glsl: Optimize "if (cond) discard;" to a conditional discard.

st_glsl_to_tgsi and ir_to_mesa have handled conditional discards for a
long time; the previous patch added that capability to i965.

i965 (Haswell) shader-db stats:

Without NIR:
total instructions in shared programs: 5792133 -> 5776360 (-0.27%)
instructions in affected programs:     737585 -> 721812 (-2.14%)
helped:                                6300
HURT:                                  68
GAINED:                                2

With NIR:
total instructions in shared programs: 5787538 -> 5769569 (-0.31%)
instructions in affected programs:     767843 -> 749874 (-2.34%)
helped:                                6522
HURT:                                  35
GAINED:                                6

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/glsl/Makefile.sources

@@ -157,6 +157,7 @@ LIBGLSL_FILES = \
 	lower_ubo_reference.cpp \
 	opt_algebraic.cpp \
 	opt_array_splitting.cpp \
+	opt_conditional_discard.cpp \
 	opt_constant_folding.cpp \
 	opt_constant_propagation.cpp \
 	opt_constant_variable.cpp \

src/glsl/glsl_parser_extras.cpp

@@ -1627,6 +1627,7 @@ do_common_optimization(exec_list *ir, bool linked,
    }
    progress = do_if_simplification(ir) || progress;
    progress = opt_flatten_nested_if_blocks(ir) || progress;
+   progress = opt_conditional_discard(ir) || progress;
    progress = do_copy_propagation(ir) || progress;
    progress = do_copy_propagation_elements(ir) || progress;
 

src/glsl/ir_optimization.h

@@ -77,6 +77,7 @@ bool do_common_optimization(exec_list *ir, bool linked,
 bool do_rebalance_tree(exec_list *instructions);
 bool do_algebraic(exec_list *instructions, bool native_integers,
                   const struct gl_shader_compiler_options *options);
+bool opt_conditional_discard(exec_list *instructions);
 bool do_constant_folding(exec_list *instructions);
 bool do_constant_variable(exec_list *instructions);
 bool do_constant_variable_unlinked(exec_list *instructions);

src/glsl/opt_conditional_discard.cpp

@@ -0,0 +1,81 @@
+/*
+ * Copyright © 2014 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 opt_conditional_discard.cpp
+ *
+ * Replace
+ *
+ *    if (cond) discard;
+ *
+ * with
+ *
+ *    (discard <condition>)
+ */
+
+#include "glsl_types.h"
+#include "ir.h"
+
+namespace {
+
+class opt_conditional_discard_visitor : public ir_hierarchical_visitor {
+public:
+   opt_conditional_discard_visitor()
+   {
+      progress = false;
+   }
+
+   ir_visitor_status visit_leave(ir_if *);
+
+   bool progress;
+};
+
+} /* anonymous namespace */
+
+bool
+opt_conditional_discard(exec_list *instructions)
+{
+   opt_conditional_discard_visitor v;
+   v.run(instructions);
+   return v.progress;
+}
+
+ir_visitor_status
+opt_conditional_discard_visitor::visit_leave(ir_if *ir)
+{
+   /* Look for "if (...) discard" with no else clause or extra statements. */
+   if (ir->then_instructions.is_empty() ||
+       !ir->then_instructions.head->next->is_tail_sentinel() ||
+       !((ir_instruction *) ir->then_instructions.head)->as_discard() ||
+       !ir->else_instructions.is_empty())
+      return visit_continue;
+
+   /* Move the condition and replace the ir_if with the ir_discard. */
+   ir_discard *discard = (ir_discard *) ir->then_instructions.head;
+   discard->condition = ir->condition;
+   ir->replace_with(discard);
+
+   progress = true;
+
+   return visit_continue;
+}