runtime: align 12-byte objects to 8 bytes on 32-bit systems

Currently on 32-bit systems 8-byte fields in a struct have an alignment
of 4 bytes, which means that atomic instructions may fault. This issue
is tracked in #36606.

Our current workaround is to allocate memory and put any such atomically
accessed fields at the beginning of the object. This workaround fails
because the tiny allocator might not align the object right. This case
specifically only happens with 12-byte objects because a type's size is
rounded up to its alignment. So if e.g. we have a type like:

type obj struct {
    a uint64
    b byte
}

then its size will be 12 bytes, because "a" will require a 4 byte
alignment. This argument may be extended to all objects of size 9-15
bytes.

So, make this workaround work by specifically aligning such objects to 8
bytes on 32-bit systems. This change leaves a TODO to remove the code
once #36606 gets resolved. It also adds a test which will presumably no
longer be necessary (the compiler should enforce the right alignment)
when it gets resolved as well.

Fixes #37262.

Change-Id: I3a34e5b014b3c37ed2e5e75e62d71d8640aa42bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/254057
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go
index 4fa14996c2..c71f856f09 100644
--- a/src/runtime/malloc.go
+++ b/src/runtime/malloc.go
@@ -1016,6 +1016,14 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 			// Align tiny pointer for required (conservative) alignment.
 			if size&7 == 0 {
 				off = alignUp(off, 8)
+			} else if sys.PtrSize == 4 && size == 12 {
+				// Conservatively align 12-byte objects to 8 bytes on 32-bit
+				// systems so that objects whose first field is a 64-bit
+				// value is aligned to 8 bytes and does not cause a fault on
+				// atomic access. See issue 37262.
+				// TODO(mknyszek): Remove this workaround if/when issue 36606
+				// is resolved.
+				off = alignUp(off, 8)
 			} else if size&3 == 0 {
 				off = alignUp(off, 4)
 			} else if size&1 == 0 {