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Qt::*_ordering: use std::bit_cast when converting to std::_ordering 

GCC, in particular, doesn't understand that the types are
fundamentally the same unless you compile with -O3, so help the
compiler by using bit_cast when it is available.

GCC now produces the same code at -O2 as previously only at -O3. Still
no tail-calls, though.

Task-number: QTBUG-118913
Change-Id: Ib1241a98f1de49c59a2e16b7d1f5cf813db4edf7
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
bb10
Marc Mutz 2023-11-16 10:38:35 +01:00 committed by Ivan Solovev
parent 9c03935c9a
commit b3819d9cc2
2 changed files with 18 additions and 3 deletions
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3
src/corelib/global/qcompare.cpp
View File

@ -11,9 +11,6 @@
QT_BEGIN_NAMESPACE
#ifdef __cpp_lib_three_way_comparison
static_assert(sizeof(std::partial_ordering) == sizeof(Qt::partial_ordering));
static_assert(sizeof(std::weak_ordering) == sizeof(Qt::weak_ordering));
static_assert(sizeof(std::strong_ordering) == sizeof(Qt::strong_ordering));
#ifdef __cpp_lib_bit_cast
#define CHECK(type, flag) \
static_assert(std::bit_cast<Qt:: type ## _ordering>(std:: type ## _ordering:: flag) \

18
src/corelib/global/qcompare.h
View File

@ -12,6 +12,9 @@
#include <QtCore/qglobal.h>
#include <QtCore/qcompare_impl.h>
#ifdef __cpp_lib_bit_cast
#include <bit>
#endif
#ifdef __cpp_lib_three_way_comparison
#include <compare>
#endif
@ -266,6 +269,10 @@ public:
constexpr Q_IMPLICIT operator std::partial_ordering() const noexcept
{
static_assert(sizeof(*this) == sizeof(std::partial_ordering));
#ifdef __cpp_lib_bit_cast
return std::bit_cast<std::partial_ordering>(*this);
#else
switch (m_order) {
case qToUnderlying(QtPrivate::Ordering::Less): return std::partial_ordering::less;
case qToUnderlying(QtPrivate::Ordering::Greater): return std::partial_ordering::greater;
@ -273,6 +280,7 @@ public:
case qToUnderlying(QtPrivate::Uncomparable::Unordered): return std::partial_ordering::unordered;
}
Q_UNREACHABLE_RETURN(std::partial_ordering::unordered);
#endif // __cpp_lib_bit_cast
}
friend constexpr bool operator==(partial_ordering lhs, std::partial_ordering rhs) noexcept
@ -414,12 +422,17 @@ public:
constexpr Q_IMPLICIT operator std::weak_ordering() const noexcept
{
static_assert(sizeof(*this) == sizeof(std::weak_ordering));
#ifdef __cpp_lib_bit_cast
return std::bit_cast<std::weak_ordering>(*this);
#else
switch (m_order) {
case qToUnderlying(QtPrivate::Ordering::Less): return std::weak_ordering::less;
case qToUnderlying(QtPrivate::Ordering::Greater): return std::weak_ordering::greater;
case qToUnderlying(QtPrivate::Ordering::Equivalent): return std::weak_ordering::equivalent;
}
Q_UNREACHABLE_RETURN(std::weak_ordering::equivalent);
#endif // __cpp_lib_bit_cast
}
friend constexpr bool operator==(weak_ordering lhs, std::weak_ordering rhs) noexcept
@ -593,12 +606,17 @@ public:
constexpr Q_IMPLICIT operator std::strong_ordering() const noexcept
{
static_assert(sizeof(*this) == sizeof(std::strong_ordering));
#ifdef __cpp_lib_bit_cast
return std::bit_cast<std::strong_ordering>(*this);
#else
switch (m_order) {
case qToUnderlying(QtPrivate::Ordering::Less): return std::strong_ordering::less;
case qToUnderlying(QtPrivate::Ordering::Greater): return std::strong_ordering::greater;
case qToUnderlying(QtPrivate::Ordering::Equal): return std::strong_ordering::equal;
}
Q_UNREACHABLE_RETURN(std::strong_ordering::equal);
#endif // __cpp_lib_bit_cast
}
friend constexpr bool operator==(strong_ordering lhs, std::strong_ordering rhs) noexcept