Add qfloat16::copySign() since we can't overload std::copysign()

Change-Id: Idfaf841b3eb3538f076ae4f0de2d7d029e1588fe Reviewed-by: Thiago Macieira <[email protected]>
dougcooper · Nov 6, 2019 · 7bf4f81 · 7bf4f81
1 parent b7858e9
commit 7bf4f81
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diff --git a/src/corelib/global/qfloat16.cpp b/src/corelib/global/qfloat16.cpp
@@ -145,6 +145,14 @@ QT_BEGIN_NAMESPACE
     \sa qIsFinite()
 */
 
+/*!
+    \since 5.15
+    \fn qfloat16::copySign(qfloat16 sign) const noexcept
+
+    Returns a qfloat16 with the sign of \a sign but the rest of its value taken
+    from this qfloat16. Serves as qfloat16's equivalent of std::copysign().
+*/
+
 /*!
     \internal
     \since 5.14

diff --git a/src/corelib/global/qfloat16.h b/src/corelib/global/qfloat16.h
@@ -84,6 +84,9 @@ class qfloat16
     bool isNaN() const noexcept { return ((b16 >> 8) & 0x7e) == 0x7e; }
     bool isFinite() const noexcept { return ((b16 >> 8) & 0x7c) != 0x7c; }
     Q_CORE_EXPORT int fpClassify() const noexcept;
+    // Can't specialize std::copysign() for qfloat16
+    qfloat16 copySign(qfloat16 sign) const noexcept
+    { return qfloat16(Wrap((sign.b16 & 0x8000) | (b16 & 0x7fff))); }
     // Support for std::numeric_limits<qfloat16>
     static constexpr qfloat16 _limit_epsilon()    noexcept { return qfloat16(Wrap(0x1400)); }
     static constexpr qfloat16 _limit_min()        noexcept { return qfloat16(Wrap(0x400)); }

diff --git a/tests/auto/corelib/global/qfloat16/tst_qfloat16.cpp b/tests/auto/corelib/global/qfloat16/tst_qfloat16.cpp
@@ -430,6 +430,13 @@ void tst_qfloat16::finite()
     QVERIFY(!qIsInf(value));
     QVERIFY(!qIsNaN(value));
     QCOMPARE(qFpClassify(value), mode);
+
+    // *NOT* using QCOMPARE() on finite qfloat16 values, since that uses fuzzy
+    // comparison, and we need exact here.
+    const qfloat16 zero(0), plus(+1), minus(-1);
+    const qfloat16 magnitude = (value < zero) ? -value : value;
+    QVERIFY(value.copySign(plus) == magnitude);
+    QVERIFY(value.copySign(minus) == -magnitude);
 }
 
 void tst_qfloat16::properties()
@@ -534,7 +541,9 @@ void tst_qfloat16::limits() // See also: qNaN() and infinity()
     QVERIFY(Bounds::denorm_min() / rose == zero);
     if (overOptimized)
         QEXPECT_FAIL("", "Over-optimized on ARM", Continue);
-    QVERIFY(-Bounds::denorm_min() / rose == -zero);
+    const qfloat16 under = (-Bounds::denorm_min()) / rose;
+    QVERIFY(under == -zero);
+    QCOMPARE(qfloat16(1).copySign(under), qfloat16(-1));
 }
 
 QTEST_APPLESS_MAIN(tst_qfloat16)