shyouhei	2018-11-15 15:17:53 +0900 (Thu, 15 Nov 2018)

  New Revision: 65737

  https://svn.ruby-lang.org/cgi-bin/viewvc.cgi?view=revision&revision=65737

  Log:
    hash.c: cast from double to unsigned is undefined
    
    When a negative double is casted into an unsigned type, that operation
    is undefined (cf: ISO/IEC 9899:1990 section 6.2.9.3).  Recent versions
    of C kindly footnotes that "The remaindering operation performed when
    a value of integer type is converted to unsigned type need not be
    performed when a value of real floating type is converted to unsigned
    type" (cf: ISO/IEC 9899:1999 section 6.3.1.4 footnote 50).
    
    So it is a wrong idea to just cast a double to st_data_t.
    
    The intention of the code is commented as "mix the actual float value
    in".  It seems we should do a reinterpret_cast and rule out
    static_cast.
    
    Confirmed this changeset does not affect `make benchmark`.

  Modified files:
    trunk/hash.c
Index: hash.c
===================================================================
--- hash.c	(revision 65736)
+++ hash.c	(revision 65737)
@@ -293,7 +293,9 @@ rb_ident_hash(st_data_t n) https://github.com/ruby/ruby/blob/trunk/hash.c#L293
      *   many integers get interpreted as 2.0 or -2.0 [Bug #10761]
      */
     if (FLONUM_P(n)) {
-	n ^= (st_data_t)rb_float_value(n);
+        union { double d; st_data_t i; } u;
+        u.d = rb_float_value(n);
+        n ^= u.i;
     }
 #endif
 

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