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qCheckedFPConversionToInteger: streamline the checks

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Use the same algorithm as convertToDouble. We don't really need
convertToDouble, as we plan to truncate anyhow (convertToDouble returns
`false` if truncation caused loss of precision, so we can't use it to
distinguish an out-of-bounds value), and these are debug-only checks.

Change-Id: I70d70c05ce3f4996bd74361e6f78c02064232a57
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
This commit is contained in:
Giuseppe D'Angelo 2025-04-03 12:14:54 +02:00
parent 07b8909360
commit a99d73c53a
1 changed files with 19 additions and 11 deletions
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30
src/corelib/global/qnumeric.h
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@ -494,19 +494,27 @@ template <typename Result,
typename std::enable_if_t<std::is_floating_point_v<FP>, bool> = true>
constexpr inline Result qCheckedFPConversionToInteger(FP value)
{
// GCC always has constexpr cmath
#if !defined(__cpp_lib_constexpr_cmath) && !defined(Q_CC_GNU_ONLY)
# ifdef QT_SUPPORTS_IS_CONSTANT_EVALUATED
#ifdef QT_SUPPORTS_IS_CONSTANT_EVALUATED
if (!q20::is_constant_evaluated())
# else
if (false)
# endif
Q_ASSERT(!std::isnan(value));
#endif
{
const FP truncatedValue = std::trunc(value);
Q_ASSERT(truncatedValue >= FP((std::numeric_limits<Result>::min)()));
Q_ASSERT(truncatedValue <= FP((std::numeric_limits<Result>::max)()));
}
constexpr Result minimal = (std::numeric_limits<Result>::min)();
constexpr Result maximal = (std::numeric_limits<Result>::max)();
// We want to check that `value > minimal-1`. `minimal` is
// precisely representable as FP (it's -2^N), but `minimal-1`
// may not be. Just rearrange the terms:
Q_ASSERT(value - FP(minimal) > FP(-1));
// Symmetrically, `maximal` may not have a precise
// representation, but `maximal+1` has, so calculate that:
constexpr FP maximalPlusOne = FP(2) * (maximal / 2 + 1);
// And check that we're below that:
Q_ASSERT(value < maximalPlusOne);
// If both checks passed, the result of truncation is representable
// as `Result`:
return Result(value);
}