QHash: double the size of the stored seed

There's now another half of the seed which will be used by the hashers.
This is not stored in QHash, so it is never changed for the lifetime of
the application (not even when QHashSeed::setDeterministicGlobalSeed()
is called). However, we will not use it when we're in deterministic
mode.

This commit uses the compiler thread-safe statics to implement the
initialization of more than one atomic word, thus freeing us from having
to have a reserved value. As a bonus, the QT_HASH_SEED warning will only
be printed once.

Change-Id: Id2983978ad544ff79911fffd16723f1673f9a5b4
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

1 files changed, 12 insertions, 0 deletions

diff --git a/tests/auto/corelib/tools/qhashseed/tst_qhashseed.cpp b/tests/auto/corelib/tools/qhashseed/tst_qhashseed.cpp
index 60f3fe9e3c..3d3b529f29 100644
--- a/tests/auto/corelib/tools/qhashseed/tst_qhashseed.cpp
+++ b/tests/auto/corelib/tools/qhashseed/tst_qhashseed.cpp
@@ -134,8 +134,12 @@ void tst_QHashSeed::reseeding()
 
 void tst_QHashSeed::quality()
 {
+    // this "bad seed" is used internally in qhash.cpp and should never leak!
+    constexpr size_t BadSeed = size_t(Q_UINT64_C(0x5555'5555'5555'5555));
+
     constexpr int Iterations = 16;
     int oneThird = 0;
+    int badSeeds = 0;
     size_t ored = 0;
 
     for (int i = 0; i < Iterations; ++i) {
@@ -146,6 +150,8 @@ void tst_QHashSeed::quality()
 
         if (bits >= std::numeric_limits<size_t>::digits / 3)
             ++oneThird;
+        if (seed == BadSeed)
+            ++badSeeds;
     }
 
     // report out
@@ -161,6 +167,12 @@ void tst_QHashSeed::quality()
 
     // at least one third of the seeds must have one third of all the bits set
     QVERIFY(oneThird > (16/3));
+
+    // at most one seed can be the bad seed, if 32-bit, none on 64-bit
+    if (std::numeric_limits<size_t>::digits > 32)
+        QCOMPARE(badSeeds, 0);
+    else
+        QVERIFY(badSeeds <= 1);
 }
 
 #if QT_VERSION < QT_VERSION_CHECK(7, 0, 0)