libgo: Update to go1.6rc1.

    
    Reviewed-on: https://go-review.googlesource.com/19200

From-SVN: r233110

1 files changed, 33 insertions, 0 deletions

diff --git a/libgo/go/runtime/fastlog2.go b/libgo/go/runtime/fastlog2.go
new file mode 100644
index 0000000..b22e825
--- /dev/null
+++ b/libgo/go/runtime/fastlog2.go
@@ -0,0 +1,33 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime
+
+import "unsafe"
+
+// fastlog2 implements a fast approximation to the base 2 log of a
+// float64. This is used to compute a geometric distribution for heap
+// sampling, without introducing dependences into package math. This
+// uses a very rough approximation using the float64 exponent and the
+// first 25 bits of the mantissa. The top 5 bits of the mantissa are
+// used to load limits from a table of constants and the rest are used
+// to scale linearly between them.
+func fastlog2(x float64) float64 {
+	const fastlogScaleBits = 20
+	const fastlogScaleRatio = 1.0 / (1 << fastlogScaleBits)
+
+	xBits := float64bits(x)
+	// Extract the exponent from the IEEE float64, and index a constant
+	// table with the first 10 bits from the mantissa.
+	xExp := int64((xBits>>52)&0x7FF) - 1023
+	xManIndex := (xBits >> (52 - fastlogNumBits)) % (1 << fastlogNumBits)
+	xManScale := (xBits >> (52 - fastlogNumBits - fastlogScaleBits)) % (1 << fastlogScaleBits)
+
+	low, high := fastlog2Table[xManIndex], fastlog2Table[xManIndex+1]
+	return float64(xExp) + low + (high-low)*float64(xManScale)*fastlogScaleRatio
+}
+
+// float64bits returns the IEEE 754 binary representation of f.
+// Taken from math.Float64bits to avoid dependences into package math.
+func float64bits(f float64) uint64 { return *(*uint64)(unsafe.Pointer(&f)) }