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`.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65737 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 3 insertions, 1 deletions

diff --git a/hash.c b/hash.c
index 83ed66d530..fc30ae9cbc 100644
--- a/hash.c
+++ b/hash.c
@@ -293,7 +293,9 @@ rb_ident_hash(st_data_t n)
      *   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