runtime: check alignment of 8-byte atomic loads and stores on 386

Currently, if we do an atomic{load,store}64 of an unaligned address on
386, we'll simply get a non-atomic load/store.  This has been the
source of myriad bugs, so add alignment checks to these two
operations.  These checks parallel the equivalent checks in
sync/atomic.

The alignment check is not necessary in cas64 because it uses a locked
instruction.  The CPU will either execute this atomically or raise an
alignment fault (#AC)---depending on the alignment check flag---either
of which is fine.

This also fixes the two places in the runtime that trip the new
checks.  One is in the runtime self-test and shouldn't have caused
real problems.  The other is in tickspersecond and could, in
principle, have caused a misread of the ticks per second during
initialization.

Change-Id: If1796667012a6154f64f5e71d043c7f5fb3dd050
Reviewed-on: https://go-review.googlesource.com/3521
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/runtime/runtime.go b/src/runtime/runtime.go
index 2ce4618f3f..ba9881fd91 100644
--- a/src/runtime/runtime.go
+++ b/src/runtime/runtime.go
@@ -10,6 +10,7 @@ import _ "unsafe" // for go:linkname
 
 var ticks struct {
 	lock mutex
+	pad  uint32 // ensure 8-byte alignment of val on 386
 	val  uint64
 }