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bitcoin/depends/patches/qt/fix-gcc16-qcompare.patch
Cory Fields d7e972a90d qt: add patch to fix build with gcc16 
Backported from:
7fccc79dd5
2026-03-04 11:31:23 +00:00

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From 7fccc79dd5744ea837ffe200bbfc9f2756870220 Mon Sep 17 00:00:00 2001
From: Thiago Macieira <thiago.macieira@intel.com>
Date: Wed, 14 Jan 2026 08:57:22 -0800
Subject: [PATCH] QtCompare: adapt to GCC 16 breaking ABI for
std::partial_ordering
They changed[1] the stored value of the unordered constant from 2 to
-SCHAR_MAX-1 (0x80). This commit hardens the check for all three
std::*_ordering types and in all compilers, even though only the
unordered value has been observed to be a problem.
For:
std::partial_ordering f(Qt::partial_ordering o) { return o; }
With GCC < 16, this remains a plain bitcast. For GCC >= 16, this now
generates:
cmpb $2, %dil
movl $-128, %eax
cmovne %edi, %eax
Hopefully, when this is used in context, the comparisons will be elided
through constant propagation.
[1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=fcb3009a32dc33906934a2360e556dfeb98980cf
Pick-to: 6.11 6.10 6.8 6.5
Change-Id: I04bc40f0aeef5c0c778dfffdaea0bf68c5719462
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
---
src/corelib/global/qcompare.cpp | 2 +
src/corelib/global/qcompare.h | 60 ++++++++++++++-----
.../corelib/global/qcompare/tst_qcompare.cpp | 2 +
3 files changed, 50 insertions(+), 14 deletions(-)
diff --git a/qtbase/src/corelib/global/qcompare.cpp b/qtbase/src/corelib/global/qcompare.cpp
index 8106a84ddcd7..7f7facafbb87 100644
--- a/qtbase/src/corelib/global/qcompare.cpp
+++ b/qtbase/src/corelib/global/qcompare.cpp
@@ -18,7 +18,9 @@ QT_BEGIN_NAMESPACE
static_assert(std::bit_cast<std:: type ## _ordering>(Qt:: type ## _ordering:: flag) \
== std:: type ## _ordering :: flag) \
/* end */
+#if !defined(__GLIBCXX__)
CHECK(partial, unordered);
+#endif
CHECK(partial, less);
CHECK(partial, greater);
CHECK(partial, equivalent);
diff --git a/qtbase/src/corelib/global/qcompare.h b/qtbase/src/corelib/global/qcompare.h
index d82cf5ab4a4e..7eee69db66a3 100644
--- a/qtbase/src/corelib/global/qcompare.h
+++ b/qtbase/src/corelib/global/qcompare.h
@@ -36,6 +36,15 @@ enum class Ordering : CompareUnderlyingType
enum class Uncomparable : CompareUnderlyingType
{
+ // We choose the value of our Uncomparable to be the same that the C++
+ // Standard Library chooses for its own std::partial_ordering::unordered,
+ // so we can convert from their type to ours via simple std::bit_cast.
