qIsNull: redo implementation

The reason for using type-punning through an integer was
to avoid compiler warnings about float comparisons. We have
now a better mechanism to suppress such warnings,
so there is no need to be "clever" and trigger UB because
of the union usage.

Drive-by change: add constexpr+noexcept because now there's no
longer UB.

Change-Id: I29f7514e39055658d4ef6c431daf5abfc660df16
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 10 insertions, 26 deletions

diff --git a/src/corelib/global/qglobal.h b/src/corelib/global/qglobal.h
index 4817acb48f..a1f191516a 100644
--- a/src/corelib/global/qglobal.h
+++ b/src/corelib/global/qglobal.h
@@ -902,38 +902,22 @@ Q_REQUIRED_RESULT Q_DECL_CONSTEXPR static inline Q_DECL_UNUSED  bool qFuzzyIsNul
     return qAbs(f) <= 0.00001f;
 }
 
-/*
-   This function tests a double for a null value. It doesn't
-   check whether the actual value is 0 or close to 0, but whether
-   it is binary 0, disregarding sign.
-*/
-Q_REQUIRED_RESULT static inline Q_DECL_UNUSED bool qIsNull(double d)
+QT_WARNING_PUSH
+QT_WARNING_DISABLE_CLANG("-Wfloat-equal")
+QT_WARNING_DISABLE_GCC("-Wfloat-equal")
+
+Q_REQUIRED_RESULT Q_DECL_CONSTEXPR static inline Q_DECL_UNUSED bool qIsNull(double d) noexcept
 {
-    union U {
-        double d;
-        quint64 u;
-    };
-    U val;
-    val.d = d;
-    return (val.u & Q_UINT64_C(0x7fffffffffffffff)) == 0;
+    return d == 0.0;
 }
 
-/*
-   This function tests a float for a null value. It doesn't
-   check whether the actual value is 0 or close to 0, but whether
-   it is binary 0, disregarding sign.
-*/
-Q_REQUIRED_RESULT static inline Q_DECL_UNUSED bool qIsNull(float f)
+Q_REQUIRED_RESULT Q_DECL_CONSTEXPR static inline Q_DECL_UNUSED bool qIsNull(float f) noexcept
 {
-    union U {
-        float f;
-        quint32 u;
-    };
-    U val;
-    val.f = f;
-    return (val.u & 0x7fffffff) == 0;
+    return f == 0.0f;
 }
 
+QT_WARNING_POP
+
 /*
    Compilers which follow outdated template instantiation rules
    require a class to have a comparison operator to exist when