i965: Add a pass to remove dead control flow.

Removes IF/ENDIF and IF/ELSE/ENDIF with no intervening instructions.

total instructions in shared programs: 1360393 -> 1360387 (-0.00%)
instructions in affected programs:     157 -> 151 (-3.82%)

(no change in vertex shaders)

Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/Makefile.sources

@@ -44,6 +44,7 @@ i965_FILES = \
 	brw_context.c \
 	brw_cubemap_normalize.cpp \
 	brw_curbe.c \
+	brw_dead_control_flow.cpp \
 	brw_device_info.c \
 	brw_disasm.c \
 	brw_draw.c \

src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp

@@ -0,0 +1,83 @@
+/*
+ * Copyright © 2013 Intel Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+/** @file brw_dead_control_flow.cpp
+ *
+ * This file implements the dead control flow elimination optimization pass.
+ */
+
+#include "brw_shader.h"
+#include "brw_cfg.h"
+
+/* Look for and eliminate dead control flow:
+ *
+ *   - if/endif
+ *   - if/else/endif
+ */
+bool
+dead_control_flow_eliminate(backend_visitor *v)
+{
+   bool progress = false;
+
+   cfg_t cfg(v);
+
+   for (int b = 0; b < cfg.num_blocks; b++) {
+      bblock_t *block = cfg.blocks[b];
+      bool found = false;
+
+      /* ENDIF instructions, by definition, can only be found at the ends of
+       * basic blocks.
+       */
+      backend_instruction *endif_inst = block->end;
+      if (endif_inst->opcode != BRW_OPCODE_ENDIF)
+         continue;
+
+      backend_instruction *if_inst = NULL, *else_inst = NULL;
+      backend_instruction *prev_inst = (backend_instruction *) endif_inst->prev;
+      if (prev_inst->opcode == BRW_OPCODE_IF) {
+         if_inst = prev_inst;
+         found = true;
+      } else if (prev_inst->opcode == BRW_OPCODE_ELSE) {
+         else_inst = prev_inst;
+
+         prev_inst = (backend_instruction *) prev_inst->prev;
+         if (prev_inst->opcode == BRW_OPCODE_IF) {
+            if_inst = prev_inst;
+            found = true;
+         }
+      }
+
+      if (found) {
+         if_inst->remove();
+         if (else_inst)
+            else_inst->remove();
+         endif_inst->remove();
+         progress = true;
+      }
+   }
+
+   if (progress)
+      v->invalidate_live_intervals();
+
+   return progress;
+}

src/mesa/drivers/dri/i965/brw_dead_control_flow.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright © 2013 Intel Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+#include "brw_shader.h"
+
+bool dead_control_flow_eliminate(backend_visitor *v);

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -46,6 +46,7 @@ extern "C" {
 #include "brw_wm.h"
 }
 #include "brw_fs.h"
+#include "brw_dead_control_flow.h"
 #include "main/uniforms.h"
 #include "brw_fs_live_variables.h"
 #include "glsl/glsl_types.h"
@@ -3264,6 +3265,7 @@ fs_visitor::run()
 	 progress = opt_copy_propagate() || progress;
 	 progress = dead_code_eliminate() || progress;
 	 progress = dead_code_eliminate_local() || progress;
+         progress = dead_control_flow_eliminate(this) || progress;
 	 progress = register_coalesce() || progress;
 	 progress = register_coalesce_2() || progress;
 	 progress = compute_to_mrf() || progress;

src/mesa/drivers/dri/i965/brw_vec4.cpp

@@ -24,6 +24,7 @@
 #include "brw_vec4.h"
 #include "brw_cfg.h"
 #include "brw_vs.h"
+#include "brw_dead_control_flow.h"
 
 extern "C" {
 #include "main/macros.h"
@@ -1498,6 +1499,7 @@ vec4_visitor::run()
    do {
       progress = false;
       progress = dead_code_eliminate() || progress;
+      progress = dead_control_flow_eliminate(this) || progress;
       progress = opt_copy_propagation() || progress;
       progress = opt_algebraic() || progress;
       progress = opt_register_coalesce() || progress;