+#if 0
+ // GCC 16 broke ABI, so we cannot use the std::*_ordering types
+ // in our ABI until we drop support for GCC 15 and earlier. When that
+ // happens and std::bit_cast is guaranteed, this can be simplified to:
+ Unordered = std::bit_cast<CompareUnderlyingType>(std::partial_ordering::unordered);
+#else
Unordered =
#if defined(_LIBCPP_VERSION) // libc++
-127
@@ -44,12 +53,26 @@ enum class Uncomparable : CompareUnderlyingType
#else // assume MSSTL
-128
#endif
+#endif // future Qt
};
} // namespace QtPrivate
namespace QtOrderingPrivate {
+using QtPrivate::Ordering;
+using QtPrivate::Uncomparable;
+
+#if defined(__cpp_lib_bit_cast) && defined(__cpp_lib_three_way_comparison)
+inline constexpr bool OrderingValuesAreEqual =
+ std::bit_cast<Ordering>(std::weak_ordering::equivalent) == Ordering::Equivalent &&
+ std::bit_cast<Ordering>(std::strong_ordering::equal) == Ordering::Equal &&
+ std::bit_cast<Ordering>(std::strong_ordering::less) == Ordering::Less &&
+ std::bit_cast<Ordering>(std::strong_ordering::greater) == Ordering::Greater;
+inline constexpr bool UnorderedValueIsEqual =
+ std::bit_cast<Uncomparable>(std::partial_ordering::unordered) == Uncomparable::Unordered;
+#endif
+
template <typename O>
constexpr O reversed(O o) noexcept
{
@@ -61,6 +84,9 @@ constexpr O reversed(O o) noexcept
} // namespace QtOrderingPrivate
+QT_WARNING_PUSH
+QT_WARNING_DISABLE_MSVC(4702) // unreachable code
+
namespace Qt {
class weak_ordering;
@@ -157,12 +183,18 @@ 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
using O = QtPrivate::Ordering;
using U = QtPrivate::Uncomparable;
using R = std::partial_ordering;
+#ifdef __cpp_lib_bit_cast
+ if constexpr (QtOrderingPrivate::OrderingValuesAreEqual) {
+ if constexpr (!QtOrderingPrivate::UnorderedValueIsEqual) {
+ if (m_order == qToUnderlying(U::Unordered))
+ return R::unordered;
+ }
+ return std::bit_cast<R>(*this);
+ }
+#endif // __cpp_lib_bit_cast
switch (m_order) {
case qToUnderlying(O::Less): return R::less;
case qToUnderlying(O::Greater): return R::greater;
@@ -170,7 +202,6 @@ public:
case qToUnderlying(U::Unordered): return R::unordered;
}
Q_UNREACHABLE_RETURN(R::unordered);
-#endif // __cpp_lib_bit_cast
}
friend constexpr bool operator==(partial_ordering lhs, std::partial_ordering rhs) noexcept
@@ -349,18 +380,18 @@ 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
using O = QtPrivate::Ordering;
using R = std::weak_ordering;
+#ifdef __cpp_lib_bit_cast
+ if constexpr (QtOrderingPrivate::OrderingValuesAreEqual)
+ return std::bit_cast<R>(*this);
+#endif // __cpp_lib_bit_cast
switch (m_order) {
case qToUnderlying(O::Less): return R::less;
case qToUnderlying(O::Greater): return R::greater;
case qToUnderlying(O::Equivalent): return R::equivalent;
}
Q_UNREACHABLE_RETURN(R::equivalent);
-#endif // __cpp_lib_bit_cast
}
friend constexpr bool operator==(weak_ordering lhs, std::weak_ordering rhs) noexcept
@@ -547,18 +578,18 @@ 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
using O = QtPrivate::Ordering;
using R = std::strong_ordering;
+#ifdef __cpp_lib_bit_cast
+ if constexpr (QtOrderingPrivate::OrderingValuesAreEqual)
+ return std::bit_cast<R>(*this);
+#endif // __cpp_lib_bit_cast
switch (m_order) {
case qToUnderlying(O::Less): return R::less;
case qToUnderlying(O::Greater): return R::greater;
case qToUnderlying(O::Equal): return R::equal;
}
Q_UNREACHABLE_RETURN(R::equal);
-#endif // __cpp_lib_bit_cast
}
friend constexpr bool operator==(strong_ordering lhs, std::strong_ordering rhs) noexcept
@@ -625,6 +656,8 @@ inline constexpr strong_ordering strong_ordering::greater(QtPrivate::Ordering::G
} // namespace Qt
+QT_WARNING_POP
+
QT_BEGIN_INCLUDE_NAMESPACE
// This is intentionally included after Qt::*_ordering types and before
diff --git a/qtbase/tests/auto/corelib/global/qcompare/tst_qcompare.cpp b/qtbase/tests/auto/corelib/global/qcompare/tst_qcompare.cpp
index b79a6661db33..ff5920134cc8 100644
--- a/qtbase/tests/auto/corelib/global/qcompare/tst_qcompare.cpp
+++ b/qtbase/tests/auto/corelib/global/qcompare/tst_qcompare.cpp
@@ -185,7 +185,9 @@ void tst_QCompare::stdQtBinaryCompatibility()
QCOMPARE_EQ(valueOf( Qt:: type ## _ordering :: flag), \
valueOf(std:: type ## _ordering :: flag)) \
/* end */
+# if !defined(__GLIBCXX__) || QT_VERSION >= QT_VERSION_CHECK(7, 0, 0)
CHECK(partial, unordered);
+# endif
CHECK(partial, less);
CHECK(partial, greater);
CHECK(partial, equivalent);
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