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convertDoubleTo: move the precision upgrade test up

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Hopefully, the compiler will realize that the suprema calculated below
are actually bigger than these limits and make the appropriate dead code
eliminations.

Change-Id: I89446ea06b5742efb194fffd16bb99f78b26eb0e
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
This commit is contained in:
Thiago Macieira 2021-11-27 20:02:57 -08:00
parent dd3c4cb9bf
commit 0ca6ad8cfc
1 changed files with 13 additions and 4 deletions
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17
src/corelib/global/qnumeric_p.h
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@ -189,6 +189,19 @@ template<typename T>
static inline bool convertDoubleTo(double v, T *value, bool allow_precision_upgrade = true) static inline bool convertDoubleTo(double v, T *value, bool allow_precision_upgrade = true)
{ {
static_assert(std::numeric_limits<T>::is_integer); static_assert(std::numeric_limits<T>::is_integer);
static_assert(std::is_integral_v<T>);
constexpr bool TypeIsLarger = std::numeric_limits<T>::digits > std::numeric_limits<double>::digits;
if constexpr (TypeIsLarger) {
using S = std::make_signed_t<T>;
constexpr S max_mantissa = S(1) << std::numeric_limits<double>::digits;
// T has more bits than double's mantissa, so don't allow "upgrading"
// to T (makes it look like the number had more precision than really
// was transmitted)
if (!allow_precision_upgrade && (v > double(max_mantissa) || v < double(-max_mantissa - 1)))
return false;
}
// The [conv.fpint] (7.10 Floating-integral conversions) section of the C++ // The [conv.fpint] (7.10 Floating-integral conversions) section of the C++
// standard says only exact conversions are guaranteed. Converting // standard says only exact conversions are guaranteed. Converting
@ -211,10 +224,6 @@ static inline bool convertDoubleTo(double v, T *value, bool allow_precision_upgr
supremum = -2.0 * std::numeric_limits<ST>::min(); // -2 * (-2^63) = 2^64, exact (for T = quint64) supremum = -2.0 * std::numeric_limits<ST>::min(); // -2 * (-2^63) = 2^64, exact (for T = quint64)
v = fabs(v); v = fabs(v);
} }
if (std::is_integral<T>::value && sizeof(T) > 4 && !allow_precision_upgrade) {
if (v > double(Q_INT64_C(1)<<53) || v < double(-((Q_INT64_C(1)<<53) + 1)))
return false;
}
*value = std::numeric_limits<T>::max(); *value = std::numeric_limits<T>::max();
if (v >= supremum) if (v >= supremum)