[release-branch.go1.8] all: merge master into release-branch.go1.8

6593d8650d go/ast: fix Object's doc comment about Data
c1730ae424 runtime: force workers out before checking mark roots
d10eddcba3 testing: make parallel t.Run safe again
2c8b70eacf crypto/x509: revert SystemCertPool implementation for Windows
fcfd91858b doc/go1.8: document Plan 9 requirements
81a61a96c9 runtime: for plugins, don't add duplicate itabs
f674537cc9 README.md: update and simplify
d8711919db cmd/go: fix bug help message
48d8edb5b2 crypto/tls: disable CBC cipher suites with SHA-256 by default
92ecd78933 cmd/compile: add ZeroWB case in writebarrier
787125abab doc: 2017 is the Year of the Gopher
5b708a6b6a cmd/compile: lvalues are only required for == when calling runtime fns
e83d506714 vendor/golang_org/x/crypto/poly1305: revendor to pick up fix for #18673
76f981c8d8 net/http: skip TestServerHijackGetsBackgroundByte on Plan 9
e395e3246a net/http: skip TestServerHijackGetsBackgroundByte_big on Plan 9
6a3c6c0de8 net/http: add another hijack-after-background-read test
467109bf56 all: test adjustments for the iOS builder
b2a3b54b95 net/http: make sure Hijack's bufio.Reader includes pre-read background byte
593ea3b360 cmd/go, misc: rework cwd handling for iOS tests
0642b8a2f1 syscall: export Fsid.X__val on s390x
4601eae6ba doc/gdb: mention GOTRACEBACK=crash
4c4c5fc7a3 misc/cgo/testplugin: test that types and itabs are unique
22689c4450 reflect: keep makeFuncImpl live across makeFuncStub
9cf06ed6cd cmd/link: only exclude C-only symbols on darwin
9c3630f578 compress/flate: avoid large stack growth in fillDeflate
4f0aac52d9 cmd/go: add comment about SIGUSR2 on iOS
333f764df3 cmd/go, misc: switch from breakpoint to SIGUSR2
39e31d5ec0 doc/go1.8: update timezone database version
08da8201ca misc/cgo/testshared: test that types and itabs are unique
fdde7ba2a2 runtime: avoid clobbering C callee-save register in cgoSigtramp
f65abf6ddc cmd/compile: hide testdclstack behind debug flag
641ef2a733 compress/gzip: skip TestGZIPFilesHaveZeroMTimes on non-builders
0724aa813f crypto/dsa: gofmt
ac05542985 net/http: deflake TestRetryIdempotentRequestsOnError
b842c9aac7 doc: remove inline styles

Change-Id: I642c056732fe1e8081e9d73e086e38ea0b2568cc

60 files changed, 855 insertions, 192 deletions

diff --git a/README.md b/README.md
index 281deecdfb..672cdf5568 100644
--- a/README.md
+++ b/README.md
@@ -5,39 +5,37 @@ reliable, and efficient software.
 
 ![Gopher image](doc/gopher/fiveyears.jpg)
 
-For documentation about how to install and use Go,
-visit https://golang.org/ or load doc/install-source.html
-in your web browser.
-
 Our canonical Git repository is located at https://go.googlesource.com/go.
 There is a mirror of the repository at https://github.com/golang/go.
 
-Go is the work of hundreds of contributors. We appreciate your help!
+Unless otherwise noted, the Go source files are distributed under the
+BSD-style license found in the LICENSE file.
 
-To contribute, please read the contribution guidelines:
-	https://golang.org/doc/contribute.html
+### Download and Install
 
-##### Note that we do not accept pull requests and that we use the issue tracker for bug reports and proposals only. Please ask questions on https://forum.golangbridge.org or https://groups.google.com/forum/#!forum/golang-nuts.
+#### Binary Distributions
 
-Unless otherwise noted, the Go source files are distributed
-under the BSD-style license found in the LICENSE file.
+Official binary distributions are available at https://golang.org/dl/.
 
---
+After downloading a binary release, visit https://golang.org/doc/install
+or load doc/install.html in your web browser for installation
+instructions.
 
-## Binary Distribution Notes
+#### Install From Source
 
-If you have just untarred a binary Go distribution, you need to set
-the environment variable $GOROOT to the full path of the go
-directory (the one containing this file).  You can omit the
-variable if you unpack it into /usr/local/go, or if you rebuild
-from sources by running all.bash (see doc/install-source.html).
-You should also add the Go binary directory $GOROOT/bin
-to your shell's path.
+If a binary distribution is not available for your combination of
+operating system and architecture, visit
+https://golang.org/doc/install/source or load doc/install-source.html
+in your web browser for source installation instructions.
 
-For example, if you extracted the tar file into $HOME/go, you might
-put the following in your .profile:
+### Contributing
 
-	export GOROOT=$HOME/go
-	export PATH=$PATH:$GOROOT/bin
+Go is the work of hundreds of contributors. We appreciate your help!
+
+To contribute, please read the contribution guidelines:
+	https://golang.org/doc/contribute.html
 
-See https://golang.org/doc/install or doc/install.html for more details.
+Note that the Go project does not use GitHub pull requests, and that
+we use the issue tracker for bug reports and proposals only. See
+https://golang.org/wiki/Questions for a list of places to ask
+questions about the Go language.
diff --git a/doc/contribute.html b/doc/contribute.html
index bcf7b25c51..f1a5b27998 100644
--- a/doc/contribute.html
+++ b/doc/contribute.html
@@ -698,7 +698,7 @@ These files will be periodically updated based on the commit logs.
 <p>Code that you contribute should use the standard copyright header:</p>
 
 <pre>
-// Copyright 2016 The Go Authors. All rights reserved.
+// Copyright 2017 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.
 </pre>
diff --git a/doc/debugging_with_gdb.html b/doc/debugging_with_gdb.html
index 52a6e76723..f0e65ea291 100644
--- a/doc/debugging_with_gdb.html
+++ b/doc/debugging_with_gdb.html
@@ -4,7 +4,8 @@
 }-->
 
 <p><i>
-This applies to the <code>gc</code> toolchain. Gccgo has native gdb support.
+This applies to the standard toolchain (the <code>gc</code> Go
+compiler and tools). Gccgo has native gdb support.
 Besides this overview you might want to consult the
 <a href="http://sourceware.org/gdb/current/onlinedocs/gdb/">GDB manual</a>.
 </i></p>
@@ -49,6 +50,14 @@ when debugging, pass the flags <code>-gcflags "-N -l"</code> to the
 debugged.
 </p>
 
+<p>
+If you want to use gdb to inspect a core dump, you can trigger a dump
+on a program crash, on systems that permit it, by setting
+<code>GOTRACEBACK=crash</code> in the environment (see the
+<a href="/pkg/runtime/#hdr-Environment_Variables"> runtime package
+documentation</a> for more info).
+</p>
+
 <h3 id="Common_Operations">Common Operations</h3>
 
 <ul>
@@ -130,7 +139,7 @@ the DWARF code.
 
 <p>
 If you're interested in what the debugging information looks like, run
-'<code>objdump -W 6.out</code>' and browse through the <code>.debug_*</code>
+'<code>objdump -W a.out</code>' and browse through the <code>.debug_*</code>
 sections.
 </p>
 
@@ -377,7 +386,9 @@ $3 = struct hchan&lt;*testing.T&gt;
 </pre>
 
 <p>
-That <code>struct hchan&lt;*testing.T&gt;</code> is the runtime-internal representation of a channel.  It is currently empty, or gdb would have pretty-printed it's contents.
+That <code>struct hchan&lt;*testing.T&gt;</code> is the
+runtime-internal representation of a channel. It is currently empty,
+or gdb would have pretty-printed its contents.
 </p>
 
 <p>
diff --git a/doc/go1.8.html b/doc/go1.8.html
index 2ac478632e..337f13d630 100644
--- a/doc/go1.8.html
+++ b/doc/go1.8.html
@@ -93,7 +93,8 @@ On OpenBSD, Go now requires OpenBSD 5.9 or later. <!-- CL 34093 -->
 <p>
 The Plan 9 port's networking support is now much more complete
 and matches the behavior of Unix and Windows with respect to deadlines
-and cancelation.
+and cancelation. For Plan 9 kernel requirements, see the
+<a href="https://golang.org/wiki/Plan9">Plan 9 wiki page</a>.
 </p>
 
 <p>
@@ -808,11 +809,6 @@ Optimizations and minor bug fixes are not listed.
 
 <dl id="crypto_x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
   <dd>
-    <p> <!-- CL 30578 -->
-      <a href="/pkg/crypto/x509/#SystemCertPool"><code>SystemCertPool</code></a>
-      is now implemented on Windows.
-    </p>
-
     <p> <!-- CL 24743 -->
       PSS signatures are now supported.
     </p>
@@ -1617,9 +1613,9 @@ crypto/x509: return error for missing SerialNumber (CL 27238)
       June 31 and July 32.
     </p>
 
-    <p> <!-- CL 33029 -->
+    <p> <!-- CL 33029 --> <!-- CL 34816 -->
       The <code>tzdata</code> database has been updated to version
-      2016i for systems that don't already have a local time zone
+      2016j for systems that don't already have a local time zone
       database.
     </p>
 
diff --git a/doc/install.html b/doc/install.html
index 2143d591cb..6bff75c5a0 100644
--- a/doc/install.html
+++ b/doc/install.html
@@ -250,7 +250,7 @@ $ <b>cd $HOME/go/src/hello</b>
 $ <b>go build</b>
 </pre>
 
-<pre class="testWindows" style="display: none">
+<pre class="testWindows">
 C:\&gt; <b>cd %USERPROFILE%\go\src\hello</b>
 C:\Users\Gopher\go\src\hello&gt; <b>go build</b>
 </pre>
@@ -267,7 +267,7 @@ $ <b>./hello</b>
 hello, world
 </pre>
 
