[gdk-pixbuf] pixops: Avoid division during alpha scaling

[Emmanuele Bassi <ebassi src gnome org>]

commit 679e9ed7b056ec3b17921863b5c7d2873a43c3b3
Author: Jan Ziak <0xe2 0x9a 0x9b gmail com>
Date:   Sat Oct 12 08:58:23 2019 +0200

    pixops: Avoid division during alpha scaling
    
    This patch significantly improves performance
    of gtksourceview-2.10.5 full-screen redraw
    on a 4K display with an alpha channel.
    
    Divisions by the common value 0xff0000 are converted into
    multiplications by an optimizing C compiler.
    For other values, 3 divisions are reduced to 1 division,
    3 multiplications and some float-to-int conversions.

 gdk-pixbuf/pixops/pixops.c | 37 ++++++++++++++++++++++++++++---------
 1 file changed, 28 insertions(+), 9 deletions(-)
---
diff --git a/gdk-pixbuf/pixops/pixops.c b/gdk-pixbuf/pixops/pixops.c
index a1c3c9bfd..623dbd09e 100644
--- a/gdk-pixbuf/pixops/pixops.c
+++ b/gdk-pixbuf/pixops/pixops.c
@@ -956,11 +956,20 @@ scale_pixel (guchar *dest, int dest_x, int dest_channels, int dest_has_alpha,
 {
   if (src_has_alpha)
     {
-      if (a)
+      if (a == 0xff0000)
        {
-         dest[0] = r / a;
-         dest[1] = g / a;
-         dest[2] = b / a;
+         /* Division by a constant is converted into a multiplication by an optimizing C compiler */
+         dest[0] = r / 0xff0000;
+         dest[1] = g / 0xff0000;
+         dest[2] = b / 0xff0000;
+         dest[3] = 0xff0000 >> 16;
+       }
+      else if (a)
+       {
+         double a1 = 1.0 / a;
+         dest[0] = r * a1;
+         dest[1] = g * a1;
+         dest[2] = b * a1;
          dest[3] = a >> 16;
        }
       else
@@ -991,6 +1000,7 @@ scale_line (int *weights, int n_x, int n_y, guchar *dest, int dest_x,
 {
   int x = x_init;
   int i, j;
+  const int n_xy = n_x * n_y;
 
   while (dest < dest_end)
     {
@@ -998,7 +1008,7 @@ scale_line (int *weights, int n_x, int n_y, guchar *dest, int dest_x,
       int *pixel_weights;
 
       pixel_weights = weights +
-        ((x >> (SCALE_SHIFT - SUBSAMPLE_BITS)) & SUBSAMPLE_MASK) * n_x * n_y;
+        ((x >> (SCALE_SHIFT - SUBSAMPLE_BITS)) & SUBSAMPLE_MASK) * n_xy;
 
       if (src_has_alpha)
        {
@@ -1022,11 +1032,20 @@ scale_line (int *weights, int n_x, int n_y, guchar *dest, int dest_x,
                }
            }
 
-         if (a)
+         if (a == 0xff0000)
+           {
+             /* Division by a constant is converted into a multiplication by an optimizing C compiler */
+             dest[0] = r / 0xff0000;
+             dest[1] = g / 0xff0000;
+             dest[2] = b / 0xff0000;
+             dest[3] = 0xff0000 >> 16;
+           }
+         else if (a)
            {
-             dest[0] = r / a;
-             dest[1] = g / a;
-             dest[2] = b / a;
+             double a1 = 1.0 / a;
+             dest[0] = r * a1;
+             dest[1] = g * a1;
+             dest[2] = b * a1;
              dest[3] = a >> 16;
            }
          else