Featurize support for signaling NaN

One of our compilers for emscripten coerces all signaling NaNs to
quiet ones, so won't do any actual signaling. Anyone relying on them
to do so shall be disappointed, so it's better that they know about it
at compile-time - or, at least, have the ability to find it out.

Put the signaling NaN producers (and remaining (test) code using them)
under the control of a feature that's disabled when numeric_limits
claims double has no signaling NaN. Assume the bootstrap library
doesn't need signaling NaNs. Sadly, until C++20 <bit>, there's no
contexpr way to test that alleged signalling and quiet NaNs are
actually distinct.

Added some auto-tests for signaling NaN, including that it's distinct
from quiet NaN. Any platform on which the last fails should disable
this feature.

Task-number: QTBUG-77967
Change-Id: I57e9d14bfe276732cd313887adc9acc354d88f08
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

2 files changed, 17 insertions, 0 deletions

diff --git a/tests/auto/corelib/global/qnumeric/tst_qnumeric.cpp b/tests/auto/corelib/global/qnumeric/tst_qnumeric.cpp
index 5697d21547..a6d600e125 100644
--- a/tests/auto/corelib/global/qnumeric/tst_qnumeric.cpp
+++ b/tests/auto/corelib/global/qnumeric/tst_qnumeric.cpp
@@ -44,6 +44,9 @@ private slots:
     void fuzzyCompare();
     void rawNaN_data();
     void rawNaN();
+#if QT_CONFIG(signaling_nan)
+    void distinctNaN();
+#endif
     void generalNaN_data();
     void generalNaN();
     void infinity();
@@ -139,6 +142,9 @@ void tst_QNumeric::rawNaN_data()
     QTest::addColumn<double>("nan");
 
     QTest::newRow("quiet") << qQNaN();
+#if QT_CONFIG(signaling_nan)
+    QTest::newRow("signaling") << qSNaN();
+#endif
 }
 
 void tst_QNumeric::rawNaN()
@@ -147,6 +153,15 @@ void tst_QNumeric::rawNaN()
     checkNaN(nan);
 }
 
+#if QT_CONFIG(signaling_nan)
+void tst_QNumeric::distinctNaN()
+{
+    const double qnan = qQNaN();
+    const double snan = qSNaN();
+    QVERIFY(memcmp(&qnan, &snan, sizeof(double)) != 0);
+}
+#endif
+
 void tst_QNumeric::generalNaN_data()
 {
     QTest::addColumn<int>("most");
diff --git a/tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp b/tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp
index 6fa82ea681..c6733205e5 100644
--- a/tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp
+++ b/tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp
@@ -1413,10 +1413,12 @@ void tst_QCborValue::toCbor_data()
     // The rest of these tests are conversions whose decoding does not yield
     // back the same QCborValue.
 
+#if QT_CONFIG(signaling_nan)
     // Signalling NaN get normalized to quiet ones
     QTest::newRow("Double:snan") << QCborValue(qSNaN()) << raw("\xfb\x7f\xf8\0""\0\0\0\0\0") << QCborValue::EncodingOptions();
     QTest::newRow("Float:snan") << QCborValue(qSNaN()) << raw("\xfa\x7f\xc0\0\0") << QCborValue::EncodingOptions(QCborValue::UseFloat);
     QTest::newRow("Float16:snan") << QCborValue(qSNaN()) << raw("\xf9\x7e\0") << QCborValue::EncodingOptions(QCborValue::UseFloat16);
+#endif
 
     // Floating point written as integers are read back as integers
     QTest::newRow("UseInteger:0") << QCborValue(0.) << raw("\x00") << QCborValue::EncodingOptions(QCborValue::UseIntegers);