-<pre class="testWindows" style="display: none">
+<pre class="testWindows">
 C:\Users\Gopher\go\src\hello&gt; <b>hello</b>
 hello, world
 </pre>
diff --git a/misc/cgo/testplugin/src/iface/main.go b/misc/cgo/testplugin/src/iface/main.go
new file mode 100644
index 0000000000..5e7e4d8b48
--- /dev/null
+++ b/misc/cgo/testplugin/src/iface/main.go
@@ -0,0 +1,46 @@
+// Copyright 2017 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 main
+
+import (
+	"iface_i"
+	"log"
+	"plugin"
+)
+
+func main() {
+	a, err := plugin.Open("iface_a.so")
+	if err != nil {
+		log.Fatalf(`plugin.Open("iface_a.so"): %v`, err)
+	}
+	b, err := plugin.Open("iface_b.so")
+	if err != nil {
+		log.Fatalf(`plugin.Open("iface_b.so"): %v`, err)
+	}
+
+	af, err := a.Lookup("F")
+	if err != nil {
+		log.Fatalf(`a.Lookup("F") failed: %v`, err)
+	}
+	bf, err := b.Lookup("F")
+	if err != nil {
+		log.Fatalf(`b.Lookup("F") failed: %v`, err)
+	}
+	if af.(func() interface{})() != bf.(func() interface{})() {
+		panic("empty interfaces not equal")
+	}
+
+	ag, err := a.Lookup("G")
+	if err != nil {
+		log.Fatalf(`a.Lookup("G") failed: %v`, err)
+	}
+	bg, err := b.Lookup("G")
+	if err != nil {
+		log.Fatalf(`b.Lookup("G") failed: %v`, err)
+	}
+	if ag.(func() iface_i.I)() != bg.(func() iface_i.I)() {
+		panic("nonempty interfaces not equal")
+	}
+}
diff --git a/misc/cgo/testplugin/src/iface_a/a.go b/misc/cgo/testplugin/src/iface_a/a.go
new file mode 100644
index 0000000000..29d2e27764
--- /dev/null
+++ b/misc/cgo/testplugin/src/iface_a/a.go
@@ -0,0 +1,17 @@
+// Copyright 2017 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 main
+
+import "iface_i"
+
+//go:noinline
+func F() interface{} {
+	return (*iface_i.T)(nil)
+}
+
+//go:noinline
+func G() iface_i.I {
+	return (*iface_i.T)(nil)
+}
diff --git a/misc/cgo/testplugin/src/iface_b/b.go b/misc/cgo/testplugin/src/iface_b/b.go
new file mode 100644
index 0000000000..29d2e27764
--- /dev/null
+++ b/misc/cgo/testplugin/src/iface_b/b.go
@@ -0,0 +1,17 @@
+// Copyright 2017 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 main
+
+import "iface_i"
+
+//go:noinline
+func F() interface{} {
+	return (*iface_i.T)(nil)
+}
+
+//go:noinline
+func G() iface_i.I {
+	return (*iface_i.T)(nil)
+}
diff --git a/misc/cgo/testplugin/src/iface_i/i.go b/misc/cgo/testplugin/src/iface_i/i.go
new file mode 100644
index 0000000000..31c80387c7
--- /dev/null
+++ b/misc/cgo/testplugin/src/iface_i/i.go
@@ -0,0 +1,17 @@
+// Copyright 2017 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 iface_i
+
+type I interface {
+	M()
+}
+
+type T struct {
+}
+
+func (t *T) M() {
+}
+
+// *T implements I
diff --git a/misc/cgo/testplugin/src/issue18676/dynamodbstreamsevt/definition.go b/misc/cgo/testplugin/src/issue18676/dynamodbstreamsevt/definition.go
new file mode 100644
index 0000000000..70fd054d08
--- /dev/null
+++ b/misc/cgo/testplugin/src/issue18676/dynamodbstreamsevt/definition.go
@@ -0,0 +1,13 @@
+// Copyright 2017 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 dynamodbstreamsevt
+
+import "encoding/json"
+
+var foo json.RawMessage
+
+type Event struct{}
+
+func (e *Event) Dummy() {}
diff --git a/misc/cgo/testplugin/src/issue18676/main.go b/misc/cgo/testplugin/src/issue18676/main.go
new file mode 100644
index 0000000000..c75409dafe
--- /dev/null
+++ b/misc/cgo/testplugin/src/issue18676/main.go
@@ -0,0 +1,31 @@
+// Copyright 2017 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.
+
+// The bug happened like this:
+// 1) The main binary adds an itab for *json.UnsupportedValueError / error
+//    (concrete type / interface type).  This itab goes in hash bucket 0x111.
+// 2) The plugin adds that same itab again.  That makes a cycle in the itab
+//    chain rooted at hash bucket 0x111.
+// 3) The main binary then asks for the itab for *dynamodbstreamsevt.Event /
+//    json.Unmarshaler.  This itab happens to also live in bucket 0x111.
+//    The lookup code goes into an infinite loop searching for this itab.
+// The code is carefully crafted so that the two itabs are both from the
+// same bucket, and so that the second itab doesn't exist in
+// the itab hashmap yet (so the entire linked list must be searched).
+package main
+
+import (
+	"encoding/json"
+	"issue18676/dynamodbstreamsevt"
+	"plugin"
+)
+
+func main() {
+	plugin.Open("plugin.so")
+
+	var x interface{} = (*dynamodbstreamsevt.Event)(nil)
+	if _, ok := x.(json.Unmarshaler); !ok {
+		println("something")
+	}
+}
diff --git a/misc/cgo/testplugin/src/issue18676/plugin.go b/misc/cgo/testplugin/src/issue18676/plugin.go
new file mode 100644
index 0000000000..8a3b85a75c
--- /dev/null
+++ b/misc/cgo/testplugin/src/issue18676/plugin.go
@@ -0,0 +1,11 @@
+// Copyright 2017 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 main
+
+import "C"
+
+import "issue18676/dynamodbstreamsevt"
+
+func F(evt *dynamodbstreamsevt.Event) {}
diff --git a/misc/cgo/testplugin/test.bash b/misc/cgo/testplugin/test.bash
index fee99a758c..ab7430acc3 100755
--- a/misc/cgo/testplugin/test.bash
+++ b/misc/cgo/testplugin/test.bash
@@ -15,8 +15,8 @@ goos=$(go env GOOS)
 goarch=$(go env GOARCH)
 
 function cleanup() {
-	rm -f plugin*.so unnamed*.so
-	rm -rf host pkg sub
+	rm -f plugin*.so unnamed*.so iface*.so
+	rm -rf host pkg sub iface issue18676
 }
 trap cleanup EXIT
 
@@ -32,3 +32,15 @@ GOPATH=$(pwd) go build -buildmode=plugin unnamed2.go
 GOPATH=$(pwd) go build host
 
 LD_LIBRARY_PATH=$(pwd) ./host
+
+# Test that types and itabs get properly uniqified.
+GOPATH=$(pwd) go build -buildmode=plugin iface_a
+GOPATH=$(pwd) go build -buildmode=plugin iface_b
+GOPATH=$(pwd) go build iface
+LD_LIBRARY_PATH=$(pwd) ./iface
+
+# Test for issue 18676 - make sure we don't add the same itab twice.
+# The buggy code hangs forever, so use a timeout to check for that.
+GOPATH=$(pwd) go build -buildmode=plugin -o plugin.so src/issue18676/plugin.go
+GOPATH=$(pwd) go build -o issue18676 src/issue18676/main.go
+timeout 10s ./issue18676
diff --git a/misc/cgo/testshared/shared_test.go b/misc/cgo/testshared/shared_test.go
index af4f91550f..f0766e511e 100644
--- a/misc/cgo/testshared/shared_test.go
+++ b/misc/cgo/testshared/shared_test.go
@@ -815,3 +815,14 @@ func TestImplicitInclusion(t *testing.T) {
 	goCmd(t, "install", "-linkshared", "implicitcmd")
 	run(t, "running executable linked against library that contains same package as it", "./bin/implicitcmd")
 }
+
+// Tests to make sure that the type fields of empty interfaces and itab
+// fields of nonempty interfaces are unique even across modules,
+// so that interface equality works correctly.
+func TestInterface(t *testing.T) {
+	goCmd(t, "install", "-buildmode=shared", "-linkshared", "iface_a")
+	// Note: iface_i gets installed implicitly as a dependency of iface_a.
+	goCmd(t, "install", "-buildmode=shared", "-linkshared", "iface_b")
+	goCmd(t, "install", "-linkshared", "iface")
+	run(t, "running type/itab uniqueness tester", "./bin/iface")
+}
diff --git a/misc/cgo/testshared/src/iface/main.go b/misc/cgo/testshared/src/iface/main.go
new file mode 100644
index 0000000000..3d5b54e73b
--- /dev/null
+++ b/misc/cgo/testshared/src/iface/main.go
@@ -0,0 +1,17 @@
+// Copyright 2017 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 main
+
+import "iface_a"
+import "iface_b"
+
+func main() {
+	if iface_a.F() != iface_b.F() {
+		panic("empty interfaces not equal")
+	}
+	if iface_a.G() != iface_b.G() {
+		panic("non-empty interfaces not equal")
+	}
+}
diff --git a/misc/cgo/testshared/src/iface_a/a.go b/misc/cgo/testshared/src/iface_a/a.go
new file mode 100644
index 0000000000..e11047c166
--- /dev/null
+++ b/misc/cgo/testshared/src/iface_a/a.go
@@ -0,0 +1,17 @@
+// Copyright 2017 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 iface_a
+
+import "iface_i"
+
+//go:noinline
+func F() interface{} {
+	return (*iface_i.T)(nil)
+}
+
+//go:noinline
+func G() iface_i.I {
+	return (*iface_i.T)(nil)
+}
diff --git a/misc/cgo/testshared/src/iface_b/b.go b/misc/cgo/testshared/src/iface_b/b.go
new file mode 100644
index 0000000000..47aee2e77e
--- /dev/null
+++ b/misc/cgo/testshared/src/iface_b/b.go
@@ -0,0 +1,17 @@
+// Copyright 2017 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 iface_b
+
+import "iface_i"
+
+//go:noinline
+func F() interface{} {
+	return (*iface_i.T)(nil)
+}
+
+//go:noinline
+func G() iface_i.I {
+	return (*iface_i.T)(nil)
+}
diff --git a/misc/cgo/testshared/src/iface_i/i.go b/misc/cgo/testshared/src/iface_i/i.go
new file mode 100644
index 0000000000..31c80387c7
--- /dev/null
+++ b/misc/cgo/testshared/src/iface_i/i.go
@@ -0,0 +1,17 @@
+// Copyright 2017 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 iface_i
+
+type I interface {
+	M()
+}
+
+type T struct {
+}
+
+func (t *T) M() {
+}
+
+// *T implements I
diff --git a/misc/ios/go_darwin_arm_exec.go b/misc/ios/go_darwin_arm_exec.go
index 1eeb289c7d..3de341b9c5 100644
--- a/misc/ios/go_darwin_arm_exec.go
+++ b/misc/ios/go_darwin_arm_exec.go
@@ -99,7 +99,7 @@ func main() {
 	// Approximately 1 in a 100 binaries fail to start. If it happens,
 	// try again. These failures happen for several reasons beyond
 	// our control, but all of them are safe to retry as they happen
-	// before lldb encounters the initial getwd breakpoint. As we
+	// before lldb encounters the initial SIGUSR2 stop. As we
 	// know the tests haven't started, we are not hiding flaky tests
 	// with this retry.
 	for i := 0; i < 5; i++ {
@@ -204,6 +204,11 @@ func run(bin string, args []string) (err error) {
 	var opts options
 	opts, args = parseArgs(args)
 
