nobu	2017-09-17 18:21:47 +0900 (Sun, 17 Sep 2017)

  New Revision: 59945

  https://svn.ruby-lang.org/cgi-bin/viewvc.cgi?view=revision&revision=59945

  Log:
    compile.c: optimize unnecessary concatstrings
    
    * compile.c (iseq_peephole_optimize): optimize away unnecessary
      concatenation of single string, following tostring which always
      puts a String instance.
      https://github.com/ruby/ruby/pull/1626#discussion_r139285653

  Modified files:
    trunk/compile.c
Index: compile.c
===================================================================
--- compile.c	(revision 59944)
+++ compile.c	(revision 59945)
@@ -2509,6 +2509,14 @@ iseq_peephole_optimize(rb_iseq_t *iseq, https://github.com/ruby/ruby/blob/trunk/compile.c#L2509
 	}
     }
 
+    if (IS_INSN_ID(iobj, tostring)) {
+	LINK_ELEMENT *next = iobj->link.next;
+	if (IS_INSN(next) && IS_INSN_ID(next, concatstrings) &&
+	    OPERAND_AT(next, 0) == INT2FIX(1)) {
+	    REMOVE_ELEM(next);
+	}
+    }
+
     if (do_tailcallopt &&
 	(IS_INSN_ID(iobj, send) ||
 	 IS_INSN_ID(iobj, opt_aref_with) ||

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