rhi: Allow testing renderpass compatibility without the objects

Follow what has been done for QRhiShaderResourceBindings. Have a way
to retrieve an opaque blob (that just happens to be a list of integers)
so that a simple == comparison can be used to determine compatibility
even when the objects from which the blob was retrieved are no longer
alive.

The contract is the following:

bool a = rp1->isCompatible(rp2);
bool b = rp1->serializedFormat() == rp2->serializedFormat();
assert(a == b);

Pick-to: 6.2
Change-Id: I45e7d05eeb6dfa2b2de474da0a0644912aaf174a
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Christian Strømme <christian.stromme@qt.io>

1 files changed, 23 insertions, 1 deletions

diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index e924841375..072d100c59 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -2740,7 +2740,17 @@ QRhiResource::Type QRhiRenderPassDescriptor::resourceType() const
     \l{QRhiTextureRenderTarget::newCompatibleRenderPassDescriptor()}{created}
     from the same QRhiTextureRenderTarget are always compatible.
 
-    \sa newCompatibleRenderPassDescriptor()
+    Similarly to QRhiShaderResourceBindings, compatibility can also be tested
+    without having two existing objects available. Extracting the opaque blob by
+    calling serializedFormat() allows testing for compatibility by comparing the
+    returned vector to another QRhiRenderPassDescriptor's
+    serializedFormat(). This has benefits in certain situations, because it
+    allows testing the compatibility of a QRhiRenderPassDescriptor with a
+    QRhiGraphicsPipeline even when the QRhiRenderPassDescriptor the pipeline was
+    originally built was is no longer available (but the data returned from its
+    serializedFormat() still is).
+
+    \sa newCompatibleRenderPassDescriptor(), serializedFormat()
  */
 
 /*!
@@ -2765,6 +2775,18 @@ QRhiResource::Type QRhiRenderPassDescriptor::resourceType() const
  */
 
 /*!
+    \fn QVector<quint32> QRhiRenderPassDescriptor::serializedFormat() const
+
+    \return a vector of integers containing an opaque blob describing the data
+    relevant for \l{isCompatible()}{compatibility}. Given two
+    QRhiRenderPassDescriptor objects \c rp1 and \c rp2, if the data returned
+    from this function is identical, then \c{rp1->isCompatible(rp2)}, and vice
+    versa hold true as well.
+
+    \sa isCompatible()
+ */
+
+/*!
     \return a pointer to a backend-specific QRhiNativeHandles subclass, such as
     QRhiVulkanRenderPassNativeHandles. The returned value is \nullptr when exposing
     the underlying native resources is not supported by the backend.