[Mesa-dev] [PATCH 2/3] glsl: Implement the GLSL 1.30+ discard control flow rule in GLSL IR.

[Eric Anholt]

Previously, I tried implementing this in the i965 driver, but did so
in a way that violated the intent of the spec, and broke Tropics.
---
 src/glsl/Makefile.sources       |    1 +
 src/glsl/ir_optimization.h      |    1 +
 src/glsl/linker.cpp             |   11 +++
 src/glsl/lower_discard_flow.cpp |  144 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 157 insertions(+)
 create mode 100644 src/glsl/lower_discard_flow.cpp

diff --git a/src/glsl/Makefile.sources b/src/glsl/Makefile.sources
index b8f7e5f..fba0360 100644
--- a/src/glsl/Makefile.sources
+++ b/src/glsl/Makefile.sources
@@ -51,6 +51,7 @@ LIBGLSL_CXX_FILES := \
 	loop_unroll.cpp \
 	lower_clip_distance.cpp \
 	lower_discard.cpp \
+	lower_discard_flow.cpp \
 	lower_if_to_cond_assign.cpp \
 	lower_instructions.cpp \
 	lower_jumps.cpp \
diff --git a/src/glsl/ir_optimization.h b/src/glsl/ir_optimization.h
index 3567835..cca6382 100644
--- a/src/glsl/ir_optimization.h
+++ b/src/glsl/ir_optimization.h
@@ -66,6 +66,7 @@ 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_discard_flow(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,
diff --git a/src/glsl/linker.cpp b/src/glsl/linker.cpp
index 6ba2972..bdab499 100644
--- a/src/glsl/linker.cpp
+++ b/src/glsl/linker.cpp
@@ -2296,6 +2296,17 @@ link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
       prog->LinkStatus = true;
    }
 
+   /* Implement the GLSL 1.30+ rule for discard vs infinite loops Do
+    * it before optimization because we want most of the checks to get
+    * dropped thanks to constant propagation.
+    */
+   if (max_version >= 130) {
+      struct gl_shader *sh = prog->_LinkedShaders[MESA_SHADER_FRAGMENT];
+      if (sh) {
+	 lower_discard_flow(sh->ir);
+      }
+   }
+
    /* Do common optimization before assigning storage for attributes,
     * uniforms, and varyings.  Later optimization could possibly make
     * some of that unused.
diff --git a/src/glsl/lower_discard_flow.cpp b/src/glsl/lower_discard_flow.cpp
new file mode 100644
index 0000000..d385c14
--- /dev/null
+++ b/src/glsl/lower_discard_flow.cpp
@@ -0,0 +1,144 @@
+/*
+ * 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_flow.cpp
+ *
+ * Implements the GLSL 1.30 revision 9 rule for fragment shader
+ * discard handling:
+ *
+ *     "Control flow exits the shader, and subsequent implicit or
+ *      explicit derivatives are undefined when this control flow is
+ *      non-uniform (meaning different fragments within the primitive
+ *      take different control paths)."
+ *
+ * There seem to be two conflicting things here.  "Control flow exits
+ * the shader" sounds like the discarded fragments should effectively
+ * jump to the end of the shader, but that breaks derivatives in the
+ * case of uniform control flow and causes rendering failure in the
+ * bushes in Unigine Tropics.
+ *
+ * The question, then, is whether the intent was "loops stop at the
+ * point that the only active channels left are discarded pixels" or
+ * "discarded pixels become inactive at the point that control flow
+ * returns to the top of a loop".  This implements the second
+ * interpretation.
+ */
+
+#include "glsl_types.h"
+#include "ir.h"
+#include "program/hash_table.h"
+
+class lower_discard_flow_visitor : public ir_hierarchical_visitor {
+public:
+   lower_discard_flow_visitor(ir_variable *discarded)
+   : discarded(discarded)
+   {
+      mem_ctx = ralloc_parent(discarded);
+   }
+
+   ~lower_discard_flow_visitor()
+   {
+   }
+
+   ir_visitor_status visit_enter(ir_discard *ir);
+   ir_visitor_status visit_enter(ir_loop_jump *ir);
+   ir_visitor_status visit_enter(ir_loop *ir);
+   ir_visitor_status visit_enter(ir_function_signature *ir);
+
+   ir_if *generate_discard_break();
+
+   ir_variable *discarded;
+   void *mem_ctx;
+};
+
+ir_visitor_status
+lower_discard_flow_visitor::visit_enter(ir_loop_jump *ir)
+{
+   if (ir->mode != ir_loop_jump::jump_continue)
+      return visit_continue;
+
+   ir->insert_before(generate_discard_break());
+
+   return visit_continue;
+}
+
+ir_visitor_status
+lower_discard_flow_visitor::visit_enter(ir_discard *ir)
+{
+   ir_dereference *lhs = new(mem_ctx) ir_dereference_variable(discarded);
+   ir_rvalue *rhs = new(mem_ctx) ir_constant(true);
+   ir_assignment *assign = new(mem_ctx) ir_assignment(lhs, rhs);
+   ir->insert_before(assign);
+
+   return visit_continue;
+}
+
+ir_visitor_status
+lower_discard_flow_visitor::visit_enter(ir_loop *ir)
+{
+   ir->body_instructions.push_tail(generate_discard_break());
+
+   return visit_continue;
+}
+
+ir_visitor_status
+lower_discard_flow_visitor::visit_enter(ir_function_signature *ir)
+{
+   if (strcmp(ir->function_name(), "main") != 0)
+      return visit_continue;
+
+   ir_dereference *lhs = new(mem_ctx) ir_dereference_variable(discarded);
+   ir_rvalue *rhs = new(mem_ctx) ir_constant(false);
+   ir_assignment *assign = new(mem_ctx) ir_assignment(lhs, rhs);
+   ir->body.push_head(assign);
+
+   return visit_continue;
+}
+
+ir_if *
+lower_discard_flow_visitor::generate_discard_break()
+{
+   ir_rvalue *if_condition = new(mem_ctx) ir_dereference_variable(discarded);
+   ir_if *if_inst = new(mem_ctx) ir_if(if_condition);
+
+   ir_instruction *br = new(mem_ctx) ir_loop_jump(ir_loop_jump::jump_break);
+   if_inst->then_instructions.push_tail(br);
+
+   return if_inst;
+}
+
+void
+lower_discard_flow(exec_list *ir)
+{
+   void *mem_ctx = ir;
+
+   ir_variable *var = new(mem_ctx) ir_variable(glsl_type::bool_type,
+					       "discarded",
+					       ir_var_temporary);
+
+   ir->push_head(var);
+
+   lower_discard_flow_visitor v(var);
+
+   visit_list_elements(&v, ir);
+}
-- 
1.7.10