Make large inputs to qNextPowerOfTwo give undefined output

Fixing documentation and removing tests.

[ChangeLog][Important Behavior Changes] The qNextPowerOfTwo()
functions now have preconditions.

Change-Id: If6d5e8bee66826910e89be7cac388a1f0422ebfd
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

3 files changed, 6 insertions, 19 deletions

diff --git a/src/corelib/kernel/qmath.h b/src/corelib/kernel/qmath.h
index 2171a4ed93..ef0784a947 100644
--- a/src/corelib/kernel/qmath.h
+++ b/src/corelib/kernel/qmath.h
@@ -370,9 +370,8 @@ constexpr inline quint64 qConstexprNextPowerOfTwo(qint64 v)
 
 constexpr inline quint32 qNextPowerOfTwo(quint32 v)
 {
+    Q_ASSERT(static_cast<qint32>(v) >= 0); // There is a next power of two
 #if defined(__cpp_lib_int_pow2) && __cpp_lib_int_pow2 >= 202002L
-    if (static_cast<qint32>(v) < 0)
-        return 0; // std::bit_ceil() is undefined for values that would overflow, but we document them to be 0
     return std::bit_ceil(v + 1);
 #elif defined(QT_HAS_BUILTIN_CLZ)
     if (v == 0)
@@ -385,9 +384,8 @@ constexpr inline quint32 qNextPowerOfTwo(quint32 v)
 
 constexpr inline quint64 qNextPowerOfTwo(quint64 v)
 {
+    Q_ASSERT(static_cast<qint64>(v) >= 0); // There is a next power of two
 #if defined(__cpp_lib_int_pow2) && __cpp_lib_int_pow2 >= 202002L
-    if (static_cast<qint64>(v) < 0)
-        return 0; // std::bit_ceil() is undefined for values that would overflow, but we document them to be 0
     return std::bit_ceil(v + 1);
 #elif defined(QT_HAS_BUILTIN_CLZLL)
     if (v == 0)
diff --git a/src/corelib/kernel/qmath.qdoc b/src/corelib/kernel/qmath.qdoc
index 4c07e40765..7a55bf96e5 100644
--- a/src/corelib/kernel/qmath.qdoc
+++ b/src/corelib/kernel/qmath.qdoc
@@ -322,7 +322,7 @@
     \relates <QtMath>
     \since 5.4
 
-    This function returns the nearest power of two greater than \a value. For 0 it returns 1, and for values larger than or equal to 2^31 it returns 0.
+    This function returns the nearest power of two greater than \a value. For 0 it returns 1, and for values larger than or equal to 2^31 the result is undefined.
 */
 
 /*!
@@ -331,7 +331,7 @@
     \since 5.4
     \overload
 
-    This function returns the nearest power of two greater than \a value. For negative values it returns 0.
+    This function returns the nearest power of two greater than \a value. For negative values the result is undefined.
 */
 
 /*!
@@ -339,7 +339,7 @@
     \relates <QtMath>
     \since 5.4
 
-    This function returns the nearest power of two greater than \a value. For 0 it returns 1, and for values larger than or equal to 2^63 it returns 0.
+    This function returns the nearest power of two greater than \a value. For 0 it returns 1, and for values larger than or equal to 2^63 the result is undefined.
 */
 
 /*!
@@ -348,5 +348,5 @@
     \since 5.4
     \overload
 
-    This function returns the nearest power of two greater than \a value. For negative values it returns 0.
+    This function returns the nearest power of two greater than \a value. For negative values the result is undefined.
 */
diff --git a/tests/auto/corelib/kernel/qmath/tst_qmath.cpp b/tests/auto/corelib/kernel/qmath/tst_qmath.cpp
index a94b984d47..348b65e012 100644
--- a/tests/auto/corelib/kernel/qmath/tst_qmath.cpp
+++ b/tests/auto/corelib/kernel/qmath/tst_qmath.cpp
@@ -290,9 +290,6 @@ void tst_QMath::qNextPowerOfTwo32S_data()
     QTest::newRow("2^30") << (1 << 30) << (1U << 31);
     QTest::newRow("2^30 + 1") << (1 << 30) + 1 << (1U << 31);
     QTest::newRow("2^31 - 1") << 0x7FFFFFFF << (1U<<31);
-    QTest::newRow("-1") << -1 << 0U;
-    QTest::newRow("-128") << -128 << 0U;
-    QTest::newRow("-(2^31)") << int(0x80000000) << 0U;
 }
 
 void tst_QMath::qNextPowerOfTwo32S()
@@ -318,8 +315,6 @@ void tst_QMath::qNextPowerOfTwo32U_data()
     QTest::newRow("2^30") << (1U << 30) << (1U << 31);
     QTest::newRow("2^30 + 1") << (1U << 30) + 1 << (1U << 31);
     QTest::newRow("2^31 - 1") << 2147483647U << 2147483648U;
-    QTest::newRow("2^31") << 2147483648U << 0U;
-    QTest::newRow("2^31 + 1") << 2147483649U << 0U;
 }
 
 void tst_QMath::qNextPowerOfTwo32U()
@@ -346,10 +341,6 @@ void tst_QMath::qNextPowerOfTwo64S_data()
     QTest::newRow("2^31") << Q_INT64_C(2147483648) << Q_UINT64_C(0x100000000);
     QTest::newRow("2^31 + 1") << Q_INT64_C(2147483649) << Q_UINT64_C(0x100000000);
     QTest::newRow("2^63 - 1") << Q_INT64_C(0x7FFFFFFFFFFFFFFF) << Q_UINT64_C(0x8000000000000000);
-    QTest::newRow("-1") << Q_INT64_C(-1) << Q_UINT64_C(0);
-    QTest::newRow("-128") << Q_INT64_C(-128) << Q_UINT64_C(0);
-    QTest::newRow("-(2^31)") << -Q_INT64_C(0x80000000) << Q_UINT64_C(0);
-    QTest::newRow("-(2^63)") << (qint64)Q_INT64_C(0x8000000000000000) << Q_UINT64_C(0);
 }
 
 void tst_QMath::qNextPowerOfTwo64S()
@@ -373,8 +364,6 @@ void tst_QMath::qNextPowerOfTwo64U_data()
     QTest::newRow("65535") << Q_UINT64_C(65535) << Q_UINT64_C(65536);
     QTest::newRow("65536") << Q_UINT64_C(65536) << Q_UINT64_C(131072);
     QTest::newRow("2^63 - 1") << Q_UINT64_C(0x7FFFFFFFFFFFFFFF)  << Q_UINT64_C(0x8000000000000000);
-    QTest::newRow("2^63") << Q_UINT64_C(0x8000000000000000) << Q_UINT64_C(0);
-    QTest::newRow("2^63 + 1") << Q_UINT64_C(0x8000000000000001) << Q_UINT64_C(0);
 }
 
 void tst_QMath::qNextPowerOfTwo64U()