From ce9f717045ef551d1cf58166f52034d40b4ceb6e Mon Sep 17 00:00:00 2001
From: Jules Blok <jules.blok@gmail.com>
Date: Sat, 29 Oct 2016 17:24:10 +0200
Subject: [PATCH] OGL: Fall back to the old dual-source blending behaviour.

---
 Source/Core/VideoBackends/OGL/Render.cpp   |  7 +++++--
 Source/Core/VideoCommon/PixelShaderGen.cpp | 22 ++++++++++------------
 2 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/Source/Core/VideoBackends/OGL/Render.cpp b/Source/Core/VideoBackends/OGL/Render.cpp
index f1e263d8db..ab6576915e 100644
--- a/Source/Core/VideoBackends/OGL/Render.cpp
+++ b/Source/Core/VideoBackends/OGL/Render.cpp
@@ -445,8 +445,7 @@ Renderer::Renderer()
 
   g_Config.backend_info.bSupportsDualSourceBlend =
       (GLExtensions::Supports("GL_ARB_blend_func_extended") ||
-       GLExtensions::Supports("GL_EXT_blend_func_extended")) &&
-      !DriverDetails::HasBug(DriverDetails::BUG_BROKEN_DUAL_SOURCE_BLENDING);
+       GLExtensions::Supports("GL_EXT_blend_func_extended"));
   g_Config.backend_info.bSupportsPrimitiveRestart =
       !DriverDetails::HasBug(DriverDetails::BUG_PRIMITIVERESTART) &&
       ((GLExtensions::Version() >= 310) || GLExtensions::Supports("GL_NV_primitive_restart"));
@@ -1244,6 +1243,10 @@ void Renderer::SetBlendMode(bool forceUpdate)
   bool useDstAlpha = bpmem.dstalpha.enable && bpmem.blendmode.alphaupdate && target_has_alpha;
   bool useDualSource = g_ActiveConfig.backend_info.bSupportsDualSourceBlend;
 
+  // Only use dual-source blending when required on drivers that don't support it very well.
+  if (DriverDetails::HasBug(DriverDetails::BUG_BROKEN_DUAL_SOURCE_BLENDING) && !useDstAlpha)
+    useDualSource = false;
+
   const GLenum glSrcFactors[8] = {
       GL_ZERO,
       GL_ONE,
diff --git a/Source/Core/VideoCommon/PixelShaderGen.cpp b/Source/Core/VideoCommon/PixelShaderGen.cpp
index ab772c718f..6ad2220353 100644
--- a/Source/Core/VideoCommon/PixelShaderGen.cpp
+++ b/Source/Core/VideoCommon/PixelShaderGen.cpp
@@ -507,7 +507,9 @@ ShaderCode GeneratePixelShaderCode(APIType ApiType, const pixel_shader_uid_data*
 
   if (ApiType == APIType::OpenGL || ApiType == APIType::Vulkan)
   {
-    if (g_ActiveConfig.backend_info.bSupportsDualSourceBlend)
+    if (g_ActiveConfig.backend_info.bSupportsDualSourceBlend &&
+        (!DriverDetails::HasBug(DriverDetails::BUG_BROKEN_DUAL_SOURCE_BLENDING) ||
+         uid_data->dstAlphaMode == DSTALPHA_DUAL_SOURCE_BLEND))
     {
       if (DriverDetails::HasBug(DriverDetails::BUG_BROKEN_FRAGMENT_SHADER_INDEX_DECORATION))
       {
@@ -523,6 +525,7 @@ ShaderCode GeneratePixelShaderCode(APIType ApiType, const pixel_shader_uid_data*
     else
     {
       out.Write("FRAGMENT_OUTPUT_LOCATION(0) out vec4 ocol0;\n");
+      out.Write("vec4 ocol1;\n");  // Consume the output we don't use
     }
 
     if (uid_data->per_pixel_depth)
@@ -1207,8 +1210,7 @@ static void WriteAlphaTest(ShaderCode& out, const pixel_shader_uid_data* uid_dat
     out.Write(")) {\n");
 
   out.Write("\t\tocol0 = float4(0.0, 0.0, 0.0, 0.0);\n");
-  if (g_ActiveConfig.backend_info.bSupportsDualSourceBlend)
-    out.Write("\t\tocol1 = float4(0.0, 0.0, 0.0, 0.0);\n");
+  out.Write("\t\tocol1 = float4(0.0, 0.0, 0.0, 0.0);\n");
   if (per_pixel_depth)
   {
     out.Write("\t\tdepth = %s;\n",
@@ -1304,8 +1306,7 @@ static void WriteColor(ShaderCode& out, const pixel_shader_uid_data* uid_data)
   if (uid_data->dstAlphaMode == DSTALPHA_NONE)
   {
     out.Write("\tocol0.a = float(prev.a >> 2) / 63.0;\n");
-    if (g_ActiveConfig.backend_info.bSupportsDualSourceBlend)
-      out.Write("\tocol1.a = float(prev.a) / 255.0;\n");
+    out.Write("\tocol1.a = float(prev.a) / 255.0;\n");
   }
   else
   {
@@ -1313,12 +1314,9 @@ static void WriteColor(ShaderCode& out, const pixel_shader_uid_data* uid_data)
     out.Write("\tocol0.a = float(" I_ALPHA ".a >> 2) / 63.0;\n");
 
     // Use dual-source color blending to perform dst alpha in a single pass
-    if (g_ActiveConfig.backend_info.bSupportsDualSourceBlend)
-    {
-      if (uid_data->dstAlphaMode == DSTALPHA_DUAL_SOURCE_BLEND)
-        out.Write("\tocol1.a = float(prev.a) / 255.0;\n");
-      else
-        out.Write("\tocol1.a = float(" I_ALPHA ".a) / 255.0;\n");
-    }
+    if (uid_data->dstAlphaMode == DSTALPHA_DUAL_SOURCE_BLEND)
+      out.Write("\tocol1.a = float(prev.a) / 255.0;\n");
+    else
+      out.Write("\tocol1.a = float(" I_ALPHA ".a) / 255.0;\n");
   }
 }