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hash.c: cast from double to unsigned is undefined

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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
This commit is contained in:
shyouhei 2018-11-15 06:17:53 +00:00
parent ef5eacc0e0
commit f98004c0f2
1 changed files with 3 additions and 1 deletions
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hash.c
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@ -293,7 +293,9 @@ rb_ident_hash(st_data_t n)
* many integers get interpreted as 2.0 or -2.0 [Bug #10761] * many integers get interpreted as 2.0 or -2.0 [Bug #10761]
*/ */
if (FLONUM_P(n)) { 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 #endif