From ccd992355df7192993c666236047820244914598 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Tue, 16 Apr 2024 21:19:13 +0200 Subject: Adding upstream version 1.21.8. Signed-off-by: Daniel Baumann --- src/runtime/fastlog2.go | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+) create mode 100644 src/runtime/fastlog2.go (limited to 'src/runtime/fastlog2.go') diff --git a/src/runtime/fastlog2.go b/src/runtime/fastlog2.go new file mode 100644 index 0000000..1f251bf --- /dev/null +++ b/src/runtime/fastlog2.go @@ -0,0 +1,27 @@ +// 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 + +// 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 dependencies 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 +} -- cgit v1.2.3