Improved gradient table generation performance for two-stop gradients.

Two stops is a fairly common case so we gain quite a bit by special
casing it. Improves performance by 10 % in parcycle benchmark, and
by 90 % in a synthetic benchmark.

Reviewed-by: Andreas Kling
(cherry picked from commit 5b74a70ac630073582be56f8a0539624a1080185)

1 files changed, 78 insertions, 0 deletions

diff --git a/src/gui/painting/qpaintengine_raster.cpp b/src/gui/painting/qpaintengine_raster.cpp
index 69025436c2..8486adb64a 100644
--- a/src/gui/painting/qpaintengine_raster.cpp
+++ b/src/gui/painting/qpaintengine_raster.cpp
@@ -5033,6 +5033,84 @@ void QGradientCache::generateGradientColorTable(const QGradient& gradient, uint
 
     bool colorInterpolation = (gradient.interpolationMode() == QGradient::ColorInterpolation);
 
+    if (stopCount == 2) {
+        uint first_color = ARGB_COMBINE_ALPHA(stops[0].second.rgba(), opacity);
+        uint second_color = ARGB_COMBINE_ALPHA(stops[1].second.rgba(), opacity);
+
+        qreal first_stop = stops[0].first;
+        qreal second_stop = stops[1].first;
+
+        if (second_stop < first_stop) {
+            qSwap(first_color, second_color);
+            qSwap(first_stop, second_stop);
+        }
+
+        if (colorInterpolation) {
+            first_color = PREMUL(first_color);
+            second_color = PREMUL(second_color);
+        }
+
+        int first_index = qRound(first_stop * (GRADIENT_STOPTABLE_SIZE-1));
+        int second_index = qRound(second_stop * (GRADIENT_STOPTABLE_SIZE-1));
+
+        uint red_first = qRed(first_color) << 16;
+        uint green_first = qGreen(first_color) << 16;
+        uint blue_first = qBlue(first_color) << 16;
+        uint alpha_first = qAlpha(first_color) << 16;
+
+        uint red_second = qRed(second_color) << 16;
+        uint green_second = qGreen(second_color) << 16;
+        uint blue_second = qBlue(second_color) << 16;
+        uint alpha_second = qAlpha(second_color) << 16;
+
+        int i = 0;
+        for (; i <= qMin(GRADIENT_STOPTABLE_SIZE, first_index); ++i) {
+            if (colorInterpolation)
+                colorTable[i] = first_color;
+            else
+                colorTable[i] = PREMUL(first_color);
+        }
+
+        if (i < second_index) {
+            qreal reciprocal = qreal(1) / (second_index - first_index);
+
+            int red_delta = qRound(int(red_second - red_first) * reciprocal);
+            int green_delta = qRound(int(green_second - green_first) * reciprocal);
+            int blue_delta = qRound(int(blue_second - blue_first) * reciprocal);
+            int alpha_delta = qRound(int(alpha_second - alpha_first) * reciprocal);
+
+            // rounding
+            red_first += 1 << 15;
+            green_first += 1 << 15;
+            blue_first += 1 << 15;
+            alpha_first += 1 << 15;
+
+            for (; i < qMin(GRADIENT_STOPTABLE_SIZE, second_index); ++i) {
+                red_first += red_delta;
+                green_first += green_delta;
+                blue_first += blue_delta;
+                alpha_first += alpha_delta;
+
+                const uint color = ((alpha_first << 8) & 0xff000000) | (red_first & 0xff0000)
+                                 | ((green_first >> 8) & 0xff00) | (blue_first >> 16);
+
+                if (colorInterpolation)
+                    colorTable[i] = color;
+                else
+                    colorTable[i] = PREMUL(color);
+            }
+        }
+
+        for (; i < GRADIENT_STOPTABLE_SIZE; ++i) {
+            if (colorInterpolation)
+                colorTable[i] = second_color;
+            else
+                colorTable[i] = PREMUL(second_color);
+        }
+
+        return;
+    }
+
     uint current_color = ARGB_COMBINE_ALPHA(stops[0].second.rgba(), opacity);
     if (stopCount == 1) {
         current_color = PREMUL(current_color);