rhi: Improve srb layout serialization helpers

Be idiomatic and return the output iterator one past the last element.
Otherwise passing in a plain pointer (as exercised by the autotest now)
fails to function because we write over the same 4 elements again and
again for each binding.

Change-Id: If74463fa5140ffa2b1d5be97b71868848ad46614
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
(cherry picked from commit ce9d0491f2a3836991c3c44fa70c32f1f606ab28)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

3 files changed, 13 insertions, 3 deletions

diff --git a/src/gui/rhi/qrhi.cpp b/src/gui/rhi/qrhi.cpp
index 420e70cecf..64168b4dc6 100644
--- a/src/gui/rhi/qrhi.cpp
+++ b/src/gui/rhi/qrhi.cpp
@@ -3058,7 +3058,7 @@ void QRhiImplementation::updateLayoutDesc(QRhiShaderResourceBindings *srb)
         const QRhiShaderResourceBinding::Data *d = b.data();
         srb->m_layoutDescHash ^= uint(d->binding) ^ uint(d->stage) ^ uint(d->type)
             ^ uint(d->type == QRhiShaderResourceBinding::SampledTexture ? d->u.stex.count : 1);
-        d->serialize(layoutDescAppender);
+        layoutDescAppender = d->serialize(layoutDescAppender);
     }
 }
 
diff --git a/src/gui/rhi/qrhi_p.h b/src/gui/rhi/qrhi_p.h
index 81b00cd07f..527d72e486 100644
--- a/src/gui/rhi/qrhi_p.h
+++ b/src/gui/rhi/qrhi_p.h
@@ -413,13 +413,14 @@ public:
         } u;
 
         template<typename Output>
-        void serialize(Output dst) const
+        Output 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);
+            return dst;
         }
     };
 
@@ -434,7 +435,7 @@ public:
                                            Output dst)
     {
         while (first != last) {
-            first->data()->serialize(dst);
+            dst = first->data()->serialize(dst);
             ++first;
         }
     }
diff --git a/tests/auto/gui/rhi/qrhi/tst_qrhi.cpp b/tests/auto/gui/rhi/qrhi/tst_qrhi.cpp
index 603acc03de..8398dac3eb 100644
--- a/tests/auto/gui/rhi/qrhi/tst_qrhi.cpp
+++ b/tests/auto/gui/rhi/qrhi/tst_qrhi.cpp
@@ -3273,6 +3273,7 @@ void tst_QRhi::srbLayoutCompatibility()
         QVERIFY(srb2->isLayoutCompatible(srb1.data()));
 
         QCOMPARE(srb1->serializedLayoutDescription(), srb2->serializedLayoutDescription());
+        QVERIFY(srb1->serializedLayoutDescription().count() == 0);
     }
 
     // different count (not compatible)
@@ -3290,6 +3291,8 @@ void tst_QRhi::srbLayoutCompatibility()
         QVERIFY(!srb2->isLayoutCompatible(srb1.data()));
 
         QVERIFY(srb1->serializedLayoutDescription() != srb2->serializedLayoutDescription());
+        QVERIFY(srb1->serializedLayoutDescription().count() == 0);
+        QVERIFY(srb2->serializedLayoutDescription().count() == 1 * QRhiShaderResourceBinding::LAYOUT_DESC_ENTRIES_PER_BINDING);
     }
 
     // full match (compatible)
@@ -3314,6 +3317,7 @@ void tst_QRhi::srbLayoutCompatibility()
         QVERIFY(!srb1->serializedLayoutDescription().isEmpty());
         QVERIFY(!srb2->serializedLayoutDescription().isEmpty());
         QCOMPARE(srb1->serializedLayoutDescription(), srb2->serializedLayoutDescription());
+        QVERIFY(srb1->serializedLayoutDescription().count() == 2 * QRhiShaderResourceBinding::LAYOUT_DESC_ENTRIES_PER_BINDING);
 
         // 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)
@@ -3323,6 +3327,11 @@ void tst_QRhi::srbLayoutCompatibility()
         QVector<quint32> layoutDesc2;
         QRhiShaderResourceBinding::serializeLayoutDescription(srb2->cbeginBindings(), srb2->cendBindings(), std::back_inserter(layoutDesc2));
         QCOMPARE(layoutDesc2, srb2->serializedLayoutDescription());
+
+        // exercise with an "output iterator" different from back_inserter
+        quint32 layoutDesc3[2 * QRhiShaderResourceBinding::LAYOUT_DESC_ENTRIES_PER_BINDING];
+        QRhiShaderResourceBinding::serializeLayoutDescription(srb1->cbeginBindings(), srb1->cendBindings(), layoutDesc3);
+        QVERIFY(!memcmp(layoutDesc3, layoutDesc1.constData(), sizeof(quint32) * 2 * QRhiShaderResourceBinding::LAYOUT_DESC_ENTRIES_PER_BINDING));
     }
 
     // different visibility (not compatible)