From: Austin Clements <austin@google.com>
Date: Thu, 21 Jan 2016 18:44:16 +0000 (-0500)
Subject: runtime: eliminate a conditional branch from heapBits.bits
X-Git-Tag: go1.7beta1~1709
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=4eb33f6b8d6d1de70d7c08ab1258dd2852ecebe4;p=gostls13.git

runtime: eliminate a conditional branch from heapBits.bits

Change-Id: I1fa5e629b2890a8509559ce4ea17b74f47d71925
Reviewed-on: https://go-review.googlesource.com/19637
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
---

diff --git a/src/runtime/mbitmap.go b/src/runtime/mbitmap.go
index 04992890a1..154fc3e0f3 100644
--- a/src/runtime/mbitmap.go
+++ b/src/runtime/mbitmap.go
@@ -290,7 +290,9 @@ func (h heapBits) forward(n uintptr) heapBits {
 // The result includes in its higher bits the bits for subsequent words
 // described by the same bitmap byte.
 func (h heapBits) bits() uint32 {
-	return uint32(*h.bitp) >> h.shift
+	// The (shift & 31) eliminates a test and conditional branch
+	// from the generated code.
+	return uint32(*h.bitp) >> (h.shift & 31)
 }
 
 // isMarked reports whether the heap bits have the marked bit set.