From 6234cb98d1b2b3b3cd39b18f69ee8cf4aa7efe32 Mon Sep 17 00:00:00 2001
From: Pekka Paalanen <pekka.paalanen@collabora.com>
Date: Thu, 2 Jun 2022 11:24:52 +0300
Subject: [PATCH] gl-renderer: fix performance regression in frag

When color management is disabled, the fragment shader was still first
ensuring straight alpha and then immediately just going back to
pre-multiplied. This is near-impossible for a shader compiler to
optimize out, I guess because of the if-statement to handle division by
zero. Having view alpha applied in between certainly didn't make it
easier.

That causes extra fragment computations that are unnecessary. In the
issue report this was found to cause a notable performance regression.

Fix the performance regression by introducing special-case paths for
when straight alpha is not needed. This skips the unnecessary
computations.

Fixes: https://gitlab.freedesktop.org/wayland/weston/-/issues/623
Fixes: 9a6a4e7032669be727c965ca19e3e30098c892e7

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
---
 libweston/renderer-gl/fragment.glsl | 44 ++++++++++++++++++++---------
 1 file changed, 31 insertions(+), 13 deletions(-)

diff --git a/libweston/renderer-gl/fragment.glsl b/libweston/renderer-gl/fragment.glsl
index c49a6cd4..e8dae27d 100644
--- a/libweston/renderer-gl/fragment.glsl
+++ b/libweston/renderer-gl/fragment.glsl
@@ -77,6 +77,10 @@ compile_const bool c_green_tint = DEF_GREEN_TINT;
 compile_const int c_color_pre_curve = DEF_COLOR_PRE_CURVE;
 compile_const int c_color_mapping = DEF_COLOR_MAPPING;
 
+compile_const bool c_need_color_pipeline =
+	c_color_pre_curve != SHADER_COLOR_CURVE_IDENTITY ||
+	c_color_mapping != SHADER_COLOR_MAPPING_IDENTITY;
+
 vec4
 yuva2rgba(vec4 yuva)
 {
@@ -245,9 +249,6 @@ color_mapping(vec3 color)
 vec4
 color_pipeline(vec4 color)
 {
-	/* View alpha (opacity) */
-	color.a *= alpha;
-
 	color.rgb = color_pre_curve(color.rgb);
 	color.rgb = color_mapping(color.rgb);
 
@@ -262,18 +263,35 @@ main()
 	/* Electrical (non-linear) RGBA values, may be premult or not */
 	color = sample_input_texture();
 
-	/* Ensure straight alpha */
-	if (c_input_is_premult) {
-		if (color.a == 0.0)
-			color.rgb = vec3(0, 0, 0);
-		else
-			color.rgb *= 1.0 / color.a;
-	}
+	if (c_need_color_pipeline) {
+		/* Ensure straight alpha */
+		if (c_input_is_premult) {
+			if (color.a == 0.0)
+				color.rgb = vec3(0, 0, 0);
+			else
+				color.rgb *= 1.0 / color.a;
+		}
 
-	color = color_pipeline(color);
+		color = color_pipeline(color);
 
-	/* pre-multiply for blending */
-	color.rgb *= color.a;
+		/* View alpha (opacity) */
+		color.a *= alpha;
+
+		/* pre-multiply for blending */
+		color.rgb *= color.a;
+	} else {
+		/* Fast path for disabled color management */
+
+		if (c_input_is_premult) {
+			/* View alpha (opacity) */
+			color *= alpha;
+		} else {
+			/* View alpha (opacity) */
+			color.a *= alpha;
+			/* pre-multiply for blending */
+			color.rgb *= color.a;
+		}
+	}
 
 	if (c_green_tint)
 		color = vec4(0.0, 0.3, 0.0, 0.2) + color * 0.8;