runtime: add prefetch functions

The Go 1.8 GC uses prefetch functions.  Add versions for gccgo that
call __builtin_prefetch.  Uncomment the test for them in testAtomic64.
Don't force the check function to return early, as now taking the
address of a local variable in the runtime package does not force it
onto the heap.

Change-Id: I48be1af93586a6c89a53b569217dcb6043466d66
Reviewed-by: Than McIntosh <>
2 files changed