cmd/compile/internal/reflectdata: fix divide by zero for zero-size array elements

When generating equality signatures for arrays with zero-size ASPECIAL
elements (e.g., [3]struct{_ [0]float64}), the compiler crashed with
a divide by zero error when computing the loop unroll factor.

Skip comparison code generation for zero-size elements since they
need no comparison.

Fixes #77303

Change-Id: Ib432cfece22b1cb714de4f0a0b0d1a2d89bb0d33
Reviewed-on: https://go-review.googlesource.com/c/go/+/738841
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/test/fixedbugs/issue77303.go b/test/fixedbugs/issue77303.go
new file mode 100644
index 0000000000..995e73b034
--- /dev/null
+++ b/test/fixedbugs/issue77303.go
@@ -0,0 +1,19 @@
+// compile
+
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 77303: compiler crash on array of zero-size ASPECIAL elements.
+
+package p
+
+type zeroSizeSpecial struct {
+	_ [0]float64
+}
+
+var x [3]zeroSizeSpecial
+
+func f() bool {
+	return x == x
+}