diff --git a/src/runtime/lfstack.go b/src/runtime/lfstack.go
index 8a2d519402c..1261f54d972 100644
--- a/src/runtime/lfstack.go
+++ b/src/runtime/lfstack.go
@@ -41,38 +41,3 @@ func lfstackpop(head *uint64) unsafe.Pointer {
 		}
 	}
 }
-
-const (
-	addrBits = 48
-	cntBits  = 64 - addrBits + 3
-)
-
-func lfstackPack(node *lfnode, cnt uintptr) uint64 {
-	if unsafe.Sizeof(uintptr(0)) == 4 {
-		// On 32-bit systems, the stored uint64 has a 32-bit pointer and 32-bit count.
-		return uint64(uintptr(unsafe.Pointer(node)))<<32 | uint64(cnt)
-	}
-	// On ppc64, 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.
-	//
-	// On mips64, Linux limits the user address space to 40 bits (see
-	// TASK_SIZE64 in the Linux kernel).  This has grown over time,
-	// so here we allow 48 bit addresses.
-	//
-	// On AMD64, virtual addresses are 48-bit numbers sign extended to 64.
-	// We shift the address left 16 to eliminate the sign extended part and make
-	// room in the bottom for the count.
-	//
-	// 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.
-	return uint64(uintptr(unsafe.Pointer(node)))<<(64-addrBits) | uint64(cnt&(1<<cntBits-1))
-}
-
-func lfstackUnpack(val uint64) *lfnode {
-	if unsafe.Sizeof(uintptr(0)) == 4 {
-		return (*lfnode)(unsafe.Pointer(uintptr(val >> 32)))
-	}
-	return (*lfnode)(unsafe.Pointer(uintptr(val >> cntBits << 3)))
-}
diff --git a/src/runtime/lfstack_32bit.go b/src/runtime/lfstack_32bit.go
new file mode 100644
index 00000000000..2f59e0212ef
--- /dev/null
+++ b/src/runtime/lfstack_32bit.go
@@ -0,0 +1,19 @@
+// Copyright 2014 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.
+
+// +build 386 arm nacl
+
+package runtime
+
+import "unsafe"
+
+// On 32-bit systems, the stored uint64 has a 32-bit pointer and 32-bit count.
+
+func lfstackPack(node *lfnode, cnt uintptr) uint64 {
+	return uint64(uintptr(unsafe.Pointer(node)))<<32 | uint64(cnt)
+}
+
+func lfstackUnpack(val uint64) *lfnode {
+	return (*lfnode)(unsafe.Pointer(uintptr(val >> 32)))
+}
diff --git a/src/runtime/lfstack_64bit.go b/src/runtime/lfstack_64bit.go
new file mode 100644
index 00000000000..07c2a141f02
--- /dev/null
+++ b/src/runtime/lfstack_64bit.go
@@ -0,0 +1,33 @@
+// Copyright 2014 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.
+
+// +build arm64 mips64 mips64le ppc64 ppc64le
+
+package runtime
+
+import "unsafe"
+
+// On ppc64, 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.
+//
+// On mips64, Linux limits the user address space to 40 bits (see
+// TASK_SIZE64 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)))<<(64-addrBits) | uint64(cnt&(1<<cntBits-1))
+}
+
+func lfstackUnpack(val uint64) *lfnode {
+	return (*lfnode)(unsafe.Pointer(uintptr(val >> cntBits << 3)))
+}
diff --git a/src/runtime/lfstack_amd64.go b/src/runtime/lfstack_amd64.go
new file mode 100644
index 00000000000..6397e1d47ff
--- /dev/null
+++ b/src/runtime/lfstack_amd64.go
@@ -0,0 +1,22 @@
+// Copyright 2014 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
+
+import "unsafe"
+
+// On AMD64, virtual addresses are 48-bit numbers sign extended to 64.
+// We shift the address left 16 to eliminate the sign extended part and make
+// room in the bottom for the count.
+// 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.
+
+func lfstackPack(node *lfnode, cnt uintptr) uint64 {
+	return uint64(uintptr(unsafe.Pointer(node)))<<16 | uint64(cnt&(1<<19-1))
+}
+
+func lfstackUnpack(val uint64) *lfnode {
+	return (*lfnode)(unsafe.Pointer(uintptr(int64(val) >> 19 << 3)))
+}