Improve performance of combine_over_u

The generic C over_u combiner can be a lot faster with the
addition of special shortcuts for 0xFF and 0x00 alpha/mask
values. This is already implemented in C and SSE2 fast paths.

Profiling the run of cairo-perf-trace benchmarks with PIXMAN_DISABLE
environment variable set to "fast mmx sse2" on Intel Core i7:

=== before ===

37.32%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] combine_over_u
21.37%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] bits_image_fetch_bilinear_no_repeat_8888
13.51%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] bits_image_fetch_bilinear_affine_none_a8r8g8b8
 2.96%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] radial_compute_color
 2.74%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] fetch_scanline_a8
 2.71%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] fetch_scanline_x8r8g8b8
 2.17%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] _pixman_gradient_walker_pixel
 1.86%  cairo-perf-trac  libcairo.so.2.11200.0 [.] _cairo_tor_scan_converter_generate
 1.57%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] bits_image_fetch_bilinear_affine_pad_a8r8g8b8
 0.97%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] combine_in_reverse_u
 0.96%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] combine_over_ca

=== after ===

28.79%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] bits_image_fetch_bilinear_no_repeat_8888
18.44%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] bits_image_fetch_bilinear_affine_none_a8r8g8b8
15.54%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] combine_over_u
 3.94%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] radial_compute_color
 3.69%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] fetch_scanline_a8
 3.69%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] fetch_scanline_x8r8g8b8
 2.94%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] _pixman_gradient_walker_pixel
 2.52%  cairo-perf-trac  libcairo.so.2.11200.0 [.] _cairo_tor_scan_converter_generate
 2.08%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] bits_image_fetch_bilinear_affine_pad_a8r8g8b8
 1.31%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] combine_in_reverse_u
 1.29%  cairo-perf-trac  libpixman-1.so.0.29.1 [.] combine_over_ca

1 files changed, 51 insertions, 7 deletions

diff --git a/pixman/pixman-combine32.c b/pixman/pixman-combine32.c
index 54cc877..3ac7576 100644
--- a/pixman/pixman-combine32.c
+++ b/pixman/pixman-combine32.c
@@ -196,14 +196,58 @@ combine_over_u (pixman_implementation_t *imp,
 {
     int i;
 
-    for (i = 0; i < width; ++i)
+    if (!mask)
     {
-	uint32_t s = combine_mask (src, mask, i);
-	uint32_t d = *(dest + i);
-	uint32_t ia = ALPHA_8 (~s);
-
-	UN8x4_MUL_UN8_ADD_UN8x4 (d, ia, s);
-	*(dest + i) = d;
+	for (i = 0; i < width; ++i)
+	{
+	    uint32_t s = *(src + i);
+	    uint32_t a = ALPHA_8 (s);
+	    if (a == 0xFF)
+	    {
+		*(dest + i) = s;
+	    }
+	    else if (s)
+	    {
+		uint32_t d = *(dest + i);
+		uint32_t ia = a ^ 0xFF;
+		UN8x4_MUL_UN8_ADD_UN8x4 (d, ia, s);
+		*(dest + i) = d;
+	    }
+	}
+    }
+    else
+    {
+	for (i = 0; i < width; ++i)
+	{
+	    uint32_t m = ALPHA_8 (*(mask + i));
+	    if (m == 0xFF)
+	    {
+		uint32_t s = *(src + i);
+		uint32_t a = ALPHA_8 (s);
+		if (a == 0xFF)
+		{
+		    *(dest + i) = s;
+		}
+		else if (s)
+		{
+		    uint32_t d = *(dest + i);
+		    uint32_t ia = a ^ 0xFF;
+		    UN8x4_MUL_UN8_ADD_UN8x4 (d, ia, s);
+		    *(dest + i) = d;
+		}
+	    }
+	    else if (m)
+	    {
+		uint32_t s = *(src + i);
+		if (s)
+		{
+		    uint32_t d = *(dest + i);
+		    UN8x4_MUL_UN8 (s, m);
+		    UN8x4_MUL_UN8_ADD_UN8x4 (d, ALPHA_8 (~s), s);
+		    *(dest + i) = d;
+		}
+	    }
+	}
     }
 }