+	// Pass the suffix for the current working directory as the
+	// first argument to the test. For iOS, cmd/go generates
+	// special handling of this argument.
+	args = append([]string{"cwdSuffix=" + pkgpath}, args...)
+
 	// ios-deploy invokes lldb to give us a shell session with the app.
 	s, err := newSession(appdir, args, opts)
 	if err != nil {
@@ -224,6 +229,7 @@ func run(bin string, args []string) (err error) {
 	s.do(`process handle SIGHUP  --stop false --pass true --notify false`)
 	s.do(`process handle SIGPIPE --stop false --pass true --notify false`)
 	s.do(`process handle SIGUSR1 --stop false --pass true --notify false`)
+	s.do(`process handle SIGUSR2 --stop true --pass false --notify true`) // sent by test harness
 	s.do(`process handle SIGCONT --stop false --pass true --notify false`)
 	s.do(`process handle SIGSEGV --stop false --pass true --notify false`) // does not work
 	s.do(`process handle SIGBUS  --stop false --pass true --notify false`) // does not work
@@ -236,20 +242,9 @@ func run(bin string, args []string) (err error) {
 		return nil
 	}
 
-	s.do(`breakpoint set -n getwd`) // in runtime/cgo/gcc_darwin_arm.go
-
 	started = true
 
-	s.doCmd("run", "stop reason = breakpoint", 20*time.Second)
-
-	// Move the current working directory into the faux gopath.
-	if pkgpath != "src" {
-		s.do(`breakpoint delete 1`)
-		s.do(`expr char* $mem = (char*)malloc(512)`)
-		s.do(`expr $mem = (char*)getwd($mem, 512)`)
-		s.do(`expr $mem = (char*)strcat($mem, "/` + pkgpath + `")`)
-		s.do(`call (void)chdir($mem)`)
-	}
+	s.doCmd("run", "stop reason = signal SIGUSR2", 20*time.Second)
 
 	startTestsLen := s.out.Len()
 	fmt.Fprintln(s.in, `process continue`)
@@ -520,13 +515,11 @@ func copyLocalData(dstbase string) (pkgpath string, err error) {
 
 	// Copy timezone file.
 	//
-	// Typical apps have the zoneinfo.zip in the root of their app bundle,
+	// Apps have the zoneinfo.zip in the root of their app bundle,
 	// read by the time package as the working directory at initialization.
-	// As we move the working directory to the GOROOT pkg directory, we
-	// install the zoneinfo.zip file in the pkgpath.
 	if underGoRoot {
 		err := cp(
-			filepath.Join(dstbase, pkgpath),
+			dstbase,
 			filepath.Join(cwd, "lib", "time", "zoneinfo.zip"),
 		)
 		if err != nil {
diff --git a/src/cmd/compile/internal/gc/alg.go b/src/cmd/compile/internal/gc/alg.go
index 8113710e39..d4f3d9884e 100644
--- a/src/cmd/compile/internal/gc/alg.go
+++ b/src/cmd/compile/internal/gc/alg.go
@@ -303,7 +303,9 @@ func genhash(sym *Sym, t *Type) {
 	typecheckslice(fn.Nbody.Slice(), Etop)
 	Curfn = nil
 	popdcl()
-	testdclstack()
+	if debug_dclstack != 0 {
+		testdclstack()
+	}
 
 	// Disable safemode while compiling this code: the code we
 	// generate internally can refer to unsafe.Pointer.
@@ -493,7 +495,9 @@ func geneq(sym *Sym, t *Type) {
 	typecheckslice(fn.Nbody.Slice(), Etop)
 	Curfn = nil
 	popdcl()
-	testdclstack()
+	if debug_dclstack != 0 {
+		testdclstack()
+	}
 
 	// Disable safemode while compiling this code: the code we
 	// generate internally can refer to unsafe.Pointer.
diff --git a/src/cmd/compile/internal/gc/bimport.go b/src/cmd/compile/internal/gc/bimport.go
index 1d668412a1..94c1184138 100644
--- a/src/cmd/compile/internal/gc/bimport.go
+++ b/src/cmd/compile/internal/gc/bimport.go
@@ -217,7 +217,9 @@ func Import(in *bufio.Reader) {
 	typecheckok = tcok
 	resumecheckwidth()
 
-	testdclstack() // debugging only
+	if debug_dclstack != 0 {
+		testdclstack()
+	}
 }
 
 func formatErrorf(format string, args ...interface{}) {
diff --git a/src/cmd/compile/internal/gc/main.go b/src/cmd/compile/internal/gc/main.go
index 75f58a731c..1690944b3d 100644
--- a/src/cmd/compile/internal/gc/main.go
+++ b/src/cmd/compile/internal/gc/main.go
@@ -30,11 +30,12 @@ var (
 )
 
 var (
-	Debug_append  int
-	Debug_closure int
-	Debug_panic   int
-	Debug_slice   int
-	Debug_wb      int
+	Debug_append   int
+	Debug_closure  int
+	debug_dclstack int
+	Debug_panic    int
+	Debug_slice    int
+	Debug_wb       int
 )
 
 // Debug arguments.
@@ -48,6 +49,7 @@ var debugtab = []struct {
 	{"append", &Debug_append},         // print information about append compilation
 	{"closure", &Debug_closure},       // print information about closure compilation
 	{"disablenil", &disable_checknil}, // disable nil checks
+	{"dclstack", &debug_dclstack},     // run internal dclstack checks
 	{"gcprog", &Debug_gcprog},         // print dump of GC programs
 	{"nil", &Debug_checknil},          // print information about nil checks
 	{"panic", &Debug_panic},           // do not hide any compiler panic
@@ -325,7 +327,6 @@ func Main() {
 	timings.Stop()
 	timings.AddEvent(int64(lexlineno-lexlineno0), "lines")
 
-	testdclstack()
 	mkpackage(localpkg.Name) // final import not used checks
 	finishUniverse()
 
diff --git a/src/cmd/compile/internal/gc/noder.go b/src/cmd/compile/internal/gc/noder.go
index ca99adea27..ce18297ac3 100644
--- a/src/cmd/compile/internal/gc/noder.go
+++ b/src/cmd/compile/internal/gc/noder.go
@@ -34,6 +34,7 @@ func parseFile(filename string) {
 	}
 
 	if nsyntaxerrors == 0 {
+		// Always run testdclstack here, even when debug_dclstack is not set, as a sanity measure.
 		testdclstack()
 	}
 }
diff --git a/src/cmd/compile/internal/gc/subr.go b/src/cmd/compile/internal/gc/subr.go
index 9b9a3f1210..7d008dfa65 100644
--- a/src/cmd/compile/internal/gc/subr.go
+++ b/src/cmd/compile/internal/gc/subr.go
@@ -1833,7 +1833,9 @@ func genwrapper(rcvr *Type, method *Field, newnam *Sym, iface int) {
 	funcbody(fn)
 	Curfn = fn
 	popdcl()
-	testdclstack()
+	if debug_dclstack != 0 {
+		testdclstack()
+	}
 
 	// wrappers where T is anonymous (struct or interface) can be duplicated.
 	if rcvr.IsStruct() || rcvr.IsInterface() || rcvr.IsPtr() && rcvr.Elem().IsStruct() {
diff --git a/src/cmd/compile/internal/gc/walk.go b/src/cmd/compile/internal/gc/walk.go
index efe2016e46..7c2e2ab442 100644
--- a/src/cmd/compile/internal/gc/walk.go
+++ b/src/cmd/compile/internal/gc/walk.go
@@ -3117,12 +3117,12 @@ func walkcompare(n *Node, init *Nodes) *Node {
 		cmpr = cmpr.Left
 	}
 
