rhi: Enable serializing a layout description without baking an srb

Pick-to: 6.2
Change-Id: I66d28cc9d5417bcd5d192fa100c21f69fd42fd6b
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

src/gui/rhi/qrhi.cpp

@@ -2996,7 +2996,7 @@ QRhiResource::Type QRhiTextureRenderTarget::resourceType() const
 QRhiShaderResourceBindings::QRhiShaderResourceBindings(QRhiImplementation *rhi)
     : QRhiResource(rhi)
 {
-    m_layoutDesc.reserve(BINDING_PREALLOC * LAYOUT_DESC_FIELD_COUNT);
+    m_layoutDesc.reserve(BINDING_PREALLOC * QRhiShaderResourceBinding::LAYOUT_DESC_ENTRIES_PER_BINDING);
 }
 
 /*!
@@ -3047,13 +3047,12 @@ void QRhiImplementation::updateLayoutDesc(QRhiShaderResourceBindings *srb)
 {
     srb->m_layoutDescHash = 0;
     srb->m_layoutDesc.clear();
+    auto layoutDescAppender = std::back_inserter(srb->m_layoutDesc);
     for (const QRhiShaderResourceBinding &b : qAsConst(srb->m_bindings)) {
         const QRhiShaderResourceBinding::Data *d = b.data();
-        // the logic must match QRhiShaderResourceBinding::isLayoutCompatible()
+        srb->m_layoutDescHash ^= uint(d->binding) ^ uint(d->stage) ^ uint(d->type)
-        const int count = d->type == QRhiShaderResourceBinding::SampledTexture ? d->u.stex.count : 1;
+            ^ uint(d->type == QRhiShaderResourceBinding::SampledTexture ? d->u.stex.count : 1);
-        // the number of entries here should match LAYOUT_DESC_FIELD_COUNT
+        d->serialize(layoutDescAppender);
-        srb->m_layoutDescHash ^= uint(d->binding) ^ uint(d->stage) ^ uint(d->type) ^ uint(count);
-        srb->m_layoutDesc << uint(d->binding) << uint(d->stage) << uint(d->type) << uint(count);
     }
 }
 

src/gui/rhi/qrhi_p.h

@@ -411,11 +411,34 @@ public:
             StorageImageData simage;
             StorageBufferData sbuf;
         } u;
+
+        template<typename Output>
+        void serialize(Output dst) const
+        {
+            // must write out exactly LAYOUT_DESC_ENTRIES_PER_BINDING elements here
+            *dst++ = quint32(binding);
+            *dst++ = quint32(stage);
+            *dst++ = quint32(type);
+            *dst++ = quint32(type == QRhiShaderResourceBinding::SampledTexture ? u.stex.count : 1);
+        }
     };
 
     Data *data() { return &d; }
     const Data *data() const { return &d; }
 
+    static const int LAYOUT_DESC_ENTRIES_PER_BINDING = 4;
+
+    template<typename Output>
+    static void serializeLayoutDescription(const QRhiShaderResourceBinding *first,
+                                           const QRhiShaderResourceBinding *last,
+                                           Output dst)
+    {
+        while (first != last) {
+            first->data()->serialize(dst);
+            ++first;
+        }
+    }
+
 private:
     Data d;
 };
@@ -1037,7 +1060,7 @@ public:
 
     bool isLayoutCompatible(const QRhiShaderResourceBindings *other) const;
 
-    QVector<uint> serializedLayoutDescription() const { return m_layoutDesc; }
+    QVector<quint32> serializedLayoutDescription() const { return m_layoutDesc; }
 
     virtual bool create() = 0;
 
@@ -1050,14 +1073,13 @@ public:
 
 protected:
     static const int BINDING_PREALLOC = 12;
-    static const int LAYOUT_DESC_FIELD_COUNT = 4;
     QRhiShaderResourceBindings(QRhiImplementation *rhi);
     QVarLengthArray<QRhiShaderResourceBinding, BINDING_PREALLOC> m_bindings;
-    uint m_layoutDescHash = 0;
+    size_t m_layoutDescHash = 0;
     // Intentionally not using QVLA for m_layoutDesc: clients like Qt Quick are much
     // better served with an implicitly shared container here, because they will likely
     // throw this directly into structs serving as cache keys.
-    QVector<uint> m_layoutDesc;
+    QVector<quint32> m_layoutDesc;
     friend class QRhiImplementation;
 #ifndef QT_NO_DEBUG_STREAM
     friend Q_GUI_EXPORT QDebug operator<<(QDebug, const QRhiShaderResourceBindings &);

tests/auto/gui/rhi/qrhi/tst_qrhi.cpp

@@ -3314,6 +3314,15 @@ void tst_QRhi::srbLayoutCompatibility()
         QVERIFY(!srb1->serializedLayoutDescription().isEmpty());
         QVERIFY(!srb2->serializedLayoutDescription().isEmpty());
         QCOMPARE(srb1->serializedLayoutDescription(), srb2->serializedLayoutDescription());
+
+        // see what we would get if a binding list got serialized "manually", without pulling it out from the srb after building
+        // (the results should be identical)
+        QVector<quint32> layoutDesc1;
+        QRhiShaderResourceBinding::serializeLayoutDescription(srb1->cbeginBindings(), srb1->cendBindings(), std::back_inserter(layoutDesc1));
+        QCOMPARE(layoutDesc1, srb1->serializedLayoutDescription());
+        QVector<quint32> layoutDesc2;
+        QRhiShaderResourceBinding::serializeLayoutDescription(srb2->cbeginBindings(), srb2->cendBindings(), std::back_inserter(layoutDesc2));
+        QCOMPARE(layoutDesc2, srb2->serializedLayoutDescription());
     }
 
     // different visibility (not compatible)