cmd/compile: fix crash on write barrier insertion

The compiler can crash if the compiled code tries to
unconditionally read from a nil pointer. This should cause
the generated binary to panic, not the compiler.

Fixes #33438

Change-Id: Ic8fa89646d6968e2cc4e27da0ad9286662f8bc49
Reviewed-on: https://go-review.googlesource.com/c/go/+/188760
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/compile/internal/ssa/writebarrier.go

@@ -520,6 +520,10 @@ func IsReadOnlyGlobalAddr(v *Value) bool {
 	if !IsGlobalAddr(v) {
 		return false
 	}
+	if v.Op == OpConst64 || v.Op == OpConst32 {
+		// Nil pointers are read only. See issue 33438.
+		return true
+	}
 	// See TODO in OpAddr case in IsSanitizerSafeAddr below.
 	return strings.HasPrefix(v.Aux.(*obj.LSym).Name, `""..stmp_`)
 }

test/fixedbugs/issue33438.go

@@ -0,0 +1,19 @@
+// compile
+
+// Copyright 2019 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.
+
+package p
+
+type hasPtrs struct {
+        x [2]*int
+	// Note: array size needs to be >1 to force this type to be not SSAable.
+	// The bug triggers only for OpMove, which is only used for unSSAable types.
+}
+
+func main() {
+        var x *hasPtrs       // Can be local, global, or arg; nil or non-nil.
+        var y *hasPtrs = nil // Must initialize to nil.
+        *x = *y
+}