-	if !islvalue(cmpl) || !islvalue(cmpr) {
-		Fatalf("arguments of comparison must be lvalues - %v %v", cmpl, cmpr)
-	}
-
 	// Chose not to inline. Call equality function directly.
 	if !inline {
+		if !islvalue(cmpl) || !islvalue(cmpr) {
+			Fatalf("arguments of comparison must be lvalues - %v %v", cmpl, cmpr)
+		}
+
 		// eq algs take pointers
 		pl := temp(ptrto(t))
 		al := nod(OAS, pl, nod(OADDR, cmpl, nil))
diff --git a/src/cmd/compile/internal/ssa/writebarrier.go b/src/cmd/compile/internal/ssa/writebarrier.go
index 1eb4d7bb1a..054ba1f85c 100644
--- a/src/cmd/compile/internal/ssa/writebarrier.go
+++ b/src/cmd/compile/internal/ssa/writebarrier.go
@@ -35,7 +35,7 @@ func writebarrier(f *Func) {
 	valueLoop:
 		for i, v := range b.Values {
 			switch v.Op {
-			case OpStoreWB, OpMoveWB, OpMoveWBVolatile:
+			case OpStoreWB, OpMoveWB, OpMoveWBVolatile, OpZeroWB:
 				if IsStackAddr(v.Args[0]) {
 					switch v.Op {
 					case OpStoreWB:
diff --git a/src/cmd/go/bug.go b/src/cmd/go/bug.go
index cbd258b80b..658f6dabd9 100644
--- a/src/cmd/go/bug.go
+++ b/src/cmd/go/bug.go
@@ -20,11 +20,10 @@ import (
 var cmdBug = &Command{
 	Run:       runBug,
 	UsageLine: "bug",
-	Short:     "print information for bug reports",
+	Short:     "start a bug report",
 	Long: `
-Bug prints information that helps file effective bug reports.
-
-Bugs may be reported at https://golang.org/issue/new.
+Bug opens the default browser and starts a new bug report.
+The report includes useful system information.
 	`,
 }
 
diff --git a/src/cmd/go/test.go b/src/cmd/go/test.go
index cdb167de75..6482f0fd32 100644
--- a/src/cmd/go/test.go
+++ b/src/cmd/go/test.go
@@ -894,9 +894,13 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 
 	if buildContext.GOOS == "darwin" {
 		if buildContext.GOARCH == "arm" || buildContext.GOARCH == "arm64" {
-			t.NeedCgo = true
+			t.IsIOS = true
+			t.NeedOS = true
 		}
 	}
+	if t.TestMain == nil {
+		t.NeedOS = true
+	}
 
 	for _, cp := range pmain.imports {
 		if len(cp.coverVars) > 0 {
@@ -1343,7 +1347,8 @@ type testFuncs struct {
 	NeedTest    bool
 	ImportXtest bool
 	NeedXtest   bool
-	NeedCgo     bool
+	NeedOS      bool
+	IsIOS       bool
 	Cover       []coverInfo
 }
 
@@ -1444,7 +1449,7 @@ var testmainTmpl = template.Must(template.New("main").Parse(`
 package main
 
 import (
-{{if not .TestMain}}
+{{if .NeedOS}}
 	"os"
 {{end}}
 	"testing"
@@ -1460,8 +1465,10 @@ import (
 	_cover{{$i}} {{$p.Package.ImportPath | printf "%q"}}
 {{end}}
 
-{{if .NeedCgo}}
+{{if .IsIOS}}
+	"os/signal"
 	_ "runtime/cgo"
+	"syscall"
 {{end}}
 )
 
@@ -1523,6 +1530,32 @@ func coverRegisterFile(fileName string, counter []uint32, pos []uint32, numStmts
 {{end}}
 
 func main() {
+{{if .IsIOS}}
+	// Send a SIGUSR2, which will be intercepted by LLDB to
+	// tell the test harness that installation was successful.
+	// See misc/ios/go_darwin_arm_exec.go.
+	signal.Notify(make(chan os.Signal), syscall.SIGUSR2)
+	syscall.Kill(0, syscall.SIGUSR2)
+	signal.Reset(syscall.SIGUSR2)
+
+	// The first argument supplied to an iOS test is an offset
+	// suffix for the current working directory.
+	// Process it here, and remove it from os.Args.
+	const hdr = "cwdSuffix="
+	if len(os.Args) < 2 || len(os.Args[1]) <= len(hdr) || os.Args[1][:len(hdr)] != hdr {
+		panic("iOS test not passed a working directory suffix")
+	}
+	suffix := os.Args[1][len(hdr):]
+	dir, err := os.Getwd()
+	if err != nil {
+		panic(err)
+	}
+	if err := os.Chdir(dir + "/" + suffix); err != nil {
+		panic(err)
+	}
+	os.Args = append([]string{os.Args[0]}, os.Args[2:]...)
+{{end}}
+
 {{if .CoverEnabled}}
 	testing.RegisterCover(testing.Cover{
 		Mode: {{printf "%q" .CoverMode}},
diff --git a/src/cmd/link/internal/ld/pcln.go b/src/cmd/link/internal/ld/pcln.go
index 1ebd7de662..479425f211 100644
--- a/src/cmd/link/internal/ld/pcln.go
+++ b/src/cmd/link/internal/ld/pcln.go
@@ -168,7 +168,7 @@ func container(s *Symbol) int {
 	if s == nil {
 		return 0
 	}
-	if Buildmode == BuildmodePlugin && onlycsymbol(s) {
+	if Buildmode == BuildmodePlugin && Headtype == obj.Hdarwin && onlycsymbol(s) {
 		return 1
 	}
 	// We want to generate func table entries only for the "lowest level" symbols,
diff --git a/src/compress/bzip2/bzip2_test.go b/src/compress/bzip2/bzip2_test.go
index a6c3080db3..95fb189585 100644
--- a/src/compress/bzip2/bzip2_test.go
+++ b/src/compress/bzip2/bzip2_test.go
@@ -204,12 +204,6 @@ func TestMTF(t *testing.T) {
 	}
 }
 
-var (
-	digits = mustLoadFile("testdata/e.txt.bz2")
-	twain  = mustLoadFile("testdata/Mark.Twain-Tom.Sawyer.txt.bz2")
-	random = mustLoadFile("testdata/random.data.bz2")
-)
-
 func benchmarkDecode(b *testing.B, compressed []byte) {
 	// Determine the uncompressed size of testfile.
 	uncompressedSize, err := io.Copy(ioutil.Discard, NewReader(bytes.NewReader(compressed)))
@@ -227,6 +221,18 @@ func benchmarkDecode(b *testing.B, compressed []byte) {
 	}
 }
 
-func BenchmarkDecodeDigits(b *testing.B) { benchmarkDecode(b, digits) }
-func BenchmarkDecodeTwain(b *testing.B)  { benchmarkDecode(b, twain) }
-func BenchmarkDecodeRand(b *testing.B)   { benchmarkDecode(b, random) }
+func BenchmarkDecodeDigits(b *testing.B) {
+	digits := mustLoadFile("testdata/e.txt.bz2")
+	b.ResetTimer()
+	benchmarkDecode(b, digits)
+}
+func BenchmarkDecodeTwain(b *testing.B) {
+	twain := mustLoadFile("testdata/Mark.Twain-Tom.Sawyer.txt.bz2")
+	b.ResetTimer()
+	benchmarkDecode(b, twain)
+}
+func BenchmarkDecodeRand(b *testing.B) {
+	random := mustLoadFile("testdata/random.data.bz2")
+	b.ResetTimer()
+	benchmarkDecode(b, random)
+}
diff --git a/src/compress/flate/deflate.go b/src/compress/flate/deflate.go
index 97265b3ca2..4d6a5357d8 100644
--- a/src/compress/flate/deflate.go
+++ b/src/compress/flate/deflate.go
@@ -136,14 +136,17 @@ func (d *compressor) fillDeflate(b []byte) int {
 			delta := d.hashOffset - 1
 			d.hashOffset -= delta
 			d.chainHead -= delta
-			for i, v := range d.hashPrev {
+
+			// Iterate over slices instead of arrays to avoid copying
+			// the entire table onto the stack (Issue #18625).
+			for i, v := range d.hashPrev[:] {
 				if int(v) > delta {
 					d.hashPrev[i] = uint32(int(v) - delta)
 				} else {
 					d.hashPrev[i] = 0
 				}
 			}
-			for i, v := range d.hashHead {
+			for i, v := range d.hashHead[:] {
 				if int(v) > delta {
 					d.hashHead[i] = uint32(int(v) - delta)
 				} else {
diff --git a/src/compress/flate/deflate_test.go b/src/compress/flate/deflate_test.go
index 521a260365..fbea761721 100644
--- a/src/compress/flate/deflate_test.go
+++ b/src/compress/flate/deflate_test.go
@@ -12,6 +12,7 @@ import (
 	"io"
 	"io/ioutil"
 	"reflect"
+	"runtime/debug"
 	"sync"
 	"testing"
 )
@@ -864,3 +865,33 @@ func TestBestSpeedMaxMatchOffset(t *testing.T) {
 		}
 	}
 }
+
+func TestMaxStackSize(t *testing.T) {
+	// This test must not run in parallel with other tests as debug.SetMaxStack
+	// affects all goroutines.
+	n := debug.SetMaxStack(1 << 16)
+	defer debug.SetMaxStack(n)
+
+	var wg sync.WaitGroup
+	defer wg.Wait()
+
+	b := make([]byte, 1<<20)
+	for level := HuffmanOnly; level <= BestCompression; level++ {
+		// Run in separate goroutine to increase probability of stack regrowth.
+		wg.Add(1)
+		go func(level int) {
+			defer wg.Done()
+			zw, err := NewWriter(ioutil.Discard, level)
+			if err != nil {
+				t.Errorf("level %d, NewWriter() = %v, want nil", level, err)
+			}
+			if n, err := zw.Write(b); n != len(b) || err != nil {
+				t.Errorf("level %d, Write() = (%d, %v), want (%d, nil)", level, n, err, len(b))
+			}
+			if err := zw.Close(); err != nil {
+				t.Errorf("level %d, Close() = %v, want nil", level, err)
+			}
+			zw.Reset(ioutil.Discard)
+		}(level)
+	}
+}
diff --git a/src/compress/flate/deflatefast.go b/src/compress/flate/deflatefast.go
index a1636a37d6..08298b76bb 100644
--- a/src/compress/flate/deflatefast.go
+++ b/src/compress/flate/deflatefast.go
@@ -60,7 +60,7 @@ func newDeflateFast() *deflateFast {
 func (e *deflateFast) encode(dst []token, src []byte) []token {
 	// Ensure that e.cur doesn't wrap.
 	if e.cur > 1<<30 {
-		*e = deflateFast{cur: maxStoreBlockSize, prev: e.prev[:0]}
+		e.resetAll()
 	}
 
