glsl: Add an optimization pass to flatten simple nested if blocks.

GLBenchmark 2.7's shaders contain conditional blocks like:

if (x) {
    if (y) {
        ...
    }
}

where the outer conditional's then clause contains exactly one statement
(the nested if) and there are no else clauses.  This can easily be
optimized into:

if (x && y) {
    ...
}

This saves a few instructions in GLBenchmark 2.7:

    total instructions in shared programs: 11833 -> 11649 (-1.55%)
    instructions in affected programs:     8234 -> 8050 (-2.23%)

It also helps CS:GO slightly (-0.05%/-0.22%).  More importantly,
however, it simplifies the control flow graph, which could enable other
optimizations.

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

src/glsl/Makefile.sources

@@ -80,6 +80,7 @@ LIBGLSL_FILES = \
 	$(GLSL_SRCDIR)/opt_dead_code.cpp \
 	$(GLSL_SRCDIR)/opt_dead_code_local.cpp \
 	$(GLSL_SRCDIR)/opt_dead_functions.cpp \
+	$(GLSL_SRCDIR)/opt_flatten_nested_if_blocks.cpp \
 	$(GLSL_SRCDIR)/opt_function_inlining.cpp \
 	$(GLSL_SRCDIR)/opt_if_simplification.cpp \
 	$(GLSL_SRCDIR)/opt_noop_swizzle.cpp \

src/glsl/glsl_parser_extras.cpp

@@ -1218,6 +1218,7 @@ do_common_optimization(exec_list *ir, bool linked,
       progress = do_structure_splitting(ir) || progress;
    }
    progress = do_if_simplification(ir) || progress;
+   progress = opt_flatten_nested_if_blocks(ir) || progress;
    progress = do_copy_propagation(ir) || progress;
    progress = do_copy_propagation_elements(ir) || progress;
    if (linked)

src/glsl/ir_optimization.h

@@ -82,6 +82,7 @@ bool do_function_inlining(exec_list *instructions);
 bool do_lower_jumps(exec_list *instructions, bool pull_out_jumps = true, bool lower_sub_return = true, bool lower_main_return = false, bool lower_continue = false, bool lower_break = false);
 bool do_lower_texture_projection(exec_list *instructions);
 bool do_if_simplification(exec_list *instructions);
+bool opt_flatten_nested_if_blocks(exec_list *instructions);
 bool do_discard_simplification(exec_list *instructions);
 bool lower_if_to_cond_assign(exec_list *instructions, unsigned max_depth = 0);
 bool do_mat_op_to_vec(exec_list *instructions);

src/glsl/opt_flatten_nested_if_blocks.cpp

@@ -0,0 +1,103 @@
+/*
+ * 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 opt_flatten_nested_if_blocks.cpp
+ *
+ * Flattens nested if blocks such as:
+ *
+ * if (x) {
+ *    if (y) {
+ *       ...
+ *    }
+ * }
+ *
+ * into a single if block with a combined condition:
+ *
+ * if (x && y) {
+ *    ...
+ * }
+ */
+
+#include "ir.h"
+#include "ir_builder.h"
+
+using namespace ir_builder;
+
+namespace {
+
+class nested_if_flattener : public ir_hierarchical_visitor {
+public:
+   nested_if_flattener()
+   {
+      progress = false;
+   }
+
+   ir_visitor_status visit_leave(ir_if *);
+   ir_visitor_status visit_enter(ir_assignment *);
+
+   bool progress;
+};
+
+} /* unnamed namespace */
+
+/* We only care about the top level "if" instructions, so don't
+ * descend into expressions.
+ */
+ir_visitor_status
+nested_if_flattener::visit_enter(ir_assignment *ir)
+{
+   (void) ir;
+   return visit_continue_with_parent;
+}
+
+bool
+opt_flatten_nested_if_blocks(exec_list *instructions)
+{
+   nested_if_flattener v;
+
+   v.run(instructions);
+   return v.progress;
+}
+
+
+ir_visitor_status
+nested_if_flattener::visit_leave(ir_if *ir)
+{
+   /* Only handle a single ir_if within the then clause of an ir_if.  No extra
+    * instructions, no else clauses, nothing.
+    */
+   if (ir->then_instructions.is_empty() || !ir->else_instructions.is_empty())
+      return visit_continue;
+
+   ir_if *inner = ((ir_instruction *) ir->then_instructions.head)->as_if();
+   if (!inner || !inner->next->is_tail_sentinel() ||
+       !inner->else_instructions.is_empty())
+      return visit_continue;
+
+   ir->condition = logic_and(ir->condition, inner->condition);
+   inner->then_instructions.move_nodes_to(&ir->then_instructions);
+
+   progress = true;
+   return visit_continue;
+}