1berry/qtbase
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

convertDoubleTo: move the precision upgrade test up

Browse Source 
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
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
src/corelib/global/qnumeric_p.h
View File

@ -189,6 +189,19 @@ template<typename T>
static inline bool convertDoubleTo(double v, T *value, bool allow_precision_upgrade = true)
{
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++
// 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)
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();
if (v >= supremum)