runtime: merge lfstack{Pack,Unpack} into one file

Merge the remaining lfstack{Pack,Unpack} implemetations into one file.

unsafe.Sizeof(uintptr(0)) == 4 is a constant comparison so this branch
folds away at compile time.

Dmitry confirmed that the upper 17 bits of an address will be zero for a
user mode pointer, so there is no need to sign extend on amd64 during
unpack, so we can reuse the same implementation as all othe 64 bit
archs.

Change-Id: I99f589416d8b181ccde5364c9c2e78e4a5efc7f1
Reviewed-on: https://go-review.googlesource.com/21597
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

1 files changed, 35 insertions, 0 deletions

diff --git a/src/runtime/lfstack.go b/src/runtime/lfstack.go
index 1261f54d97..8a2d519402 100644
--- a/src/runtime/lfstack.go
+++ b/src/runtime/lfstack.go
@@ -41,3 +41,38 @@ 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)))
+}