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QRect: fix UB (int overflow) in center()

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QRect::center() should be defined for any
  QRect(x1,y1,x2,x2), INT_MIN <= x1, x2, y1, y2 <= INT_MAX
because the average of two signed integers is always
representable as a signed integer.

But not when it's calculated as (x1+x2)/2, since that
expression overflows when x1 > INT_MAX - x2.

Instead of playing games with Hacker's Delight-style
expressions, or use Google's patented algorithm, which
requires two divisions, take advantage of the fact that
int is not intmax_t and perform the calculation in the
qint64 domain. The cast back to int is always well-
defined since, as mentioned, the result is always
representable in an int.

Fix a test-case that expected a nonsensical result due
to overflow.

[ChangeLog][QtCore][QRect] Fixed integer overflow in
center(). This fixes the result for some corner-cases
like a 1x1 rectangle at (INT_MIN, INT_MIN), for which
the previous implementation could return anything
(due to invoking undefined behavior), but commonly
returned (0, 0).

Change-Id: I1a885ca6dff770327dd31655c3eb473fcfeb8878
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
This commit is contained in:
Marc Mutz 2016-03-14 15:18:11 +01:00
parent 1dcc53f6fa
commit e4c6d73f92
2 changed files with 2 additions and 2 deletions
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2
src/corelib/tools/qrect.h
View File

@ -245,7 +245,7 @@ Q_DECL_CONSTEXPR inline QPoint QRect::bottomLeft() const Q_DECL_NOTHROW
{ return QPoint(x1, y2); }
Q_DECL_CONSTEXPR inline QPoint QRect::center() const Q_DECL_NOTHROW
{ return QPoint((x1+x2)/2, (y1+y2)/2); }
{ return QPoint(int((qint64(x1)+x2)/2), int((qint64(y1)+y2)/2)); } // cast avoids overflow on addition
Q_DECL_CONSTEXPR inline int QRect::width() const Q_DECL_NOTHROW
{ return x2 - x1 + 1; }

2
tests/auto/corelib/tools/qrect/tst_qrect.cpp
View File

@ -2349,7 +2349,7 @@ void tst_QRect::center_data()
QTest::newRow( "SmallestQRect" ) << getQRectCase( SmallestQRect ) << QPoint(1,1);
QTest::newRow( "MiddleQRect" ) << getQRectCase( MiddleQRect ) << QPoint(0,0);
QTest::newRow( "LargestQRect" ) << getQRectCase( LargestQRect ) << QPoint(INT_MAX/2,INT_MAX/2);
QTest::newRow( "SmallestCoordQRect" ) << getQRectCase( SmallestCoordQRect ) << QPoint(0,0);
QTest::newRow( "SmallestCoordQRect" ) << getQRectCase( SmallestCoordQRect ) << QPoint(INT_MIN, INT_MIN);
QTest::newRow( "LargestCoordQRect" ) << getQRectCase( LargestCoordQRect ) << QPoint(0,0);
QTest::newRow( "RandomQRect" ) << getQRectCase( RandomQRect ) << QPoint(105,207);
QTest::newRow( "NegativeSizeQRect" ) << getQRectCase( NegativeSizeQRect ) << QPoint(-4,-4);