rhi: Allow testing QRhiRenderPassDescriptors for compatibility

For Metal and Vulkan this needs actual work because that's where
the concept of renderpass descriptors is relevant. GL and D3D can
just return true always.

The big benefit of this is that Qt Quick can now compare renderpass
descriptors via isCompatible() for its pipeline cache (similarly to
how it is already using isLayoutCompatible() for srbs), and so
renderpass descriptors for layers (Item.layer, ShaderEffect) will
typically be compatible and so can pick up pipelines created by other
layers from the cache.

Also add autotests for shader resource binding and renderpass descriptor
compatibility.

Task-number: QTBUG-80318
Change-Id: I0008bc51c4ee13b0113d2c8caf799e1257f18a18
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

1 files changed, 18 insertions, 0 deletions

diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index ac5836fc68..ece5190ff7 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -2319,6 +2319,24 @@ QRhiResource::Type QRhiRenderPassDescriptor::resourceType() const
 }
 
 /*!
+    \fn bool QRhiRenderPassDescriptor::isCompatible(const QRhiRenderPassDescriptor *other) const;
+
+    \return true if the \a other QRhiRenderPassDescriptor is compatible with
+    this one, meaning \c this and \a other can be used interchangebly in
+    QRhiGraphicsPipeline::setRenderPassDescriptor().
+
+    The concept of the compatibility of renderpass descriptors is similar to
+    the \l{QRhiShaderResourceBindings::isLayoutCompatible}{layout
+    compatibility} of QRhiShaderResourceBindings instances. They allow better
+    reuse of QRhiGraphicsPipeline instances: for example, a
+    QRhiGraphicsPipeline instance cache is expected to use these functions to
+    look for a matching pipeline, instead of just comparing pointers, thus
+    allowing a different QRhiRenderPassDescriptor and
+    QRhiShaderResourceBindings to be used in combination with the pipeline, as
+    long as they are compatible.
+ */
+
+/*!
     \return a pointer to a backend-specific QRhiNativeHandles subclass, such as
     QRhiVulkanRenderPassNativeHandles. The returned value is null when exposing
     the underlying native resources is not supported by the backend.