Fix T78803: Bad widget drawing with the R600g driver

The SB back-end optimizer for the mesa R600g driver corrupts the vertex
shader for widget drawing. This will not be fixed upstream because SB is
getting replaced as part of the new NIR path. This was thought to be an
issue with instancing and an attempted fix was submitted in D8374, but
it did not fix the issue.

This patch reimplements the array look-up part of the code using switch
case as a workaround and removes the old workaround implemented as part
of D8374.

Reviewed By: Clement Foucault

Differential Revision: https://developer.blender.org/D10967

2 files changed, 21 insertions, 11 deletions

diff --git a/source/blender/editors/interface/interface_widgets.c b/source/blender/editors/interface/interface_widgets.c
index 0c6be7b1196..e351cd30c14 100644
--- a/source/blender/editors/interface/interface_widgets.c
+++ b/source/blender/editors/interface/interface_widgets.c
@@ -1225,12 +1225,6 @@ static bool draw_widgetbase_batch_skip_draw_cache(void)
     return true;
   }
 
-  /* There are also reports that some AMD and Mesa driver configuration suffer from the
-   * same issue, T78803. */
-  if (GPU_type_matches(GPU_DEVICE_ATI, GPU_OS_UNIX, GPU_DRIVER_OPENSOURCE)) {
-    return true;
-  }
-
   return false;
 }
 
diff --git a/source/blender/gpu/shaders/gpu_shader_2D_widget_base_vert.glsl b/source/blender/gpu/shaders/gpu_shader_2D_widget_base_vert.glsl
index 81e0965fad3..b5036b51d9d 100644
--- a/source/blender/gpu/shaders/gpu_shader_2D_widget_base_vert.glsl
+++ b/source/blender/gpu/shaders/gpu_shader_2D_widget_base_vert.glsl
@@ -61,11 +61,27 @@ vec2 do_widget(void)
   const vec2 ofs = vec2(0.5, -0.5);
   lineWidth = abs(rect.x - recti.x);
   vec2 emboss_ofs = vec2(0.0, -lineWidth);
-  vec2 v_pos[4] = vec2[4](rect.xz + emboss_ofs + ofs.yy,
-                          rect.xw + ofs.yx,
-                          rect.yz + emboss_ofs + ofs.xy,
-                          rect.yw + ofs.xx);
-  vec2 pos = v_pos[gl_VertexID];
+
+  vec2 pos;
+  switch (gl_VertexID) {
+    default:
+    case 0: {
+      pos = rect.xz + emboss_ofs + ofs.yy;
+      break;
+    }
+    case 1: {
+      pos = rect.xw + ofs.yx;
+      break;
+    }
+    case 2: {
+      pos = rect.yz + emboss_ofs + ofs.xy;
+      break;
+    }
+    case 3: {
+      pos = rect.yw + ofs.xx;
+      break;
+    }
+  }
 
   uvInterp = pos - rect.xz;
   outRectSize = rect.yw - rect.xz;