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Introduce always constexpr variants of qCountTrailingZeroBits

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qt_builtin_ctz is unfortunately not constexpr on MSVC, so the whole
function cannot be used in constexpr contexts, unless v is exactly 0. As we
don't have std::is_costant_evaluated until C++20, we need to introduce a new
helper function, which always works in constexpr contexts.

Change-Id: I36290acb61e430d6bdb3da112a9b2263be4b9fbf
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
This commit is contained in:
Fabian Kosmale 2020-03-09 11:11:32 +01:00
parent cb8eb0f86c
commit e464e1eb8e
1 changed files with 46 additions and 19 deletions
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65
src/corelib/tools/qalgorithms.h
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@ -744,11 +744,9 @@ Q_DECL_CONST_FUNCTION QT_POPCOUNT_CONSTEXPR inline uint qPopulationCount(long un
#endif
#undef QT_POPCOUNT_CONSTEXPR
Q_DECL_RELAXED_CONSTEXPR inline uint qCountTrailingZeroBits(quint32 v) noexcept
namespace QtPrivate {
constexpr inline uint qConstexprCountTrailingZeroBits(quint32 v) noexcept
{
#if defined(QT_HAS_BUILTIN_CTZ)
return v ? QAlgorithmsPrivate::qt_builtin_ctz(v) : 32U;
#else
// see http://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightParallel
unsigned int c = 32; // c will be the number of zero bits on the right
v &= -signed(v);
@ -759,14 +757,17 @@ Q_DECL_RELAXED_CONSTEXPR inline uint qCountTrailingZeroBits(quint32 v) noexcept
if (v & 0x33333333) c -= 2;
if (v & 0x55555555) c -= 1;
return c;
#endif
}
Q_DECL_RELAXED_CONSTEXPR inline uint qCountTrailingZeroBits(quint8 v) noexcept
constexpr inline uint qConstexprCountTrailingZeroBits(quint64 v) noexcept
{
quint32 x = static_cast<quint32>(v);
return x ? qConstexprCountTrailingZeroBits(x)
: 32 + qConstexprCountTrailingZeroBits(static_cast<quint32>(v >> 32));
}
constexpr inline uint qConstexprCountTrailingZeroBits(quint8 v) noexcept
{
#if defined(QT_HAS_BUILTIN_CTZ)
return v ? QAlgorithmsPrivate::qt_builtin_ctz(v) : 8U;
#else
unsigned int c = 8; // c will be the number of zero bits on the right
v &= -signed(v);
if (v) c--;
@ -774,14 +775,10 @@ Q_DECL_RELAXED_CONSTEXPR inline uint qCountTrailingZeroBits(quint8 v) noexcept
if (v & 0x00000033) c -= 2;
if (v & 0x00000055) c -= 1;
return c;
#endif
}
Q_DECL_RELAXED_CONSTEXPR inline uint qCountTrailingZeroBits(quint16 v) noexcept
constexpr inline uint qConstexprCountTrailingZeroBits(quint16 v) noexcept
{
#if defined(QT_HAS_BUILTIN_CTZS)
return v ? QAlgorithmsPrivate::qt_builtin_ctzs(v) : 16U;
#else
unsigned int c = 16; // c will be the number of zero bits on the right
v &= -signed(v);
if (v) c--;
@ -790,21 +787,51 @@ Q_DECL_RELAXED_CONSTEXPR inline uint qCountTrailingZeroBits(quint16 v) noexcept
if (v & 0x00003333) c -= 2;
if (v & 0x00005555) c -= 1;
return c;
}
constexpr inline uint qConstexprCountTrailingZeroBits(unsigned long v) noexcept
{
return qConstexprCountTrailingZeroBits(QIntegerForSizeof<long>::Unsigned(v));
}
}
constexpr inline uint qCountTrailingZeroBits(quint32 v) noexcept
{
#if defined(QT_HAS_BUILTIN_CTZ)
return v ? QAlgorithmsPrivate::qt_builtin_ctz(v) : 32U;
#else
return QtPrivate::qConstexprCountTrailingZeroBits(v);
#endif
}
Q_DECL_RELAXED_CONSTEXPR inline uint qCountTrailingZeroBits(quint64 v) noexcept
constexpr inline uint qCountTrailingZeroBits(quint8 v) noexcept
{
#if defined(QT_HAS_BUILTIN_CTZ)
return v ? QAlgorithmsPrivate::qt_builtin_ctz(v) : 8U;
#else
return QtPrivate::qConstexprCountTrailingZeroBits(v);
#endif
}
constexpr inline uint qCountTrailingZeroBits(quint16 v) noexcept
{
#if defined(QT_HAS_BUILTIN_CTZS)
return v ? QAlgorithmsPrivate::qt_builtin_ctzs(v) : 16U;
#else
return QtPrivate::qConstexprCountTrailingZeroBits(v);
#endif
}
constexpr inline uint qCountTrailingZeroBits(quint64 v) noexcept
{
#if defined(QT_HAS_BUILTIN_CTZLL)
return v ? QAlgorithmsPrivate::qt_builtin_ctzll(v) : 64;
#else
quint32 x = static_cast<quint32>(v);
return x ? qCountTrailingZeroBits(x)
: 32 + qCountTrailingZeroBits(static_cast<quint32>(v >> 32));
return QtPrivate::qConstexprCountTrailingZeroBits(v);
#endif
}
Q_DECL_RELAXED_CONSTEXPR inline uint qCountTrailingZeroBits(unsigned long v) noexcept
constexpr inline uint qCountTrailingZeroBits(unsigned long v) noexcept
{
return qCountTrailingZeroBits(QIntegerForSizeof<long>::Unsigned(v));
}