rhi: Make the serialized srb layout description accessible

...by the Qt Quick renderer, for example.

A typical Qt Quick material binding set serializes to 8 uints. This
would not demand a container like QVector. However, being implicitly
shared is essential here due to the intended usage (query the
serialized blob, put it into a cache key, hash it, compare it, all
without any copying and new allocs; we can afford an extra alloc
upon each srb construction, but don't want more afterwards in the
rendering engines)

Also make it clear in the pipeline docs that the optimization Qt Quick
is (soon going to be) doing is legal. (the srb ref in the pipeline can
be dead and dangling as long as every call to setShaderResources()
specifies a layout-compatible alternative)

Pick-to: 6.2
Change-Id: I97efbea1fa3516b10c9832adbab0a21b7bc0845d
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

2 files changed, 14 insertions, 1 deletions

diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index 70c8bf5813..2da5959b56 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -2996,6 +2996,7 @@ QRhiResource::Type QRhiTextureRenderTarget::resourceType() const
 QRhiShaderResourceBindings::QRhiShaderResourceBindings(QRhiImplementation *rhi)
     : QRhiResource(rhi)
 {
+    m_layoutDesc.reserve(BINDING_PREALLOC * LAYOUT_DESC_FIELD_COUNT);
 }
 
 /*!
@@ -5614,6 +5615,13 @@ void QRhiCommandBuffer::setGraphicsPipeline(QRhiGraphicsPipeline *ps)
     srb. In this case setShaderResources() must be called even if \a srb is
     the same as in the last call.
 
+    When \a srb is not null, the QRhiShaderResourceBindings object the pipeline
+    was built with in create() is guaranteed to be not accessed in any form. In
+    fact, it does not need to be valid even at this point: destroying the
+    pipeline's associated srb after create() and instead explicitly specifying
+    another, \l{QRhiShaderResourceBindings::isLayoutCompatible()}{layout
+    compatible} one in every setShaderResources() call is valid.
+
     \a dynamicOffsets allows specifying buffer offsets for uniform buffers that
     were associated with \a srb via
     QRhiShaderResourceBinding::uniformBufferWithDynamicOffset(). This is
diff --git a/src/gui/rhi/qrhi_p.h b/src/gui/rhi/qrhi_p.h
index 1e570e8818..8c3e0fdfb2 100644
--- a/src/gui/rhi/qrhi_p.h
+++ b/src/gui/rhi/qrhi_p.h
@@ -1037,6 +1037,8 @@ public:
 
     bool isLayoutCompatible(const QRhiShaderResourceBindings *other) const;
 
+    QVector<uint> serializedLayoutDescription() const { return m_layoutDesc; }
+
     virtual bool create() = 0;
 
     enum UpdateFlag {
@@ -1052,7 +1054,10 @@ protected:
     QRhiShaderResourceBindings(QRhiImplementation *rhi);
     QVarLengthArray<QRhiShaderResourceBinding, BINDING_PREALLOC> m_bindings;
     uint m_layoutDescHash = 0;
-    QVarLengthArray<uint, BINDING_PREALLOC * LAYOUT_DESC_FIELD_COUNT> m_layoutDesc;
+    // 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;
     friend class QRhiImplementation;
 #ifndef QT_NO_DEBUG_STREAM
     friend Q_GUI_EXPORT QDebug operator<<(QDebug, const QRhiShaderResourceBindings &);