 	// This check isn't in the Snappy implementation, but there, the caller
@@ -265,6 +265,21 @@ func (e *deflateFast) reset() {
 
 	// Protect against e.cur wraparound.
 	if e.cur > 1<<30 {
-		*e = deflateFast{cur: maxStoreBlockSize, prev: e.prev[:0]}
+		e.resetAll()
+	}
+}
+
+// resetAll resets the deflateFast struct and is only called in rare
+// situations to prevent integer overflow. It manually resets each field
+// to avoid causing large stack growth.
+//
+// See https://golang.org/issue/18636.
+func (e *deflateFast) resetAll() {
+	// This is equivalent to:
+	//	*e = deflateFast{cur: maxStoreBlockSize, prev: e.prev[:0]}
+	e.cur = maxStoreBlockSize
+	e.prev = e.prev[:0]
+	for i := range e.table {
+		e.table[i] = tableEntry{}
 	}
 }
diff --git a/src/compress/gzip/issue14937_test.go b/src/compress/gzip/issue14937_test.go
index e76d47cc4e..30c1390dfd 100644
--- a/src/compress/gzip/issue14937_test.go
+++ b/src/compress/gzip/issue14937_test.go
@@ -9,11 +9,17 @@ import (
 	"testing"
 )
 
-// Per golang.org/issue/14937, check that every .gz file
-// in the tree has a zero mtime.
+// TestGZIPFilesHaveZeroMTimes checks that every .gz file in the tree
+// has a zero MTIME. This is a requirement for the Debian maintainers
+// to be able to have deterministic packages.
+//
+// See https://golang.org/issue/14937.
 func TestGZIPFilesHaveZeroMTimes(t *testing.T) {
-	if testing.Short() && testenv.Builder() == "" {
-		t.Skip("skipping in short mode")
+	// To avoid spurious false positives due to untracked GZIP files that
+	// may be in the user's GOROOT (Issue 18604), we only run this test on
+	// the builders, which should have a clean checkout of the tree.
+	if testenv.Builder() == "" {
+		t.Skip("skipping test on non-builder")
 	}
 	goroot, err := filepath.EvalSymlinks(runtime.GOROOT())
 	if err != nil {
diff --git a/src/crypto/dsa/dsa_test.go b/src/crypto/dsa/dsa_test.go
index b89aeaebea..8600059f03 100644
--- a/src/crypto/dsa/dsa_test.go
+++ b/src/crypto/dsa/dsa_test.go
@@ -95,7 +95,7 @@ func TestSignAndVerify(t *testing.T) {
 func TestSigningWithDegenerateKeys(t *testing.T) {
 	// Signing with degenerate private keys should not cause an infinite
 	// loop.
-	badKeys := []struct{
+	badKeys := []struct {
 		p, q, g, y, x string
 	}{
 		{"00", "01", "00", "00", "00"},
@@ -105,7 +105,7 @@ func TestSigningWithDegenerateKeys(t *testing.T) {
 	for i, test := range badKeys {
 		priv := PrivateKey{
 			PublicKey: PublicKey{
-				Parameters: Parameters {
+				Parameters: Parameters{
 					P: fromHex(test.p),
 					Q: fromHex(test.q),
 					G: fromHex(test.g),
diff --git a/src/crypto/tls/cipher_suites.go b/src/crypto/tls/cipher_suites.go
index 05146743ac..beb0f1926d 100644
--- a/src/crypto/tls/cipher_suites.go
+++ b/src/crypto/tls/cipher_suites.go
@@ -84,15 +84,15 @@ var cipherSuites = []*cipherSuite{
 	{TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, 16, 0, 4, ecdheECDSAKA, suiteECDHE | suiteECDSA | suiteTLS12, nil, nil, aeadAESGCM},
 	{TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384, 32, 0, 4, ecdheRSAKA, suiteECDHE | suiteTLS12 | suiteSHA384, nil, nil, aeadAESGCM},
 	{TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, 32, 0, 4, ecdheECDSAKA, suiteECDHE | suiteECDSA | suiteTLS12 | suiteSHA384, nil, nil, aeadAESGCM},
-	{TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, ecdheRSAKA, suiteECDHE | suiteTLS12, cipherAES, macSHA256, nil},
+	{TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, ecdheRSAKA, suiteECDHE | suiteTLS12 | suiteDefaultOff, cipherAES, macSHA256, nil},
 	{TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, 16, 20, 16, ecdheRSAKA, suiteECDHE, cipherAES, macSHA1, nil},
-	{TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, ecdheECDSAKA, suiteECDHE | suiteECDSA | suiteTLS12, cipherAES, macSHA256, nil},
+	{TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, ecdheECDSAKA, suiteECDHE | suiteECDSA | suiteTLS12 | suiteDefaultOff, cipherAES, macSHA256, nil},
 	{TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA, 16, 20, 16, ecdheECDSAKA, suiteECDHE | suiteECDSA, cipherAES, macSHA1, nil},
 	{TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA, 32, 20, 16, ecdheRSAKA, suiteECDHE, cipherAES, macSHA1, nil},
 	{TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA, 32, 20, 16, ecdheECDSAKA, suiteECDHE | suiteECDSA, cipherAES, macSHA1, nil},
 	{TLS_RSA_WITH_AES_128_GCM_SHA256, 16, 0, 4, rsaKA, suiteTLS12, nil, nil, aeadAESGCM},
 	{TLS_RSA_WITH_AES_256_GCM_SHA384, 32, 0, 4, rsaKA, suiteTLS12 | suiteSHA384, nil, nil, aeadAESGCM},
-	{TLS_RSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, rsaKA, suiteTLS12, cipherAES, macSHA256, nil},
+	{TLS_RSA_WITH_AES_128_CBC_SHA256, 16, 32, 16, rsaKA, suiteTLS12 | suiteDefaultOff, cipherAES, macSHA256, nil},
 	{TLS_RSA_WITH_AES_128_CBC_SHA, 16, 20, 16, rsaKA, 0, cipherAES, macSHA1, nil},
 	{TLS_RSA_WITH_AES_256_CBC_SHA, 32, 20, 16, rsaKA, 0, cipherAES, macSHA1, nil},
 	{TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA, 24, 20, 8, ecdheRSAKA, suiteECDHE, cipher3DES, macSHA1, nil},
diff --git a/src/crypto/tls/tls.go b/src/crypto/tls/tls.go
index f2e5aea2bc..615d1e5576 100644
--- a/src/crypto/tls/tls.go
+++ b/src/crypto/tls/tls.go
@@ -6,8 +6,8 @@
 package tls
 
 // BUG(agl): The crypto/tls package only implements some countermeasures
-// against Lucky13 attacks on CBC-mode encryption. See
-// http://www.isg.rhul.ac.uk/tls/TLStiming.pdf and
+// against Lucky13 attacks on CBC-mode encryption, and only on SHA1
+// variants. See http://www.isg.rhul.ac.uk/tls/TLStiming.pdf and
 // https://www.imperialviolet.org/2013/02/04/luckythirteen.html.
 
 import (
diff --git a/src/crypto/x509/cert_pool.go b/src/crypto/x509/cert_pool.go
index fea33df379..71ffbdf0e0 100644
--- a/src/crypto/x509/cert_pool.go
+++ b/src/crypto/x509/cert_pool.go
@@ -4,7 +4,11 @@
 
 package x509
 
-import "encoding/pem"
+import (
+	"encoding/pem"
+	"errors"
+	"runtime"
+)
 
 // CertPool is a set of certificates.
 type CertPool struct {
@@ -26,6 +30,11 @@ func NewCertPool() *CertPool {
 // Any mutations to the returned pool are not written to disk and do
 // not affect any other pool.
 func SystemCertPool() (*CertPool, error) {
+	if runtime.GOOS == "windows" {
+		// Issue 16736, 18609:
+		return nil, errors.New("crypto/x509: system root pool is not available on Windows")
+	}
+
 	return loadSystemRoots()
 }
 
diff --git a/src/crypto/x509/root_windows.go b/src/crypto/x509/root_windows.go
index ca2fba5cb4..a936fec7d8 100644
--- a/src/crypto/x509/root_windows.go
+++ b/src/crypto/x509/root_windows.go
@@ -226,6 +226,11 @@ func (c *Certificate) systemVerify(opts *VerifyOptions) (chains [][]*Certificate
 }
 
 func loadSystemRoots() (*CertPool, error) {
+	// TODO: restore this functionality on Windows. We tried to do
+	// it in Go 1.8 but had to revert it. See Issue 18609.
+	// Returning (nil, nil) was the old behavior, prior to CL 30578.
+	return nil, nil
+
 	const CRYPT_E_NOT_FOUND = 0x80092004
 
 	store, err := syscall.CertOpenSystemStore(0, syscall.StringToUTF16Ptr("ROOT"))
diff --git a/src/crypto/x509/x509_test.go b/src/crypto/x509/x509_test.go
index aa30d85b7d..b085dad90f 100644
--- a/src/crypto/x509/x509_test.go
+++ b/src/crypto/x509/x509_test.go
@@ -24,6 +24,7 @@ import (
 	"net"
 	"os/exec"
 	"reflect"
+	"runtime"
 	"strings"
 	"testing"
 	"time"
@@ -1477,6 +1478,9 @@ func TestMultipleRDN(t *testing.T) {
 }
 
 func TestSystemCertPool(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("not implemented on Windows; Issue 16736, 18609")
+	}
 	_, err := SystemCertPool()
 	if err != nil {
 		t.Fatal(err)
diff --git a/src/go/ast/scope.go b/src/go/ast/scope.go
index 1ce5e2e84b..a400c7152a 100644
--- a/src/go/ast/scope.go
+++ b/src/go/ast/scope.go
@@ -70,10 +70,8 @@ func (s *Scope) String() string {
 // The Data fields contains object-specific data:
 //
 //	Kind    Data type         Data value
-//	Pkg	*types.Package    package scope
+//	Pkg     *Scope            package scope
 //	Con     int               iota for the respective declaration
-//	Con     != nil            constant value
-//	Typ     *Scope            (used as method scope during type checking - transient)
 //
 type Object struct {
 	Kind ObjKind
diff --git a/src/go/doc/doc_test.go b/src/go/doc/doc_test.go
index ad8ba5378f..82e63100d4 100644
--- a/src/go/doc/doc_test.go
+++ b/src/go/doc/doc_test.go
@@ -25,7 +25,7 @@ var files = flag.String("files", "", "consider only Go test files matching this
 
 const dataDir = "testdata"
 
