Add qHash() overloads for floating-point types

This implementation is based on GCC's implementation of std::hash<FP>, but only
to the extent of checking for zero before hashing the bits. The bit hasher is
the Qt one; I didn't even look what GCC uses.

The check against 0.0 is mandated by the requirement to have
  \forall x,y: x == y => qHash(x) == qHash(y)
which would be violated for x = 0.0 and y = -0.0 if we only hashed the bits.

Implemented out-of-line to avoid potential FP-comparison warnings, as well
as to be able to use the file-static hash() functions, which gets inlined
unlike qHashBits(), which cannot be.

[ChangeLog][QtCore][QHash/QSet] Allowed to use float, double and long double
as QHash/QSet keys.

Change-Id: I38cec4afb860f17e9f8be7b67544e58b330f8fff
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/tests/auto/corelib/tools/qhash/tst_qhash.cpp b/tests/auto/corelib/tools/qhash/tst_qhash.cpp
index af1c7aed15..ddb72a3c32 100644
--- a/tests/auto/corelib/tools/qhash/tst_qhash.cpp
+++ b/tests/auto/corelib/tools/qhash/tst_qhash.cpp
@@ -64,6 +64,7 @@ private slots:
     void rehash_isnt_quadratic();
     void dont_need_default_constructor();
     void qhash();
+    void fp_qhash_of_zero_is_zero();
     void qmultihash_specific();
 
     void compare();
@@ -1043,6 +1044,20 @@ void tst_QHash::qhash()
     }
 }
 
+void tst_QHash::fp_qhash_of_zero_is_zero()
+{
+    QCOMPARE(qHash(-0.0f), 0U);
+    QCOMPARE(qHash( 0.0f), 0U);
+
+    QCOMPARE(qHash(-0.0 ), 0U);
+    QCOMPARE(qHash( 0.0 ), 0U);
+
+#ifndef Q_OS_DARWIN
+    QCOMPARE(qHash(-0.0L), 0U);
+    QCOMPARE(qHash( 0.0L), 0U);
+#endif
+}
+
 void tst_QHash::qmultihash_specific()
 {
     QMultiHash<int, int> hash1;