE(offset + 0, noAssert);
+  const x0 = this.readUInt32BE(offset + 4, noAssert);
+  return toDouble(x0, x1);
 };
 
 
-Buffer.prototype.readFloatBE = function readFloatBE(offset, noAssert) {
-  offset = offset >>> 0;
-  if (!noAssert)
-    checkOffset(offset, 4, this.length);
-  return _readFloatBE(this, offset);
+Buffer.prototype.readDoubleLE = function(offset, noAssert) {
+  const x0 = this.readUInt32LE(offset + 0, noAssert);
+  const x1 = this.readUInt32LE(offset + 4, noAssert);
+  return toDouble(x0, x1);
 };
 
 
-Buffer.prototype.readDoubleLE = function readDoubleLE(offset, noAssert) {
-  offset = offset >>> 0;
-  if (!noAssert)
-    checkOffset(offset, 8, this.length);
-  return _readDoubleLE(this, offset);
+Buffer.prototype.readFloatBE = function(offset, noAssert) {
+  return toFloat(this.readUInt32BE(offset, noAssert));
 };
 
 
-Buffer.prototype.readDoubleBE = function readDoubleBE(offset, noAssert) {
-  offset = offset >>> 0;
-  if (!noAssert)
-    checkOffset(offset, 8, this.length);
-  return _readDoubleBE(this, offset);
+Buffer.prototype.readFloatLE = function(offset, noAssert) {
+  return toFloat(this.readUInt32LE(offset, noAssert));
 };
 
 

src/node_buffer.cc

@@ -719,49 +719,6 @@ static inline void Swizzle(char* start, unsigned int len) {
 }
 
 
-template <typename T, enum Endianness endianness>
-void ReadFloatGeneric(const FunctionCallbackInfo<Value>& args) {
-  THROW_AND_RETURN_UNLESS_BUFFER(Environment::GetCurrent(args), args[0]);
-  SPREAD_BUFFER_ARG(args[0], ts_obj);
-
-  uint32_t offset = args[1]->Uint32Value();
-  CHECK_LE(offset + sizeof(T), ts_obj_length);
-
-  union NoAlias {
-    T val;
-    char bytes[sizeof(T)];
-  };
-
-  union NoAlias na;
-  const char* ptr = static_cast<const char*>(ts_obj_data) + offset;
-  memcpy(na.bytes, ptr, sizeof(na.bytes));
-  if (endianness != GetEndianness())
-    Swizzle(na.bytes, sizeof(na.bytes));
-
-  args.GetReturnValue().Set(na.val);
-}
-
-
-void ReadFloatLE(const FunctionCallbackInfo<Value>& args) {
-  ReadFloatGeneric<float, kLittleEndian>(args);
-}
-
-
-void ReadFloatBE(const FunctionCallbackInfo<Value>& args) {
-  ReadFloatGeneric<float, kBigEndian>(args);
-}
-
-
-void ReadDoubleLE(const FunctionCallbackInfo<Value>& args) {
-  ReadFloatGeneric<double, kLittleEndian>(args);
-}
-
-
-void ReadDoubleBE(const FunctionCallbackInfo<Value>& args) {
-  ReadFloatGeneric<double, kBigEndian>(args);
-}
-
-
 template <typename T, enum Endianness endianness>
 void WriteFloatGeneric(const FunctionCallbackInfo<Value>& args) {
   Environment* env = Environment::GetCurrent(args);
@@ -1276,11 +1233,6 @@ void Initialize(Local<Object> target,
   env->SetMethod(target, "indexOfNumber", IndexOfNumber);
   env->SetMethod(target, "indexOfString", IndexOfString);
 
-  env->SetMethod(target, "readDoubleBE", ReadDoubleBE);
-  env->SetMethod(target, "readDoubleLE", ReadDoubleLE);
-  env->SetMethod(target, "readFloatBE", ReadFloatBE);
-  env->SetMethod(target, "readFloatLE", ReadFloatLE);
-
   env->SetMethod(target, "writeDoubleBE", WriteDoubleBE);
   env->SetMethod(target, "writeDoubleLE", WriteDoubleLE);
   env->SetMethod(target, "writeFloatBE", WriteFloatBE);