Don't fetch destination in RGB64 mode when not necessary

Extends the short-cut of not fetching destination to also apply to
the RGB64 painting. This saves reading and converting destination pixels
when they will be fully replaced with source pixels.

Since ARGB32 was switched to using the RGB64 drawhelpers, and ARGB32 is
particularly expensive to read, this change is important to avoid
performance regression.

Change-Id: If3a2439140d6364e8429783cfa786bd000cfab45
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

1 files changed, 21 insertions, 20 deletions

diff --git a/src/gui/painting/qdrawhelper.cpp b/src/gui/painting/qdrawhelper.cpp
index 4671c5cecf..654be5690b 100644
--- a/src/gui/painting/qdrawhelper.cpp
+++ b/src/gui/painting/qdrawhelper.cpp
@@ -1144,6 +1144,11 @@ static QRgba64 *QT_FASTCALL destFetch64uint32(QRgba64 *buffer, QRasterBuffer *ra
     return const_cast<QRgba64 *>(layout->convertToARGB64PM(buffer, src, length, 0, 0));
 }
 
+static QRgba64 * QT_FASTCALL destFetch64Undefined(QRgba64 *buffer, QRasterBuffer *, int, int, int)
+{
+    return buffer;
+}
+
 static DestFetchProc destFetchProc[QImage::NImageFormats] =
 {
     0,                  // Format_Invalid
@@ -1176,8 +1181,8 @@ static DestFetchProc destFetchProc[QImage::NImageFormats] =
 static DestFetchProc64 destFetchProc64[QImage::NImageFormats] =
 {
     0,                  // Format_Invalid
-    destFetch64,        // Format_Mono,
-    destFetch64,        // Format_MonoLSB
+    0,                  // Format_Mono,
+    0,                  // Format_MonoLSB
     0,                  // Format_Indexed8
     destFetch64uint32,  // Format_RGB32
     destFetch64uint32,  // Format_ARGB32,
@@ -3737,27 +3742,23 @@ static inline Operator getOperator(const QSpanData *data, const QSpan *spans, in
 
     op.destFetch = destFetchProc[data->rasterBuffer->format];
     op.destFetch64 = destFetchProc64[data->rasterBuffer->format];
-    if (op.mode == QPainter::CompositionMode_Source) {
-        switch (data->rasterBuffer->format) {
-        case QImage::Format_RGB32:
-        case QImage::Format_ARGB32_Premultiplied:
-            // don't clear destFetch as it sets up the pointer correctly to save one copy
-            break;
-        default: {
-            if (data->type == QSpanData::Texture && data->texture.const_alpha != 256)
+    if (op.mode == QPainter::CompositionMode_Source &&
+            (data->type != QSpanData::Texture || data->texture.const_alpha == 256)) {
+        const QSpan *lastSpan = spans + spanCount;
+        bool alphaSpans = false;
+        while (spans < lastSpan) {
+            if (spans->coverage != 255) {
+                alphaSpans = true;
                 break;
-            const QSpan *lastSpan = spans + spanCount;
-            bool alphaSpans = false;
-            while (spans < lastSpan) {
-                if (spans->coverage != 255) {
-                    alphaSpans = true;
-                    break;
-                }
-                ++spans;
             }
-            if (!alphaSpans)
-                op.destFetch = 0;
+            ++spans;
         }
+        if (!alphaSpans) {
+            // If all spans are opaque we do not need to fetch dest.
+            // But don't clear passthrough destFetch as they are just as fast and save destStore.
+            if (op.destFetch != destFetchARGB32P)
+                op.destFetch = 0;
+            op.destFetch64 = destFetch64Undefined;
         }
     }