glsl: Add a lowering pass to move discards out of if-statements.

This should allow lower_if_to_cond_assign to work in the presence of
discards, fixing bug #31690 and likely #31983.

NOTE: This is a candidate for the 7.9 branch.

src/glsl/Makefile

@@ -52,6 +52,7 @@ CXX_SOURCES = \
 	loop_analysis.cpp \
 	loop_controls.cpp \
 	loop_unroll.cpp \
+	lower_discard.cpp \
 	lower_if_to_cond_assign.cpp \
 	lower_instructions.cpp \
 	lower_jumps.cpp \

src/glsl/SConscript

@@ -49,6 +49,7 @@ sources = [
     'loop_analysis.cpp',
     'loop_controls.cpp',
     'loop_unroll.cpp',
+    'lower_discard.cpp',
     'lower_if_to_cond_assign.cpp',
     'lower_instructions.cpp',
     'lower_jumps.cpp',

src/glsl/ir_optimization.h

@@ -62,6 +62,7 @@ bool do_swizzle_swizzle(exec_list *instructions);
 bool do_tree_grafting(exec_list *instructions);
 bool do_vec_index_to_cond_assign(exec_list *instructions);
 bool do_vec_index_to_swizzle(exec_list *instructions);
+bool lower_discard(exec_list *instructions);
 bool lower_instructions(exec_list *instructions, unsigned what_to_lower);
 bool lower_noise(exec_list *instructions);
 bool lower_variable_index_to_cond_assign(exec_list *instructions,

src/glsl/lower_discard.cpp

@@ -0,0 +1,198 @@
+/*
+ * 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 lower_discard.cpp
+ *
+ * This pass moves discards out of if-statements.
+ *
+ * Case 1: The "then" branch contains a conditional discard:
+ * ---------------------------------------------------------
+ *
+ *    if (cond1) {
+ *	 s1;
+ *	 discard cond2;
+ *	 s2;
+ *    } else {
+ *	 s3;
+ *    }
+ *
+ * becomes:
+ *
+ *    temp = false;
+ *    if (cond1) {
+ *	 s1;
+ *	 temp = cond2;
+ *	 s2;
+ *    } else {
+ *	 s3;
+ *    }
+ *    discard temp;
+ *
+ * Case 2: The "else" branch contains a conditional discard:
+ * ---------------------------------------------------------
+ *
+ *    if (cond1) {
+ *	 s1;
+ *    } else {
+ *	 s2;
+ *	 discard cond2;
+ *	 s3;
+ *    }
+ *
+ * becomes:
+ *
+ *    temp = false;
+ *    if (cond1) {
+ *	 s1;
+ *    } else {
+ *	 s2;
+ *	 temp = cond2;
+ *	 s3;
+ *    }
+ *    discard temp;
+ *
+ * Case 3: Both branches contain a conditional discard:
+ * ----------------------------------------------------
+ *
+ *    if (cond1) {
+ *	 s1;
+ *	 discard cond2;
+ *	 s2;
+ *    } else {
+ *	 s3;
+ *	 discard cond3;
+ *	 s4;
+ *    }
+ *
+ * becomes:
+ *
+ *    temp = false;
+ *    if (cond1) {
+ *	 s1;
+ *	 temp = cond2;
+ *	 s2;
+ *    } else {
+ *	 s3;
+ *	 temp = cond3;
+ *	 s4;
+ *    }
+ *    discard temp;
+ *
+ * If there are multiple conditional discards, we need only deal with one of
+ * them.  Repeatedly applying this pass will take care of the others.
+ *
+ * Unconditional discards are treated as having a condition of "true".
+ */
+
+#include "glsl_types.h"
+#include "ir.h"
+
+class lower_discard_visitor : public ir_hierarchical_visitor {
+public:
+   lower_discard_visitor()
+   {
+      this->progress = false;
+   }
+
+   ir_visitor_status visit_leave(ir_if *);
+
+   bool progress;
+};
+
+
+bool
+lower_discard(exec_list *instructions)
+{
+   lower_discard_visitor v;
+
+   visit_list_elements(&v, instructions);
+
+   return v.progress;
+}
+
+
+static ir_discard *
+find_discard(exec_list &instructions)
+{
+   foreach_list(n, &instructions) {
+      ir_discard *ir = ((ir_instruction *) n)->as_discard();
+      if (ir != NULL)
+	 return ir;
+   }
+   return NULL;
+}
+
+
+static void
+replace_discard(void *mem_ctx, ir_variable *var, ir_discard *ir)
+{
+   ir_rvalue *condition = ir->condition;
+
+   /* For unconditional discards, use "true" as the condition. */
+   if (condition == NULL)
+      condition = new(mem_ctx) ir_constant(true);
+
+   ir_assignment *assignment =
+      new(mem_ctx) ir_assignment(new(mem_ctx) ir_dereference_variable(var),
+				 condition, NULL);
+
+   ir->replace_with(assignment);
+}
+
+
+ir_visitor_status
+lower_discard_visitor::visit_leave(ir_if *ir)
+{
+   ir_discard *then_discard = find_discard(ir->then_instructions);
+   ir_discard *else_discard = find_discard(ir->else_instructions);
+
+   if (then_discard == NULL && else_discard == NULL)
+      return visit_continue;
+
+   void *mem_ctx = talloc_parent(ir);
+
+   ir_variable *temp = new(mem_ctx) ir_variable(glsl_type::bool_type,
+						"discard_cond_temp",
+						ir_var_temporary);
+   ir_assignment *temp_initializer =
+      new(mem_ctx) ir_assignment(new(mem_ctx) ir_dereference_variable(temp),
+				 new(mem_ctx) ir_constant(false), NULL);
+
+   ir->insert_before(temp);
+   ir->insert_before(temp_initializer);
+
+   if (then_discard != NULL)
+      replace_discard(mem_ctx, temp, then_discard);
+
+   if (else_discard != NULL)
+      replace_discard(mem_ctx, temp, else_discard);
+
+   ir_discard *discard = then_discard != NULL ? then_discard : else_discard;
+   discard->condition = new(mem_ctx) ir_dereference_variable(temp);
+   ir->insert_after(discard);
+
+   this->progress = true;
+
+   return visit_continue;
+}

src/mesa/program/ir_to_mesa.cpp

@@ -2858,8 +2858,10 @@ _mesa_ir_link_shader(struct gl_context *ctx, struct gl_shader_program *prog)
 
 	 progress = lower_quadop_vector(ir, true) || progress;
 
-	 if (options->EmitNoIfs)
+	 if (options->EmitNoIfs) {
+	    progress = lower_discard(ir) || progress;
 	    progress = do_if_to_cond_assign(ir) || progress;
+	 }
 
 	 if (options->EmitNoNoise)
 	    progress = lower_noise(ir) || progress;