runtime: Add prefetch to allocation code

The routine mallocgc retrieves objects from freelists. Prefetch
the object that will be returned in the next call to mallocgc.
Experiments indicate that this produces a 1% improvement when using
prefetchnta and less when using prefetcht0, prefetcht1, or prefetcht2.

Benchmark numbers indicate a 1% improvement over no
prefetch, much less over prefetcht0, prefetcht1, and prefetcht2.
These numbers were for the garbage benchmark with MAXPROCS=4
no prefetch                          >> 5.96 / 5.77 / 5.89
prefetcht0(uintptr(v.ptr().next))    >> 5.88 / 6.17 / 5.84
prefetcht1(uintptr(v.ptr().next))    >> 5.88 / 5.89 / 5.91
prefetcht2(uintptr(v.ptr().next))    >> 5.87 / 6.47 / 5.92
prefetchnta(uintptr(v.ptr().next))   >> 5.72 / 5.84 / 5.85

Change-Id: I54e07172081cccb097d5b5ce8789d74daa055ed9
Reviewed-on: https://go-review.googlesource.com/5350Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/malloc.go

@@ -553,7 +553,8 @@ func mallocgc(size uintptr, typ *_type, flags uint32) unsafe.Pointer {
 			}
 			s.freelist = v.ptr().next
 			s.ref++
-			//TODO: prefetch v.next
+			// prefetchnta offers best performance, see change list message.
+			prefetchnta(uintptr(v.ptr().next))
 			x = unsafe.Pointer(v)
 			(*[2]uint64)(x)[0] = 0
 			(*[2]uint64)(x)[1] = 0
@@ -584,7 +585,8 @@ func mallocgc(size uintptr, typ *_type, flags uint32) unsafe.Pointer {
 			}
 			s.freelist = v.ptr().next
 			s.ref++
-			//TODO: prefetch
+			// prefetchnta offers best performance, see change list message.
+			prefetchnta(uintptr(v.ptr().next))
 			x = unsafe.Pointer(v)
 			if flags&flagNoZero == 0 {
 				v.ptr().next = 0