-var templateTxt = readTemplate("template.txt")
+var templateTxt *template.Template
 
 func readTemplate(filename string) *template.Template {
 	t := template.New(filename)
@@ -96,6 +96,9 @@ func test(t *testing.T, mode Mode) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	if templateTxt == nil {
+		templateTxt = readTemplate("template.txt")
+	}
 
 	// test packages
 	for _, pkg := range pkgs {
diff --git a/src/go/parser/performance_test.go b/src/go/parser/performance_test.go
index f2732c0e2b..b2e1c11e9d 100644
--- a/src/go/parser/performance_test.go
+++ b/src/go/parser/performance_test.go
@@ -10,17 +10,12 @@ import (
 	"testing"
 )
 
-var src = readFile("parser.go")
-
-func readFile(filename string) []byte {
-	data, err := ioutil.ReadFile(filename)
+func BenchmarkParse(b *testing.B) {
+	src, err := ioutil.ReadFile("parser.go")
 	if err != nil {
-		panic(err)
+		b.Fatal(err)
 	}
-	return data
-}
-
-func BenchmarkParse(b *testing.B) {
+	b.ResetTimer()
 	b.SetBytes(int64(len(src)))
 	for i := 0; i < b.N; i++ {
 		if _, err := ParseFile(token.NewFileSet(), "", src, ParseComments); err != nil {
diff --git a/src/net/http/serve_test.go b/src/net/http/serve_test.go
index 072da2552b..681dff193a 100644
--- a/src/net/http/serve_test.go
+++ b/src/net/http/serve_test.go
@@ -5173,3 +5173,142 @@ func TestServerDuplicateBackgroundRead(t *testing.T) {
 	}
 	wg.Wait()
 }
+
+// Test that the bufio.Reader returned by Hijack includes any buffered
+// byte (from the Server's backgroundRead) in its buffer. We want the
+// Handler code to be able to tell that a byte is available via
+// bufio.Reader.Buffered(), without resorting to Reading it
+// (potentially blocking) to get at it.
+func TestServerHijackGetsBackgroundByte(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping test; see https://golang.org/issue/18657")
+	}
+	setParallel(t)
+	defer afterTest(t)
+	done := make(chan struct{})
+	inHandler := make(chan bool, 1)
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		defer close(done)
+
+		// Tell the client to send more data after the GET request.
+		inHandler <- true
+
+		// Wait until the HTTP server sees the extra data
+		// after the GET request. The HTTP server fires the
+		// close notifier here, assuming it's a pipelined
+		// request, as documented.
+		select {
+		case <-w.(CloseNotifier).CloseNotify():
+		case <-time.After(5 * time.Second):
+			t.Error("timeout")
+			return
+		}
+
+		conn, buf, err := w.(Hijacker).Hijack()
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		defer conn.Close()
+		n := buf.Reader.Buffered()
+		if n != 1 {
+			t.Errorf("buffered data = %d; want 1", n)
+		}
+		peek, err := buf.Reader.Peek(3)
+		if string(peek) != "foo" || err != nil {
+			t.Errorf("Peek = %q, %v; want foo, nil", peek, err)
+		}
+	}))
+	defer ts.Close()
+
+	cn, err := net.Dial("tcp", ts.Listener.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cn.Close()
+	if _, err := cn.Write([]byte("GET / HTTP/1.1\r\nHost: e.com\r\n\r\n")); err != nil {
+		t.Fatal(err)
+	}
+	<-inHandler
+	if _, err := cn.Write([]byte("foo")); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := cn.(*net.TCPConn).CloseWrite(); err != nil {
+		t.Fatal(err)
+	}
+	select {
+	case <-done:
+	case <-time.After(2 * time.Second):
+		t.Error("timeout")
+	}
+}
+
+// Like TestServerHijackGetsBackgroundByte above but sending a
+// immediate 1MB of data to the server to fill up the server's 4KB
+// buffer.
+func TestServerHijackGetsBackgroundByte_big(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping test; see https://golang.org/issue/18657")
+	}
+	setParallel(t)
+	defer afterTest(t)
+	done := make(chan struct{})
+	const size = 8 << 10
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		defer close(done)
+
+		// Wait until the HTTP server sees the extra data
+		// after the GET request. The HTTP server fires the
+		// close notifier here, assuming it's a pipelined
+		// request, as documented.
+		select {
+		case <-w.(CloseNotifier).CloseNotify():
+		case <-time.After(5 * time.Second):
+			t.Error("timeout")
+			return
+		}
+
+		conn, buf, err := w.(Hijacker).Hijack()
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		defer conn.Close()
+		slurp, err := ioutil.ReadAll(buf.Reader)
+		if err != nil {
+			t.Error("Copy: %v", err)
+		}
+		allX := true
+		for _, v := range slurp {
+			if v != 'x' {
+				allX = false
+			}
+		}
+		if len(slurp) != size {
+			t.Errorf("read %d; want %d", len(slurp), size)
+		} else if !allX {
+			t.Errorf("read %q; want %d 'x'", slurp, size)
+		}
+	}))
+	defer ts.Close()
+
+	cn, err := net.Dial("tcp", ts.Listener.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cn.Close()
+	if _, err := fmt.Fprintf(cn, "GET / HTTP/1.1\r\nHost: e.com\r\n\r\n%s",
+		strings.Repeat("x", size)); err != nil {
+		t.Fatal(err)
+	}
+	if err := cn.(*net.TCPConn).CloseWrite(); err != nil {
+		t.Fatal(err)
+	}
+
+	select {
+	case <-done:
+	case <-time.After(2 * time.Second):
+		t.Error("timeout")
+	}
+}
diff --git a/src/net/http/server.go b/src/net/http/server.go
index 96236489bd..df70a15193 100644
--- a/src/net/http/server.go
+++ b/src/net/http/server.go
@@ -164,7 +164,7 @@ type Flusher interface {
 // should always test for this ability at runtime.
 type Hijacker interface {
 	// Hijack lets the caller take over the connection.
-	// After a call to Hijack(), the HTTP server library
+	// After a call to Hijack the HTTP server library
 	// will not do anything else with the connection.
 	//
 	// It becomes the caller's responsibility to manage
@@ -174,6 +174,9 @@ type Hijacker interface {
 	// already set, depending on the configuration of the
 	// Server. It is the caller's responsibility to set
 	// or clear those deadlines as needed.
+	//
+	// The returned bufio.Reader may contain unprocessed buffered
+	// data from the client.
 	Hijack() (net.Conn, *bufio.ReadWriter, error)
 }
 
@@ -293,6 +296,11 @@ func (c *conn) hijackLocked() (rwc net.Conn, buf *bufio.ReadWriter, err error) {
 	rwc.SetDeadline(time.Time{})
 
 	buf = bufio.NewReadWriter(c.bufr, bufio.NewWriter(rwc))
+	if c.r.hasByte {
+		if _, err := c.bufr.Peek(c.bufr.Buffered() + 1); err != nil {
+			return nil, nil, fmt.Errorf("unexpected Peek failure reading buffered byte: %v", err)
+		}
+	}
 	c.setState(rwc, StateHijacked)
 	return
 }
diff --git a/src/net/http/transport_test.go b/src/net/http/transport_test.go
index d5ddf6a123..a58b1839cc 100644
--- a/src/net/http/transport_test.go
+++ b/src/net/http/transport_test.go
@@ -36,6 +36,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"time"
 )
@@ -2545,6 +2546,13 @@ type closerFunc func() error
 
 func (f closerFunc) Close() error { return f() }
 
+type writerFuncConn struct {
+	net.Conn
+	write func(p []byte) (n int, err error)
+}
+
+func (c writerFuncConn) Write(p []byte) (n int, err error) { return c.write(p) }
+
 // Issue 4677. If we try to reuse a connection that the server is in the
 // process of closing, we may end up successfully writing out our request (or a
 // portion of our request) only to find a connection error when we try to read
@@ -2557,66 +2565,78 @@ func (f closerFunc) Close() error { return f() }
 func TestRetryIdempotentRequestsOnError(t *testing.T) {
 	defer afterTest(t)
 
+	var (
+		mu     sync.Mutex
+		logbuf bytes.Buffer
+	)
+	logf := func(format string, args ...interface{}) {
+		mu.Lock()
+		defer mu.Unlock()
+		fmt.Fprintf(&logbuf, format, args...)
+		logbuf.WriteByte('\n')
+	}
+
 	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		logf("Handler")
+		w.Header().Set("X-Status", "ok")
 	}))
 	defer ts.Close()
 
