cmd/compile/internal/escape: escape values with >PtrSize alignment

After CL 381317 there exist values that may have an alignment greater
than the pointer size for that platform. Specifically, atomic.{Ui|I}nt64
may be aligned to 8 bytes on a 32-bit platform. If such a value, or
a container for the value, gets stack-allocated, it's possible that it
won't be aligned correctly, because the maximum alignment we enforce on
stacks is governed by the pointer size. Changing that would be a
significant undertaking, so just escape these values to the heap
instead, where we're sure they'll actually be aligned correctly.

Change is by rsc@, I'm just shepherding it through code review.

For #50860.

Change-Id: I51669561c0a13ecb84f821020e144c58cb528418
Reviewed-on: https://go-review.googlesource.com/c/go/+/410131
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

diff --git a/src/cmd/compile/internal/escape/utils.go b/src/cmd/compile/internal/escape/utils.go
index 2c6e9bcbebda07143061a9ddb631e837685db432..b481d8e4b6479f900784d62cbfe9888d799aff5f 100644 (file)
--- a/src/cmd/compile/internal/escape/utils.go
+++ b/src/cmd/compile/internal/escape/utils.go
@@ -7,6 +7,7 @@ package escape
 import (
        "cmd/compile/internal/ir"
        "cmd/compile/internal/typecheck"
+       "cmd/compile/internal/types"
 )
 
 func isSliceSelfAssign(dst, src ir.Node) bool {
@@ -185,10 +186,16 @@ func HeapAllocReason(n ir.Node) string {
        if n.Type().Size() > ir.MaxStackVarSize {
                return "too large for stack"
        }
+       if n.Type().Alignment() > int64(types.PtrSize) {
+               return "too aligned for stack"
+       }
 
        if (n.Op() == ir.ONEW || n.Op() == ir.OPTRLIT) && n.Type().Elem().Size() > ir.MaxImplicitStackVarSize {
                return "too large for stack"
        }
+       if (n.Op() == ir.ONEW || n.Op() == ir.OPTRLIT) && n.Type().Elem().Alignment() > int64(types.PtrSize) {
+               return "too aligned for stack"
+       }
 
        if n.Op() == ir.OCLOSURE && typecheck.ClosureType(n.(*ir.ClosureExpr)).Size() > ir.MaxImplicitStackVarSize {
                return "too large for stack"