QHash: suppress annoying GCC warning about allocation size too big

GCC is right, the maximum memory allocation is half the VM size,
not the full VM size or multiple times that. QHashPrivate::Span
is big.

 qhash.h:552:17: warning: argument 1 value ‘18446744073709551615’ exceeds maximum object size 9223372036854775807 [-Walloc-size-larger-than=]

Change-Id: Ie72b0dd0fbe84d2caae0fffd16a071ffb5d0c70f
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 31 insertions, 20 deletions

diff --git a/src/corelib/tools/qhash.h b/src/corelib/tools/qhash.h
index 9145891faf..a738865a17 100644
--- a/src/corelib/tools/qhash.h
+++ b/src/corelib/tools/qhash.h
@@ -102,26 +102,6 @@ size_t calculateHash(const T &t, size_t seed = 0)
     }
 }
 
-// QHash uses a power of two growth policy.
-namespace GrowthPolicy {
-inline constexpr size_t maxNumBuckets() noexcept
-{
-    return size_t(1) << (8 * sizeof(size_t) - 1);
-}
-inline constexpr size_t bucketsForCapacity(size_t requestedCapacity) noexcept
-{
-    if (requestedCapacity <= 8)
-        return 16;
-    if (requestedCapacity >= maxNumBuckets())
-        return maxNumBuckets();
-    return qNextPowerOfTwo(QIntegerForSize<sizeof(size_t)>::Unsigned(2 * requestedCapacity - 1));
-}
-inline constexpr size_t bucketForHash(size_t nBuckets, size_t hash) noexcept
-{
-    return hash & (nBuckets - 1);
-}
-}
-
 template <typename Key, typename T>
 struct Node
 {
@@ -453,6 +433,37 @@ struct Span {
     }
 };
 
+// QHash uses a power of two growth policy.
+namespace GrowthPolicy {
+inline constexpr size_t maxNumBuckets() noexcept
+{
+    // ensure the size of a Span does not depend on the template parameters
+    using Node1 = Node<int, int>;
+    using Node2 = Node<char, void *>;
+    using Node3 = Node<qsizetype, QHashDummyValue>;
+    static_assert(sizeof(Span<Node1>) == sizeof(Span<Node2>));
+    static_assert(sizeof(Span<Node1>) == sizeof(Span<Node3>));
+    static_assert(int(Span<Node1>::NEntries) == int(Span<Node2>::NEntries));
+    static_assert(int(Span<Node1>::NEntries) == int(Span<Node3>::NEntries));
+
+    // Maximum is 2^31-1 or 2^63-1 bytes (limited by qsizetype and ptrdiff_t)
+    size_t max = (std::numeric_limits<ptrdiff_t>::max)();
+    return max / sizeof(Span<Node1>) * Span<Node1>::NEntries;
+}
+inline constexpr size_t bucketsForCapacity(size_t requestedCapacity) noexcept
+{
+    if (requestedCapacity <= 8)
+        return 16;
+    if (requestedCapacity >= maxNumBuckets())
+        return maxNumBuckets();
+    return qNextPowerOfTwo(QIntegerForSize<sizeof(size_t)>::Unsigned(2 * requestedCapacity - 1));
+}
+inline constexpr size_t bucketForHash(size_t nBuckets, size_t hash) noexcept
+{
+    return hash & (nBuckets - 1);
+}
+} // namespace GrowthPolicy
+
 template <typename Node>
 struct iterator;