-	tr := &Transport{}
+	var writeNumAtomic int32
+	tr := &Transport{
+		Dial: func(network, addr string) (net.Conn, error) {
+			logf("Dial")
+			c, err := net.Dial(network, ts.Listener.Addr().String())
+			if err != nil {
+				logf("Dial error: %v", err)
+				return nil, err
+			}
+			return &writerFuncConn{
+				Conn: c,
+				write: func(p []byte) (n int, err error) {
+					if atomic.AddInt32(&writeNumAtomic, 1) == 2 {
+						logf("intentional write failure")
+						return 0, errors.New("second write fails")
+					}
+					logf("Write(%q)", p)
+					return c.Write(p)
+				},
+			}, nil
+		},
+	}
+	defer tr.CloseIdleConnections()
 	c := &Client{Transport: tr}
 
-	const N = 2
-	retryc := make(chan struct{}, N)
 	SetRoundTripRetried(func() {
-		retryc <- struct{}{}
+		logf("Retried.")
 	})
 	defer SetRoundTripRetried(nil)
 
-	for n := 0; n < 100; n++ {
-		// open 2 conns
-		errc := make(chan error, N)
-		for i := 0; i < N; i++ {
-			// start goroutines, send on errc
-			go func() {
-				res, err := c.Get(ts.URL)
-				if err == nil {
-					res.Body.Close()
-				}
-				errc <- err
-			}()
-		}
-		for i := 0; i < N; i++ {
-			if err := <-errc; err != nil {
-				t.Fatal(err)
-			}
-		}
-
-		ts.CloseClientConnections()
-		for i := 0; i < N; i++ {
-			go func() {
-				res, err := c.Get(ts.URL)
-				if err == nil {
-					res.Body.Close()
-				}
-				errc <- err
-			}()
+	for i := 0; i < 3; i++ {
+		res, err := c.Get("http://fake.golang/")
+		if err != nil {
+			t.Fatalf("i=%d: Get = %v", i, err)
 		}
+		res.Body.Close()
+	}
 
-		for i := 0; i < N; i++ {
-			if err := <-errc; err != nil {
-				t.Fatal(err)
-			}
-		}
-		for i := 0; i < N; i++ {
-			select {
-			case <-retryc:
-				// we triggered a retry, test was successful
-				t.Logf("finished after %d runs\n", n)
-				return
-			default:
-			}
-		}
+	mu.Lock()
+	got := logbuf.String()
+	mu.Unlock()
+	const want = `Dial
+Write("GET / HTTP/1.1\r\nHost: fake.golang\r\nUser-Agent: Go-http-client/1.1\r\nAccept-Encoding: gzip\r\n\r\n")
+Handler
+intentional write failure
+Retried.
+Dial
+Write("GET / HTTP/1.1\r\nHost: fake.golang\r\nUser-Agent: Go-http-client/1.1\r\nAccept-Encoding: gzip\r\n\r\n")
+Handler
+Write("GET / HTTP/1.1\r\nHost: fake.golang\r\nUser-Agent: Go-http-client/1.1\r\nAccept-Encoding: gzip\r\n\r\n")
+Handler
+`
+	if got != want {
+		t.Errorf("Log of events differs. Got:\n%s\nWant:\n%s", got, want)
 	}
-	t.Fatal("did not trigger any retries")
 }
 
 // Issue 6981
diff --git a/src/os/os_test.go b/src/os/os_test.go
index b7300cd38c..7ad9aac70e 100644
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -54,12 +54,15 @@ var sysdir = func() *sysDir {
 	case "darwin":
 		switch runtime.GOARCH {
 		case "arm", "arm64":
+			/// At this point the test harness has not had a chance
+			// to move us into the ./src/os directory, so the
+			// current working directory is the root of the app.
 			wd, err := syscall.Getwd()
 			if err != nil {
 				wd = err.Error()
 			}
 			return &sysDir{
-				filepath.Join(wd, "..", ".."),
+				wd,
 				[]string{
 					"ResourceRules.plist",
 					"Info.plist",
diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index 033a18171d..022350b322 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -26,6 +26,8 @@ import (
 	"unsafe"
 )
 
+var sink interface{}
+
 func TestBool(t *testing.T) {
 	v := ValueOf(true)
 	if v.Bool() != true {
@@ -5331,6 +5333,72 @@ func TestCallGC(t *testing.T) {
 	f2("four", "five5", "six666", "seven77", "eight888")
 }
 
+// Issue 18635 (function version).
+func TestKeepFuncLive(t *testing.T) {
+	// Test that we keep makeFuncImpl live as long as it is
+	// referenced on the stack.
+	typ := TypeOf(func(i int) {})
+	var f, g func(in []Value) []Value
+	f = func(in []Value) []Value {
+		clobber()
+		i := int(in[0].Int())
+		if i > 0 {
+			// We can't use Value.Call here because
+			// runtime.call* will keep the makeFuncImpl
+			// alive. However, by converting it to an
+			// interface value and calling that,
+			// reflect.callReflect is the only thing that
+			// can keep the makeFuncImpl live.
+			//
+			// Alternate between f and g so that if we do
+			// reuse the memory prematurely it's more
+			// likely to get obviously corrupted.
+			MakeFunc(typ, g).Interface().(func(i int))(i - 1)
+		}
+		return nil
+	}
+	g = func(in []Value) []Value {
+		clobber()
+		i := int(in[0].Int())
+		MakeFunc(typ, f).Interface().(func(i int))(i)
+		return nil
+	}
+	MakeFunc(typ, f).Call([]Value{ValueOf(10)})
+}
+
+// Issue 18635 (method version).
+type KeepMethodLive struct{}
+
+func (k KeepMethodLive) Method1(i int) {
+	clobber()
+	if i > 0 {
+		ValueOf(k).MethodByName("Method2").Interface().(func(i int))(i - 1)
+	}
+}
+
+func (k KeepMethodLive) Method2(i int) {
+	clobber()
+	ValueOf(k).MethodByName("Method1").Interface().(func(i int))(i)
+}
+
+func TestKeepMethodLive(t *testing.T) {
+	// Test that we keep methodValue live as long as it is
+	// referenced on the stack.
+	KeepMethodLive{}.Method1(10)
+}
+
+// clobber tries to clobber unreachable memory.
+func clobber() {
+	runtime.GC()
+	for i := 1; i < 32; i++ {
+		for j := 0; j < 10; j++ {
+			obj := make([]*byte, i)
+			sink = obj
+		}
+	}
+	runtime.GC()
+}
+
 type funcLayoutTest struct {
 	rcvr, t                  Type
 	size, argsize, retOffset uintptr
diff --git a/src/reflect/value.go b/src/reflect/value.go
index 042414ffe7..699ba69408 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -538,6 +538,11 @@ func callReflect(ctxt *makeFuncImpl, frame unsafe.Pointer) {
 			off += typ.size
 		}
 	}
+
+	// runtime.getArgInfo expects to be able to find ctxt on the
+	// stack when it finds our caller, makeFuncStub. Make sure it
+	// doesn't get garbage collected.
+	runtime.KeepAlive(ctxt)
 }
 
 // methodReceiver returns information about the receiver
@@ -650,6 +655,9 @@ func callMethod(ctxt *methodValue, frame unsafe.Pointer) {
 	// though it's a heap object.
 	memclrNoHeapPointers(args, frametype.size)
 	framePool.Put(args)
+
+	// See the comment in callReflect.
+	runtime.KeepAlive(ctxt)
 }
 
 // funcName returns the name of f, for use in error messages.
diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index 64a2f3abef..0b996d8950 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -1129,8 +1129,6 @@ top:
 		// sitting in the per-P work caches.
 		// Flush and disable work caches.
 
-		gcMarkRootCheck()
-
 		// Disallow caching workbufs and indicate that we're in mark 2.
 		gcBlackenPromptly = true
 
@@ -1153,6 +1151,16 @@ top:
 			})
 		})
 
+		// Check that roots are marked. We should be able to
+		// do this before the forEachP, but based on issue
+		// #16083 there may be a (harmless) race where we can
+		// enter mark 2 while some workers are still scanning
+		// stacks. The forEachP ensures these scans are done.
+		//
+		// TODO(austin): Figure out the race and fix this
+		// properly.
+		gcMarkRootCheck()
+
 		// Now we can start up mark 2 workers.
 		atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, 0xffffffff)
 		atomic.Xaddint64(&gcController.fractionalMarkWorkersNeeded, 0xffffffff)
diff --git a/src/runtime/plugin.go b/src/runtime/plugin.go
index 80869e1b1c..8edb29c9fe 100644
--- a/src/runtime/plugin.go
+++ b/src/runtime/plugin.go
@@ -56,7 +56,9 @@ func plugin_lastmoduleinit() (path string, syms map[string]interface{}, mismatch
 
 	lock(&ifaceLock)
 	for _, i := range md.itablinks {
-		additab(i, true, false)
+		if i.inhash == 0 {
+			additab(i, true, false)
+		}
 	}
 	unlock(&ifaceLock)
 
diff --git a/src/runtime/sys_linux_amd64.s b/src/runtime/sys_linux_amd64.s
index 839df16406..6ddcb30ae2 100644
--- a/src/runtime/sys_linux_amd64.s
+++ b/src/runtime/sys_linux_amd64.s
@@ -330,9 +330,9 @@ sigtrampnog:
 	// Lock sigprofCallersUse.
 	MOVL	$0, AX
 	MOVL	$1, CX
-	MOVQ	$runtime·sigprofCallersUse(SB), BX
+	MOVQ	$runtime·sigprofCallersUse(SB), R11
 	LOCK
-	CMPXCHGL	CX, 0(BX)
+	CMPXCHGL	CX, 0(R11)
 	JNZ	sigtramp  // Skip stack trace if already locked.
 
