r300g: optimize blending by conditionally disabling reads from the colorbuffer

2 files changed, 30 insertions, 3 deletions

diff --git a/src/gallium/drivers/r300/r300_reg.h b/src/gallium/drivers/r300/r300_reg.h
index 0aa1da07f8b..034bfc15cf9 100644
--- a/src/gallium/drivers/r300/r300_reg.h
+++ b/src/gallium/drivers/r300/r300_reg.h
@@ -2186,6 +2186,8 @@ USE OR OTHER DEALINGS IN THE SOFTWARE.
 #       define R300_DISCARD_SRC_PIXELS_SRC_ALPHA_1     (4 << 3)
 #       define R300_DISCARD_SRC_PIXELS_SRC_COLOR_1     (5 << 3)
 #       define R300_DISCARD_SRC_PIXELS_SRC_ALPHA_COLOR_1     (6 << 3)
+#       define R500_SRC_ALPHA_0_NO_READ                (1 << 30)
+#       define R500_SRC_ALPHA_1_NO_READ                (1 << 31)
 
 /* the following are shared between CBLEND and ABLEND */
 #       define R300_FCN_MASK                         (3  << 12)
diff --git a/src/gallium/drivers/r300/r300_state.c b/src/gallium/drivers/r300/r300_state.c
index f8cce112ee6..872a393321d 100644
--- a/src/gallium/drivers/r300/r300_state.c
+++ b/src/gallium/drivers/r300/r300_state.c
@@ -198,9 +198,36 @@ static void* r300_create_blend_state(struct pipe_context* pipe,
             srcA == PIPE_BLENDFACTOR_DST_ALPHA ||
             srcA == PIPE_BLENDFACTOR_INV_DST_COLOR ||
             srcA == PIPE_BLENDFACTOR_INV_DST_ALPHA ||
-            srcRGB == PIPE_BLENDFACTOR_SRC_ALPHA_SATURATE)
+            srcRGB == PIPE_BLENDFACTOR_SRC_ALPHA_SATURATE) {
+            /* Enable reading from the colorbuffer. */
             blend->blend_control |= R300_READ_ENABLE;
 
+            if (r300_screen(r300_context(pipe)->context.screen)->caps->is_r500) {
+                /* Optimization: Depending on incoming pixels, we can
+                 * conditionally disable the reading in hardware... */
+                if (eqRGB != PIPE_BLEND_MIN && eqA != PIPE_BLEND_MIN &&
+                    eqRGB != PIPE_BLEND_MAX && eqA != PIPE_BLEND_MAX) {
+                    /* Disable reading if SRC_ALPHA == 0. */
+                    if ((dstRGB == PIPE_BLENDFACTOR_SRC_ALPHA ||
+                         dstRGB == PIPE_BLENDFACTOR_ZERO) &&
+                        (dstA == PIPE_BLENDFACTOR_SRC_COLOR ||
+                         dstA == PIPE_BLENDFACTOR_SRC_ALPHA ||
+                         dstA == PIPE_BLENDFACTOR_ZERO)) {
+                         blend->blend_control |= R500_SRC_ALPHA_0_NO_READ;
+                    }
+
+                    /* Disable reading if SRC_ALPHA == 1. */
+                    if ((dstRGB == PIPE_BLENDFACTOR_INV_SRC_ALPHA ||
+                         dstRGB == PIPE_BLENDFACTOR_ZERO) &&
+                        (dstA == PIPE_BLENDFACTOR_INV_SRC_COLOR ||
+                         dstA == PIPE_BLENDFACTOR_INV_SRC_ALPHA ||
+                         dstA == PIPE_BLENDFACTOR_ZERO)) {
+                         blend->blend_control |= R500_SRC_ALPHA_1_NO_READ;
+                    }
+                }
+            }
+        }
+
         /* Optimization: discard pixels which don't change the colorbuffer.
          *
          * The code below is non-trivial and some math is involved.
@@ -246,8 +273,6 @@ static void* r300_create_blend_state(struct pipe_context* pipe,
             }
         }
 
-        /* XXX implement the optimization with SRC_ALPHA_?_NO_READ */
-
         /* separate alpha */
         if (srcA != srcRGB || dstA != dstRGB || eqA != eqRGB) {
             blend->blend_control |= R300_SEPARATE_ALPHA_ENABLE;