reflect: use runtime's memmove instead of its own

They will both need write barriers at some point.
But until then, no reason why we shouldn't share.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/141330043

src/reflect/value.go

@@ -15,32 +15,6 @@ const bigEndian = false // can be smarter if we find a big-endian machine
 const ptrSize = unsafe.Sizeof((*byte)(nil))
 const cannotSet = "cannot set value obtained from unexported struct field"
 
-// TODO: This will have to go away when
-// the new gc goes in.
-func memmove(adst, asrc unsafe.Pointer, n uintptr) {
-	dst := uintptr(adst)
-	src := uintptr(asrc)
-	switch {
-	case src < dst && src+n > dst:
-		// byte copy backward
-		// careful: i is unsigned
-		for i := n; i > 0; {
-			i--
-			*(*byte)(unsafe.Pointer(dst + i)) = *(*byte)(unsafe.Pointer(src + i))
-		}
-	case (n|src|dst)&(ptrSize-1) != 0:
-		// byte copy forward
-		for i := uintptr(0); i < n; i++ {
-			*(*byte)(unsafe.Pointer(dst + i)) = *(*byte)(unsafe.Pointer(src + i))
-		}
-	default:
-		// word copy forward
-		for i := uintptr(0); i < n; i += ptrSize {
-			*(*uintptr)(unsafe.Pointer(dst + i)) = *(*uintptr)(unsafe.Pointer(src + i))
-		}
-	}
-}
-
 // Value is the reflection interface to a Go value.
 //
 // Not all methods apply to all kinds of values.  Restrictions,
@@ -2703,6 +2677,9 @@ func call(fn, arg unsafe.Pointer, n uint32, retoffset uint32)
 
 func ifaceE2I(t *rtype, src interface{}, dst unsafe.Pointer)
 
+//go:noescape
+func memmove(adst, asrc unsafe.Pointer, n uintptr)
+
 // Dummy annotation marking that the value x escapes,
 // for use in cases where the reflect code is so clever that
 // the compiler cannot follow.

src/runtime/thunk.s

@@ -74,6 +74,9 @@ TEXT reflect·chansend(SB), NOSPLIT, $0-0
 TEXT reflect·chanrecv(SB), NOSPLIT, $0-0
 	JMP	runtime·reflect_chanrecv(SB)
 
+TEXT reflect·memmove(SB), NOSPLIT, $0-0
+	JMP	runtime·memmove(SB)
+
 TEXT runtime∕debug·freeOSMemory(SB), NOSPLIT, $0-0
 	JMP	runtime·freeOSMemory(SB)