[dev.cc] runtime: allow more address bits in lfstack on Power64

Previously, lfstack assumed Linux limited user space addresses
to 43 bits on Power64 based on a paper from 2001.  It turns
out the limit is now 46 bits, so lfstack was truncating
pointers.

Raise the limit to 48 bits (for some future proofing and to
make it match amd64) and add a self-test that will fail in a
useful way if ever unpack(pack(x)) != x.

With this change, dev.cc passes all.bash on power64le.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/174430043

src/runtime/lfstack.go

@@ -12,6 +12,10 @@ import "unsafe"
 func lfstackpush(head *uint64, node *lfnode) {
 	node.pushcnt++
 	new := lfstackPack(node, node.pushcnt)
+	if node1, _ := lfstackUnpack(new); node1 != node {
+		println("runtime: lfstackpush invalid packing: node=", node, " cnt=", hex(node.pushcnt), " packed=", hex(new), " -> node=", node1, "\n")
+		gothrow("lfstackpush")
+	}
 	for {
 		old := atomicload64(head)
 		node.next, _ = lfstackUnpack(old)

src/runtime/lfstack_linux_power64x.go

@@ -9,18 +9,24 @@ package runtime
 
 import "unsafe"
 
-// On Power64, Linux limits the user address space to 43 bits.
-// (https://www.kernel.org/doc/ols/2001/ppc64.pdf)
-// In addition to the 21 bits taken from the top, we can take 3 from the
-// bottom, because node must be pointer-aligned, giving a total of 24 bits
+// On Power64, Linux limits the user address space to 46 bits (see
+// TASK_SIZE_USER64 in the Linux kernel).  This has grown over time,
+// so here we allow 48 bit addresses.
+//
+// In addition to the 16 bits taken from the top, we can take 3 from the
+// bottom, because node must be pointer-aligned, giving a total of 19 bits
 // of count.
+const (
+	addrBits = 48
+	cntBits  = 64 - addrBits + 3
+)
 
 func lfstackPack(node *lfnode, cnt uintptr) uint64 {
-	return uint64(uintptr(unsafe.Pointer(node)))<<21 | uint64(cnt&(1<<24-1))
+	return uint64(uintptr(unsafe.Pointer(node)))<<(64-addrBits) | uint64(cnt&(1<<cntBits-1))
 }
 
 func lfstackUnpack(val uint64) (node *lfnode, cnt uintptr) {
-	node = (*lfnode)(unsafe.Pointer(uintptr(val >> 24 << 3)))
-	cnt = uintptr(val & (1<<24 - 1))
+	node = (*lfnode)(unsafe.Pointer(uintptr(val >> cntBits << 3)))
+	cnt = uintptr(val & (1<<cntBits - 1))
 	return
 }