leafify opt_plus

Inspired by 346aa557b3

Closes: https://github.com/ruby/ruby/pull/2321

insns.def

@@ -1080,9 +1080,6 @@ opt_plus
 (CALL_INFO ci, CALL_CACHE cc)
 (VALUE recv, VALUE obj)
 (VALUE val)
-/* Array + anything can be handled inside of opt_plus, and that
- * anything is converted into array using #to_ary. */
-// attr bool leaf = false; /* has rb_to_array_type() */
 {
     val = vm_opt_plus(recv, obj);
 

internal.h

@@ -2124,6 +2124,7 @@ char *rb_str_to_cstr(VALUE str);
 VALUE rb_str_eql(VALUE str1, VALUE str2);
 VALUE rb_obj_as_string_result(VALUE str, VALUE obj);
 const char *ruby_escaped_char(int c);
+VALUE rb_str_opt_plus(VALUE, VALUE);
 
 /* expect tail call optimization */
 static inline VALUE

string.c

@@ -1952,6 +1952,37 @@ rb_str_plus(VALUE str1, VALUE str2)
     return str3;
 }
 
+/* A variant of rb_str_plus that does not raise but return Qundef instead. */
+MJIT_FUNC_EXPORTED VALUE
+rb_str_opt_plus(VALUE str1, VALUE str2)
+{
+    assert(RBASIC_CLASS(str1) == rb_cString);
+    assert(RBASIC_CLASS(str2) == rb_cString);
+    long len1, len2;
+    MAYBE_UNUSED(char) *ptr1, *ptr2;
+    RSTRING_GETMEM(str1, ptr1, len1);
+    RSTRING_GETMEM(str2, ptr2, len2);
+    int enc1 = rb_enc_get_index(str1);
+    int enc2 = rb_enc_get_index(str2);
+
+    if (enc1 < 0) {
+        return Qundef;
+    }
+    else if (enc2 < 0) {
+        return Qundef;
+    }
+    else if (enc1 != enc2) {
+        return Qundef;
+    }
+    else if (len1 > LONG_MAX - len2) {
+        return Qundef;
+    }
+    else {
+        return rb_str_plus(str1, str2);
+    }
+
+}
+
 /*
  *  call-seq:
  *     str * integer   -> new_str

vm_insnhelper.c

@@ -3848,9 +3848,10 @@ vm_opt_plus(VALUE recv, VALUE obj)
     else if (RBASIC_CLASS(recv) == rb_cString &&
 	     RBASIC_CLASS(obj) == rb_cString &&
 	     BASIC_OP_UNREDEFINED_P(BOP_PLUS, STRING_REDEFINED_OP_FLAG)) {
-	return rb_str_plus(recv, obj);
+	return rb_str_opt_plus(recv, obj);
     }
     else if (RBASIC_CLASS(recv) == rb_cArray &&
+             RBASIC_CLASS(obj) == rb_cArray &&
 	     BASIC_OP_UNREDEFINED_P(BOP_PLUS, ARRAY_REDEFINED_OP_FLAG)) {
 	return rb_ary_plus(recv, obj);
     }