runtime: align allocations harder in GODEBUG=sbrk=1 mode

Currently, GODEBUG=sbrk=1 mode aligns allocations by their type's
alignment. You would think this would be the right thing to do, but
because 64-bit fields are only 4-byte aligned right now (see #599),
this can cause a 64-bit field of an allocated object to be 4-byte
aligned, but not 8-byte aligned. If there is an atomic access to that
unaligned 64-bit field, it will crash.

This doesn't happen in normal allocation mode because the
size-segregated allocation and the current size classes will cause any
types larger than 8 bytes to be 8 byte aligned.

We fix this by making sbrk=1 mode use alignment based on the type's
size rather than its declared alignment. This matches how the tiny
allocator aligns allocations.

This was tested with

  GOARCH=386 GODEBUG=sbrk=1 go test sync/atomic

This crashes with an unaligned access before this change, and passes
with this change.

This should be reverted when/if we fix #599.

Fixes #33159.

Change-Id: Ifc52c72c6b99c5d370476685271baa43ad907565
Reviewed-on: https://go-review.googlesource.com/c/go/+/186919
Run-TryBot: Austin Clements <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: aclements

src/runtime/malloc.go

@@ -866,7 +866,22 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 	if debug.sbrk != 0 {
 		align := uintptr(16)
 		if typ != nil {
-			align = uintptr(typ.align)
+			// TODO(austin): This should be just
+			//   align = uintptr(typ.align)
+			// but that's only 4 on 32-bit platforms,
+			// even if there's a uint64 field in typ (see #599).
+			// This causes 64-bit atomic accesses to panic.
+			// Hence, we use stricter alignment that matches
+			// the normal allocator better.
+			if size&7 == 0 {
+				align = 8
+			} else if size&3 == 0 {
+				align = 4
+			} else if size&1 == 0 {
+				align = 2
+			} else {
+				align = 1
+			}
 		}
 		return persistentalloc(size, align, &memstats.other_sys)
 	}