glsl: Add an optimization pass to simplify discards.

NOTE: This is a candidate for the 7.9 branch.
(cherry picked from commit 9a1d063c6d)

src/glsl/Makefile

@@ -77,6 +77,7 @@ CXX_SOURCES = \
 	loop_unroll.cpp \
 	lower_noise.cpp \
 	lower_variable_index_to_cond_assign.cpp \
+	opt_discard_simplification.cpp \
 	opt_redundant_jumps.cpp \
 	s_expression.cpp
 

src/glsl/SConscript

@@ -74,6 +74,7 @@ sources = [
     'loop_unroll.cpp',
     'lower_noise.cpp',
     'lower_variable_index_to_cond_assign.cpp',
+    'opt_discard_simplification.cpp',
     'opt_redundant_jumps.cpp',
     's_expression.cpp',
 ]

src/glsl/glsl_parser_extras.cpp

@@ -702,6 +702,7 @@ do_common_optimization(exec_list *ir, bool linked, unsigned max_unroll_iteration
    }
    progress = do_structure_splitting(ir) || progress;
    progress = do_if_simplification(ir) || progress;
+   progress = do_discard_simplification(ir) || progress;
    progress = do_copy_propagation(ir) || progress;
    if (linked)
       progress = do_dead_code(ir) || progress;

src/glsl/ir_optimization.h

@@ -45,6 +45,7 @@ bool do_explog_to_explog2(exec_list *instructions);
 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_if_simplification(exec_list *instructions);
+bool do_discard_simplification(exec_list *instructions);
 bool do_if_to_cond_assign(exec_list *instructions);
 bool do_mat_op_to_vec(exec_list *instructions);
 bool do_mod_to_fract(exec_list *instructions);

src/glsl/opt_discard_simplification.cpp

@@ -0,0 +1,180 @@
+/*
+ * Copyright © 2010 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_discard_simplification.cpp
+ *
+ * This pass simplifies if-statements and loops containing unconditional
+ * discards.
+ *
+ * Case 1: Both branches contain unconditional discards:
+ * -----------------------------------------------------
+ *
+ *    if (cond) {
+ *	 s1;
+ *	 discard;
+ *	 s2;
+ *    } else {
+ *	 s3;
+ *	 discard;
+ *	 s4;
+ *    }
+ *
+ * becomes:
+ *
+ *    discard
+ *
+ * Case 2: The "then" clause contains an unconditional discard:
+ * ------------------------------------------------------------
+ *
+ *    if (cond) {
+ *       s1;
+ *       discard;
+ *       s2;
+ *    } else {
+ *	 s3;
+ *    }
+ *
+ * becomes:
+ *
+ *    if (cond) {
+ *	 discard;
+ *    } else {
+ *	 s3;
+ *    }
+ *
+ * Case 3: The "else" clause contains an unconditional discard:
+ * ------------------------------------------------------------
+ *
+ *    if (cond) {
+ *       s1;
+ *    } else {
+ *       s2;
+ *       discard;
+ *	 s3;
+ *    }
+ *
+ * becomes:
+ *
+ *    if (cond) {
+ *	 s1;
+ *    } else {
+ *	 discard;
+ *    }
+ */
+
+#include "glsl_types.h"
+#include "ir.h"
+
+class discard_simplifier : public ir_hierarchical_visitor {
+public:
+   discard_simplifier()
+   {
+      this->progress = false;
+   }
+
+   ir_visitor_status visit_enter(ir_if *);
+   ir_visitor_status visit_enter(ir_loop *);
+
+   bool progress;
+};
+
+static ir_discard *
+find_unconditional_discard(exec_list &instructions)
+{
+   foreach_list(n, &instructions) {
+      ir_discard *ir = ((ir_instruction *) n)->as_discard();
+      if (ir != NULL && ir->condition == NULL)
+	 return ir;
+   }
+   return NULL;
+}
+
+static bool
+is_only_instruction(ir_discard *discard)
+{
+   return (discard->prev->is_head_sentinel() &&
+	   discard->next->is_tail_sentinel());
+}
+
+ir_visitor_status
+discard_simplifier::visit_enter(ir_if *ir)
+{
+   ir_discard *then_discard = find_unconditional_discard(ir->then_instructions);
+   ir_discard *else_discard = find_unconditional_discard(ir->else_instructions);
+
+   if (then_discard == NULL && else_discard == NULL)
+      return visit_continue;
+
+   /* If both branches result in discard, replace whole if with discard. */
+   if (then_discard != NULL && else_discard != NULL) {
+      this->progress = true;
+      ir->replace_with(then_discard);
+      return visit_continue_with_parent;
+   }
+
+   /* Otherwise, one branch has a discard. */
+   if (then_discard != NULL && !is_only_instruction(then_discard)) {
+      this->progress = true;
+      ir->then_instructions.make_empty();
+      ir->then_instructions.push_tail(then_discard);
+   } else if (else_discard != NULL && !is_only_instruction(else_discard)) {
+      this->progress = true;
+      ir->else_instructions.make_empty();
+      ir->else_instructions.push_tail(else_discard);
+   }
+
+   visit_list_elements(this, &ir->then_instructions);
+   return visit_continue_with_parent;
+}
+
+ir_visitor_status
+discard_simplifier::visit_enter(ir_loop *ir)
+{
+   ir_discard *discard = find_unconditional_discard(ir->body_instructions);
+
+   if (discard) {
+      ir->replace_with(discard);
+      return visit_continue_with_parent;
+   }
+
+   return visit_continue;
+}
+
+bool
+do_discard_simplification(exec_list *instructions)
+{
+   /* Look for a top-level unconditional discard */
+   ir_discard *discard = find_unconditional_discard(*instructions);
+   if (discard != NULL) {
+      instructions->make_empty();
+      instructions->push_tail(discard);
+      return true;
+   }
+
+   discard_simplifier v;
+
+   visit_list_elements(&v, instructions);
+
+   return v.progress;
+}