[dev.cc] runtime: convert parallel support code from C to Go

The conversion was done with an automated tool and then
modified only as necessary to make it compile and run.

[This CL is part of the removal of C code from package runtime.
See golang.org/s/dev.cc for an overview.]

LGTM=r
R=r, austin
CC=dvyukov, golang-codereviews, iant, khr
https://golang.org/cl/172250043

src/runtime/lfstack.go

@@ -0,0 +1,51 @@
+// Copyright 2012 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.
+
+// Lock-free stack.
+// The following code runs only on g0 stack.
+
+package runtime
+
+import "unsafe"
+
+const (
+	// lfPtrBits and lfCountMask are defined in lfstack_*.go.
+	lfPtrMask = 1<<lfPtrBits - 1
+)
+
+func lfstackpush(head *uint64, node *lfnode) {
+	unode := uintptr(unsafe.Pointer(node))
+	if unode&^lfPtrMask != 0 {
+		print("p=", node, "\n")
+		gothrow("lfstackpush: invalid pointer")
+	}
+
+	node.pushcnt++
+	new := uint64(unode) | (uint64(node.pushcnt)&lfCountMask)<<lfPtrBits
+	for {
+		old := atomicload64(head)
+		node.next = (*lfnode)(unsafe.Pointer(uintptr(old & lfPtrMask)))
+		if cas64(head, old, new) {
+			break
+		}
+	}
+}
+
+func lfstackpop(head *uint64) unsafe.Pointer {
+	for {
+		old := atomicload64(head)
+		if old == 0 {
+			return nil
+		}
+		node := (*lfnode)(unsafe.Pointer(uintptr(old & lfPtrMask)))
+		node2 := (*lfnode)(atomicloadp(unsafe.Pointer(&node.next)))
+		new := uint64(0)
+		if node2 != nil {
+			new = uint64(uintptr(unsafe.Pointer(node2))) | uint64(node2.pushcnt&lfCountMask)<<lfPtrBits
+		}
+		if cas64(head, old, new) {
+			return unsafe.Pointer(node)
+		}
+	}
+}