From 1c591fd9246ca776304a3c370dd2578bd886feac Mon Sep 17 00:00:00 2001 From: Edward Welbourne Date: Wed, 9 Sep 2020 14:59:32 +0200 Subject: Deal with {und,ov}erflow issues in QLine's length handling Use std::hypot() instead of sqrt() of a sum of squares. This ensures length() can't be zero when isNull() is false. Use length() in QLine::setLength() rather than duplicating that. Clarify and expand some documentation; isNull() never said what constituted validity, nor did unitVector() mention that is should not be used on a line for which isNull() is true. Make clear that lines of denormal length cannot be rescaled accurately. Given that we use fuzzy comparison to determine equality of end-points, isNull() can be false for a line with displacements less than sqrt(numeric_limits::denorm_min()) between the coordinates of its end-points (as long as these are not much bigger); squaring these would give zero, hence a zero length, where using hypot() avoids the underflow and gives a non-zero length. Having a zero length for a line with isNull() false would lead to problems in setLength(), which uses an isNull() pre-test, protecting a call to unitVector(). (It was already possible for a null line to have non-zero length; this now arises in more cases.) Restored QLine::setLength() to the form it had before a recent change to avoid division by zero (which resulted from underflow in computing the length of a non-null line) but allow for the possibility that the unit vector it computes as transient may not have length exactly one. Add tests against {ov,und}erflow problems in QLine. Reworked the test added during the divide-by-zero fix to make it part of the existing test. Pick-to: 5.15 5.12 Change-Id: I7b71d66b872ccc08a64e941acd36b45b0ea15fab Reviewed-by: Sze Howe Koh --- src/corelib/tools/qline.h | 13 +++++-------- 1 file changed, 5 insertions(+), 8 deletions(-) (limited to 'src/corelib/tools/qline.h') diff --git a/src/corelib/tools/qline.h b/src/corelib/tools/qline.h index adef545d86..8d57e09c4a 100644 --- a/src/corelib/tools/qline.h +++ b/src/corelib/tools/qline.h @@ -40,8 +40,6 @@ #ifndef QLINE_H #define QLINE_H -#include - #include QT_BEGIN_NAMESPACE @@ -372,15 +370,14 @@ constexpr inline QPointF QLineF::center() const return QPointF(0.5 * pt1.x() + 0.5 * pt2.x(), 0.5 * pt1.y() + 0.5 * pt2.y()); } -QT_WARNING_DISABLE_FLOAT_COMPARE - inline void QLineF::setLength(qreal len) { - const qreal oldlength = qSqrt(dx() * dx() + dy() * dy()); - if (!oldlength) + if (isNull()) return; - const qreal factor = len / oldlength; - pt2 = QPointF(pt1.x() + dx() * factor, pt1.y() + dy() * factor); + Q_ASSERT(length() > 0); + const QLineF v = unitVector(); + len /= v.length(); // In case it's not quite exactly 1. + pt2 = QPointF(pt1.x() + len * v.dx(), pt1.y() + len * v.dy()); } constexpr inline QPointF QLineF::pointAt(qreal t) const -- cgit v1.2.3