Particles: reduce memory allocations by flattening a set

When an affector acts on a particle, it will add it to a set
of "seen" particles. This means an allocation, per particle,
per frame.

In Qt 6 the problem is less dramatic due to the new QHash
implementation, which uses "wider" buckets, but in Qt 5 this is
hundreds of memory allocations (and deallocations) per frame.

Just reimplement a minimal flat-set API for this use case,
and replace the QSet usages with it. On a testcase with
200 active particles, this reduces memory allocations from
~20'000 per second to 0 when the scene is "stable".

Pick-to: 5.15
Change-Id: I4be1e12a23b8dffca91955148532db243e383a4c
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

1 files changed, 2 insertions, 1 deletions

diff --git a/src/particles/qquickparticlesystem_p.h b/src/particles/qquickparticlesystem_p.h
index b7812c6ea5..b1053d8ff8 100644
--- a/src/particles/qquickparticlesystem_p.h
+++ b/src/particles/qquickparticlesystem_p.h
@@ -63,6 +63,7 @@
 #include <private/qv4global_p.h>
 #include <private/qv4staticvalue_p.h>
 #include "qtquickparticlesglobal_p.h"
+#include "qquickparticleflatset_p.h"
 
 QT_BEGIN_NAMESPACE
 
@@ -410,7 +411,7 @@ public:
     int systemSync(QQuickParticlePainter* p);
 
     //Data members here for ease of related class and auto-test usage. Not "public" API. TODO: d_ptrize
-    QSet<QQuickParticleData*> needsReset;
+    QtQuickParticlesPrivate::QFlatSet<QQuickParticleData*> needsReset;
     QVector<QQuickParticleData*> bySysIdx; //Another reference to the data (data owned by group), but by sysIdx
     QQuickStochasticEngine* stateEngine;