nobu	2016-05-02 12:58:28 +0900 (Mon, 02 May 2016)

  New Revision: 54882

  https://svn.ruby-lang.org/cgi-bin/viewvc.cgi?view=revision&revision=54882

  Log:
    string.c: shortcut
    
    * string.c (rb_str_concat): shortcut concatenation to ASCII-8BIT
      as well as US-ASCII.

  Modified files:
    trunk/string.c
Index: string.c
===================================================================
--- string.c	(revision 54881)
+++ string.c	(revision 54882)
@@ -2742,6 +2742,7 @@ rb_str_concat(VALUE str1, VALUE str2) https://github.com/ruby/ruby/blob/trunk/string.c#L2742
 {
     unsigned int code;
     rb_encoding *enc = STR_ENC_GET(str1);
+    int encidx;
 
     if (FIXNUM_P(str2) || RB_TYPE_P(str2, T_BIGNUM)) {
 	if (rb_num_to_uint(str2, &code) == 0) {
@@ -2757,7 +2758,8 @@ rb_str_concat(VALUE str1, VALUE str2) https://github.com/ruby/ruby/blob/trunk/string.c#L2758
 	return rb_str_append(str1, str2);
     }
 
-    if (enc == rb_usascii_encoding()) {
+    encidx = rb_enc_to_index(enc);
+    if (encidx == ENCINDEX_ASCII || encidx == ENCINDEX_US_ASCII) {
 	/* US-ASCII automatically extended to ASCII-8BIT */
 	char buf[1];
 	buf[0] = (char)code;
@@ -2765,8 +2767,8 @@ rb_str_concat(VALUE str1, VALUE str2) https://github.com/ruby/ruby/blob/trunk/string.c#L2767
 	    rb_raise(rb_eRangeError, "%u out of char range", code);
 	}
 	rb_str_cat(str1, buf, 1);
-	if (code > 127) {
-	    rb_enc_associate_index(str1, rb_ascii8bit_encindex());
+	if (encidx == ENCINDEX_US_ASCII && code > 127) {
+	    rb_enc_associate_index(str1, ENCINDEX_ASCII);
 	    ENC_CODERANGE_SET(str1, ENC_CODERANGE_VALID);
 	}
     }

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