runtime: don't report a pointer alignment error for pointer-free base type

Fixes #37298

Change-Id: I8ba9c8b106e16cea7dd25473c7390b0f2ba9a1a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/223781
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

3 files changed, 20 insertions, 4 deletions

diff --git a/src/runtime/checkptr.go b/src/runtime/checkptr.go
index 8e401e8763..59891a06a5 100644
--- a/src/runtime/checkptr.go
+++ b/src/runtime/checkptr.go
@@ -8,8 +8,10 @@ import "unsafe"
 
 func checkptrAlignment(p unsafe.Pointer, elem *_type, n uintptr) {
 	// Check that (*[n]elem)(p) is appropriately aligned.
+	// Note that we allow unaligned pointers if the types they point to contain
+	// no pointers themselves. See issue 37298.
 	// TODO(mdempsky): What about fieldAlign?
-	if uintptr(p)&(uintptr(elem.align)-1) != 0 {
+	if elem.ptrdata != 0 && uintptr(p)&(uintptr(elem.align)-1) != 0 {
 		throw("checkptr: misaligned pointer conversion")
 	}
 
diff --git a/src/runtime/checkptr_test.go b/src/runtime/checkptr_test.go
index 1a7c253733..8ab8a4937c 100644
--- a/src/runtime/checkptr_test.go
+++ b/src/runtime/checkptr_test.go
@@ -24,7 +24,8 @@ func TestCheckPtr(t *testing.T) {
 		cmd  string
 		want string
 	}{
-		{"CheckPtrAlignment", "fatal error: checkptr: misaligned pointer conversion\n"},
+		{"CheckPtrAlignmentPtr", "fatal error: checkptr: misaligned pointer conversion\n"},
+		{"CheckPtrAlignmentNoPtr", ""},
 		{"CheckPtrArithmetic", "fatal error: checkptr: pointer arithmetic result points to invalid allocation\n"},
 		{"CheckPtrSize", "fatal error: checkptr: converted pointer straddles multiple allocations\n"},
 		{"CheckPtrSmall", "fatal error: checkptr: pointer arithmetic computed bad pointer value\n"},
@@ -38,6 +39,12 @@ func TestCheckPtr(t *testing.T) {
 			if err != nil {
 				t.Log(err)
 			}
+			if tc.want == "" {
+				if len(got) > 0 {
+					t.Errorf("output:\n%s\nwant no output", got)
+				}
+				return
+			}
 			if !strings.HasPrefix(string(got), tc.want) {
 				t.Errorf("output:\n%s\n\nwant output starting with: %s", got, tc.want)
 			}
diff --git a/src/runtime/testdata/testprog/checkptr.go b/src/runtime/testdata/testprog/checkptr.go
index 177db38e5a..45e6fb1aa5 100644
--- a/src/runtime/testdata/testprog/checkptr.go
+++ b/src/runtime/testdata/testprog/checkptr.go
@@ -7,18 +7,25 @@ package main
 import "unsafe"
 
 func init() {
-	register("CheckPtrAlignment", CheckPtrAlignment)
+	register("CheckPtrAlignmentNoPtr", CheckPtrAlignmentNoPtr)
+	register("CheckPtrAlignmentPtr", CheckPtrAlignmentPtr)
 	register("CheckPtrArithmetic", CheckPtrArithmetic)
 	register("CheckPtrSize", CheckPtrSize)
 	register("CheckPtrSmall", CheckPtrSmall)
 }
 
-func CheckPtrAlignment() {
+func CheckPtrAlignmentNoPtr() {
 	var x [2]int64
 	p := unsafe.Pointer(&x[0])
 	sink2 = (*int64)(unsafe.Pointer(uintptr(p) + 1))
 }
 
+func CheckPtrAlignmentPtr() {
+	var x [2]int64
+	p := unsafe.Pointer(&x[0])
+	sink2 = (**int64)(unsafe.Pointer(uintptr(p) + 1))
+}
+
 func CheckPtrArithmetic() {
 	var x int
 	i := uintptr(unsafe.Pointer(&x))