[Mesa-dev] [PATCH 1/2] mesa: reduce CPU overhead when updating the scissor

[Samuel Pitoiset]

Before:
DrawElements (1 VBO, 8 UBO, 8 Tex) w/ scissor change: 3.67 million (44.4%)

After:
DrawElements (1 VBO, 8 UBO, 8 Tex) w/ scissor change: 5.02 million (62.1%)

Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
---
 src/mesa/main/scissor.c | 79 ++++++++++++++++++++++++++++++-------------------
 1 file changed, 48 insertions(+), 31 deletions(-)

diff --git a/src/mesa/main/scissor.c b/src/mesa/main/scissor.c
index b38db06bc8..c2d85bae2e 100644
--- a/src/mesa/main/scissor.c
+++ b/src/mesa/main/scissor.c
@@ -29,28 +29,20 @@
 #include "main/mtypes.h"
 #include "main/scissor.h"
 
+static bool
+is_scissor_equal(struct gl_context *ctx, unsigned idx,
+                 GLint x, GLint y, GLsizei width, GLsizei height)
+{
+   return ctx->Scissor.ScissorArray[idx].X == x &&
+          ctx->Scissor.ScissorArray[idx].Y == y &&
+          ctx->Scissor.ScissorArray[idx].Width == width &&
+          ctx->Scissor.ScissorArray[idx].Height == height;
+}
 
-/**
- * Set scissor rectangle data directly in ScissorArray
- *
- * This is an internal function that performs no error checking on the
- * supplied data.  It also does \b not call \c dd_function_table::Scissor.
- *
- * \sa _mesa_set_scissor
- */
 static void
-set_scissor_no_notify(struct gl_context *ctx, unsigned idx,
-                      GLint x, GLint y, GLsizei width, GLsizei height)
+set_new_scissor(struct gl_context *ctx, unsigned idx,
+                GLint x, GLint y, GLsizei width, GLsizei height)
 {
-   if (x == ctx->Scissor.ScissorArray[idx].X &&
-       y == ctx->Scissor.ScissorArray[idx].Y &&
-       width == ctx->Scissor.ScissorArray[idx].Width &&
-       height == ctx->Scissor.ScissorArray[idx].Height)
-      return;
-
-   FLUSH_VERTICES(ctx, ctx->DriverFlags.NewScissorRect ? 0 : _NEW_SCISSOR);
-   ctx->NewDriverState |= ctx->DriverFlags.NewScissorRect;
-
    ctx->Scissor.ScissorArray[idx].X = x;
    ctx->Scissor.ScissorArray[idx].Y = y;
    ctx->Scissor.ScissorArray[idx].Width = width;
@@ -58,8 +50,19 @@ set_scissor_no_notify(struct gl_context *ctx, unsigned idx,
 }
 
 static void
+flush_scissor(struct gl_context *ctx)
+{
+   FLUSH_VERTICES(ctx, ctx->DriverFlags.NewScissorRect ? 0 : _NEW_SCISSOR);
+   ctx->NewDriverState |= ctx->DriverFlags.NewScissorRect;
+
+   if (ctx->Driver.Scissor)
+      ctx->Driver.Scissor(ctx);
+}
+
+static void
 scissor(struct gl_context *ctx, GLint x, GLint y, GLsizei width, GLsizei height)
 {
+   bool changed = false;
    unsigned i;
 
    /* The GL_ARB_viewport_array spec says:
@@ -74,11 +77,16 @@ scissor(struct gl_context *ctx, GLint x, GLint y, GLsizei width, GLsizei height)
     * Set the scissor rectangle for all of the viewports supported by the
     * implementation, but only signal the driver once at the end.
     */
-   for (i = 0; i < ctx->Const.MaxViewports; i++)
-      set_scissor_no_notify(ctx, i, x, y, width, height);
+   for (i = 0; i < ctx->Const.MaxViewports; i++) {
+      if (!changed && is_scissor_equal(ctx, i, x, y, width, height))
+         continue;
 
-   if (ctx->Driver.Scissor)
-      ctx->Driver.Scissor(ctx);
+      changed = true;
+      set_new_scissor(ctx, i, x, y, width, height);
+   }
+
+   if (changed)
+      flush_scissor(ctx);
 }
 
 /**
@@ -125,23 +133,31 @@ void
 _mesa_set_scissor(struct gl_context *ctx, unsigned idx,
                   GLint x, GLint y, GLsizei width, GLsizei height)
 {
-   set_scissor_no_notify(ctx, idx, x, y, width, height);
+   if (is_scissor_equal(ctx, idx, x, y, width, height))
+      return;
 
-   if (ctx->Driver.Scissor)
-      ctx->Driver.Scissor(ctx);
+   set_new_scissor(ctx, idx, x, y, width, height);
+   flush_scissor(ctx);
 }
 
 static void
 scissor_array(struct gl_context *ctx, GLuint first, GLsizei count,
               struct gl_scissor_rect *rect)
 {
+   bool changed = false;
+
    for (GLsizei i = 0; i < count; i++) {
-      set_scissor_no_notify(ctx, i + first, rect[i].X, rect[i].Y,
-                            rect[i].Width, rect[i].Height);
+      if (!changed && is_scissor_equal(ctx, i + first, rect[i].X, rect[i].Y,
+                                       rect[i].Width, rect[i].Height))
+         continue;
+
+      changed = true;
+      set_new_scissor(ctx, i + first, rect[i].X, rect[i].Y,
+                      rect[i].Width, rect[i].Height);
    }
 
-   if (ctx->Driver.Scissor)
-      ctx->Driver.Scissor(ctx);
+   if (changed)
+      flush_scissor(ctx);
 }
 
 /**
@@ -328,5 +344,6 @@ _mesa_init_scissor(struct gl_context *ctx)
     * so just initialize all of them.
     */
    for (i = 0; i < MAX_VIEWPORTS; i++)
-      set_scissor_no_notify(ctx, i, 0, 0, 0, 0);
+      set_new_scissor(ctx, i, 0, 0, 0, 0);
+   flush_scissor(ctx);
 }
-- 
2.13.3