 	// Jump to the traceback function in runtime/cgo.
diff --git a/src/syscall/mkpost.go b/src/syscall/mkpost.go
index 26aeec843f..e75ba1502a 100644
--- a/src/syscall/mkpost.go
+++ b/src/syscall/mkpost.go
@@ -18,6 +18,7 @@ import (
 	"log"
 	"os"
 	"regexp"
+	"strings"
 )
 
 func main() {
@@ -38,10 +39,16 @@ func main() {
 		re = regexp.MustCompile("Pad_cgo[A-Za-z0-9_]*")
 		s = re.ReplaceAllString(s, "_")
 
+		// We want to keep X__val in Fsid. Hide it and restore it later.
+		s = strings.Replace(s, "X__val", "MKPOSTFSIDVAL", 1)
+
 		// Replace other unwanted fields with blank identifiers.
 		re = regexp.MustCompile("X_[A-Za-z0-9_]*")
 		s = re.ReplaceAllString(s, "_")
 
+		// Restore X__val in Fsid.
+		s = strings.Replace(s, "MKPOSTFSIDVAL", "X__val", 1)
+
 		// Force the type of RawSockaddr.Data to [14]int8 to match
 		// the existing gccgo API.
 		re = regexp.MustCompile("(Data\\s+\\[14\\])uint8")
diff --git a/src/syscall/ztypes_linux_s390x.go b/src/syscall/ztypes_linux_s390x.go
index cdde47863f..63c4a83b19 100644
--- a/src/syscall/ztypes_linux_s390x.go
+++ b/src/syscall/ztypes_linux_s390x.go
@@ -140,7 +140,7 @@ type Dirent struct {
 }
 
 type Fsid struct {
-	_ [2]int32
+	X__val [2]int32
 }
 
 type Flock_t struct {
diff --git a/src/testing/benchmark.go b/src/testing/benchmark.go
index c033ce5fec..bcebb418c4 100644
--- a/src/testing/benchmark.go
+++ b/src/testing/benchmark.go
@@ -219,7 +219,7 @@ func (b *B) run1() bool {
 	}
 	// Only print the output if we know we are not going to proceed.
 	// Otherwise it is printed in processBench.
-	if b.hasSub || b.finished {
+	if atomic.LoadInt32(&b.hasSub) != 0 || b.finished {
 		tag := "BENCH"
 		if b.skipped {
 			tag = "SKIP"
@@ -460,10 +460,13 @@ func (ctx *benchContext) processBench(b *B) {
 //
 // A subbenchmark is like any other benchmark. A benchmark that calls Run at
 // least once will not be measured itself and will be called once with N=1.
+//
+// Run may be called simultaneously from multiple goroutines, but all such
+// calls must happen before the outer benchmark function for b returns.
 func (b *B) Run(name string, f func(b *B)) bool {
 	// Since b has subbenchmarks, we will no longer run it as a benchmark itself.
 	// Release the lock and acquire it on exit to ensure locks stay paired.
-	b.hasSub = true
+	atomic.StoreInt32(&b.hasSub, 1)
 	benchmarkLock.Unlock()
 	defer benchmarkLock.Lock()
 
diff --git a/src/testing/sub_test.go b/src/testing/sub_test.go
index 8d5d9206f0..bb7b3e0925 100644
--- a/src/testing/sub_test.go
+++ b/src/testing/sub_test.go
@@ -6,6 +6,7 @@ package testing
 
 import (
 	"bytes"
+	"fmt"
 	"regexp"
 	"strings"
 	"sync/atomic"
@@ -515,3 +516,19 @@ func TestBenchmarkOutput(t *T) {
 	Benchmark(func(b *B) { b.Error("do not print this output") })
 	Benchmark(func(b *B) {})
 }
+
+func TestParallelSub(t *T) {
+	c := make(chan int)
+	block := make(chan int)
+	for i := 0; i < 10; i++ {
+		go func(i int) {
+			<-block
+			t.Run(fmt.Sprint(i), func(t *T) {})
+			c <- 1
+		}(i)
+	}
+	close(block)
+	for i := 0; i < 10; i++ {
+		<-c
+	}
+}
diff --git a/src/testing/testing.go b/src/testing/testing.go
index c972b2737f..ddbdc25bf1 100644
--- a/src/testing/testing.go
+++ b/src/testing/testing.go
@@ -216,6 +216,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
@@ -267,8 +268,8 @@ type common struct {
 	skipped    bool         // Test of benchmark has been skipped.
 	finished   bool         // Test function has completed.
 	done       bool         // Test is finished and all subtests have completed.
-	hasSub     bool
-	raceErrors int // number of races detected during test
+	hasSub     int32        // written atomically
+	raceErrors int          // number of races detected during test
 
 	parent   *common
 	level    int       // Nesting depth of test or benchmark.
@@ -645,7 +646,7 @@ func tRunner(t *T, fn func(t *T)) {
 		// Do not lock t.done to allow race detector to detect race in case
 		// the user does not appropriately synchronizes a goroutine.
 		t.done = true
-		if t.parent != nil && !t.hasSub {
+		if t.parent != nil && atomic.LoadInt32(&t.hasSub) == 0 {
 			t.setRan()
 		}
 		t.signal <- true
@@ -659,8 +660,11 @@ func tRunner(t *T, fn func(t *T)) {
 
 // Run runs f as a subtest of t called name. It reports whether f succeeded.
 // Run will block until all its parallel subtests have completed.
+//
+// Run may be called simultaneously from multiple goroutines, but all such
+// calls must happen before the outer test function for t returns.
 func (t *T) Run(name string, f func(t *T)) bool {
-	t.hasSub = true
+	atomic.StoreInt32(&t.hasSub, 1)
 	testName, ok := t.context.match.fullName(&t.common, name)
 	if !ok {
 		return true
diff --git a/src/vendor/golang_org/x/crypto/poly1305/sum_amd64.s b/src/vendor/golang_org/x/crypto/poly1305/sum_amd64.s
index bc75c61afc..2edae63828 100644
--- a/src/vendor/golang_org/x/crypto/poly1305/sum_amd64.s
+++ b/src/vendor/golang_org/x/crypto/poly1305/sum_amd64.s
@@ -54,9 +54,9 @@
 	ADCQ  t3, h1;                  \
 	ADCQ  $0, h2
 
-DATA poly1305Mask<>+0x00(SB)/8, $0x0FFFFFFC0FFFFFFF
-DATA poly1305Mask<>+0x08(SB)/8, $0x0FFFFFFC0FFFFFFC
-GLOBL poly1305Mask<>(SB), RODATA, $16
+DATA ·poly1305Mask<>+0x00(SB)/8, $0x0FFFFFFC0FFFFFFF
+DATA ·poly1305Mask<>+0x08(SB)/8, $0x0FFFFFFC0FFFFFFC
+GLOBL ·poly1305Mask<>(SB), RODATA, $16
 
 // func poly1305(out *[16]byte, m *byte, mlen uint64, key *[32]key)
 TEXT ·poly1305(SB), $0-32
@@ -67,8 +67,8 @@ TEXT ·poly1305(SB), $0-32
 
 	MOVQ 0(AX), R11
 	MOVQ 8(AX), R12
-	ANDQ poly1305Mask<>(SB), R11   // r0
-	ANDQ poly1305Mask<>+8(SB), R12 // r1
+	ANDQ ·poly1305Mask<>(SB), R11   // r0
+	ANDQ ·poly1305Mask<>+8(SB), R12 // r1
 	XORQ R8, R8                    // h0
 	XORQ R9, R9                    // h1
 	XORQ R10, R10                  // h2
diff --git a/src/vendor/golang_org/x/crypto/poly1305/sum_arm.s b/src/vendor/golang_org/x/crypto/poly1305/sum_arm.s
index 93167b2712..f70b4ac484 100644
--- a/src/vendor/golang_org/x/crypto/poly1305/sum_arm.s
+++ b/src/vendor/golang_org/x/crypto/poly1305/sum_arm.s
@@ -9,12 +9,12 @@
 // This code was translated into a form compatible with 5a from the public
 // domain source by Andrew Moon: github.com/floodyberry/poly1305-opt/blob/master/app/extensions/poly1305.
 
-DATA poly1305_init_constants_armv6<>+0x00(SB)/4, $0x3ffffff
-DATA poly1305_init_constants_armv6<>+0x04(SB)/4, $0x3ffff03
-DATA poly1305_init_constants_armv6<>+0x08(SB)/4, $0x3ffc0ff
-DATA poly1305_init_constants_armv6<>+0x0c(SB)/4, $0x3f03fff
-DATA poly1305_init_constants_armv6<>+0x10(SB)/4, $0x00fffff
-GLOBL poly1305_init_constants_armv6<>(SB), 8, $20
+DATA ·poly1305_init_constants_armv6<>+0x00(SB)/4, $0x3ffffff
+DATA ·poly1305_init_constants_armv6<>+0x04(SB)/4, $0x3ffff03
+DATA ·poly1305_init_constants_armv6<>+0x08(SB)/4, $0x3ffc0ff
+DATA ·poly1305_init_constants_armv6<>+0x0c(SB)/4, $0x3f03fff
+DATA ·poly1305_init_constants_armv6<>+0x10(SB)/4, $0x00fffff
+GLOBL ·poly1305_init_constants_armv6<>(SB), 8, $20
 
 // Warning: the linker may use R11 to synthesize certain instructions. Please
 // take care and verify that no synthetic instructions use it.
@@ -27,7 +27,7 @@ TEXT poly1305_init_ext_armv6<>(SB), NOSPLIT, $0
 	ADD       $4, R13, R8
 	MOVM.IB   [R4-R7], (R8)
 	MOVM.IA.W (R1), [R2-R5]
-	MOVW      $poly1305_init_constants_armv6<>(SB), R7
+	MOVW      $·poly1305_init_constants_armv6<>(SB), R7
 	MOVW      R2, R8
 	MOVW      R2>>26, R9
 	MOVW      R3>>20, g
diff --git a/test/fixedbugs/issue18661.go b/test/fixedbugs/issue18661.go
new file mode 100644
index 0000000000..8c83775200
--- /dev/null
+++ b/test/fixedbugs/issue18661.go
@@ -0,0 +1,18 @@
+// compile
+
+// Copyright 2017 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 p
+
+var (
+	e interface{}
+	s = struct{ a *int }{}
+	b = e == s
+)
+
+func test(obj interface{}) {
+	if obj != struct{ a *string }{} {
+	}
+}