[mutter] cogl: Add new option COGL_DEBUG=stencilling to disable optimizations

[Marge Bot <marge-bot src gnome org>]

commit c3d64cc05be11856433f12c84ad9cb8658d6d220
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Thu May 20 18:21:04 2021 +0800

    cogl: Add new option COGL_DEBUG=stencilling to disable optimizations
    
    This means stencilling every clip stack entry even when stencilling could
    be skipped. This visibly reveals some bugs in the stencilling logic.
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1873>

 cogl/cogl/cogl-debug-options.h           | 6 ++++++
 cogl/cogl/cogl-debug.c                   | 1 +
 cogl/cogl/cogl-debug.h                   | 1 +
 cogl/cogl/driver/gl/cogl-clip-stack-gl.c | 6 ++++--
 4 files changed, 12 insertions(+), 2 deletions(-)
---
diff --git a/cogl/cogl/cogl-debug-options.h b/cogl/cogl/cogl-debug-options.h
index f6cda51a2d..e6b1ca46d7 100644
--- a/cogl/cogl/cogl-debug-options.h
+++ b/cogl/cogl/cogl-debug-options.h
@@ -189,3 +189,9 @@ OPT (TEXTURES,
      "textures",
      N_("Debug texture management"),
      N_("Logs information about texture management"))
+OPT (STENCILLING,
+     N_("Root Cause"),
+     "stencilling",
+     N_("Stencil every clip entry"),
+     N_("Disables optimizations that usually avoid stencilling when it's not "
+        "needed. This exercises more of the stencilling logic than usual."))
diff --git a/cogl/cogl/cogl-debug.c b/cogl/cogl/cogl-debug.c
index c4d648652d..056d92ec3e 100644
--- a/cogl/cogl/cogl-debug.c
+++ b/cogl/cogl/cogl-debug.c
@@ -85,6 +85,7 @@ static const GDebugKey cogl_behavioural_debug_keys[] = {
   { "disable-fast-read-pixel", COGL_DEBUG_DISABLE_FAST_READ_PIXEL},
   { "sync-primitive", COGL_DEBUG_SYNC_PRIMITIVE },
   { "sync-frame", COGL_DEBUG_SYNC_FRAME},
+  { "stencilling", COGL_DEBUG_STENCILLING },
 };
 static const int n_cogl_behavioural_debug_keys =
   G_N_ELEMENTS (cogl_behavioural_debug_keys);
diff --git a/cogl/cogl/cogl-debug.h b/cogl/cogl/cogl-debug.h
index e202cc4a4a..d818437f6f 100644
--- a/cogl/cogl/cogl-debug.h
+++ b/cogl/cogl/cogl-debug.h
@@ -75,6 +75,7 @@ typedef enum
   COGL_DEBUG_SYNC_PRIMITIVE,
   COGL_DEBUG_SYNC_FRAME,
   COGL_DEBUG_TEXTURES,
+  COGL_DEBUG_STENCILLING,
 
   COGL_DEBUG_N_FLAGS
 } CoglDebugFlags;
diff --git a/cogl/cogl/driver/gl/cogl-clip-stack-gl.c b/cogl/cogl/driver/gl/cogl-clip-stack-gl.c
index 4975f34cf6..bec4d9b453 100644
--- a/cogl/cogl/driver/gl/cogl-clip-stack-gl.c
+++ b/cogl/cogl/driver/gl/cogl-clip-stack-gl.c
@@ -500,7 +500,8 @@ _cogl_clip_stack_gl_flush (CoglClipStack *stack,
 
               /* We don't need to do anything extra if the clip for this
                  rectangle was entirely described by its scissor bounds */
-              if (!rect->can_be_scissor)
+              if (!rect->can_be_scissor ||
+                  G_UNLIKELY (COGL_DEBUG_ENABLED (COGL_DEBUG_STENCILLING)))
                 {
                   COGL_NOTE (CLIPPING, "Adding stencil clip for rectangle");
 
@@ -522,7 +523,8 @@ _cogl_clip_stack_gl_flush (CoglClipStack *stack,
               /* If nrectangles <= 1, it can be fully represented with the
                * scissor clip.
                */
-              if (cairo_region_num_rectangles (region->region) > 1)
+              if (cairo_region_num_rectangles (region->region) > 1 ||
+                  G_UNLIKELY (COGL_DEBUG_ENABLED (COGL_DEBUG_STENCILLING)))
                 {
                   COGL_NOTE (CLIPPING, "Adding stencil clip for region");