glsl2: Add module to suss out loop control variables from loop analysis data

This is the next step on the road to loop unrolling

src/glsl/Makefile

@@ -73,6 +73,7 @@ CXX_SOURCES = \
 	linker.cpp \
 	link_functions.cpp \
 	loop_analysis.cpp \
+	loop_controls.cpp \
 	s_expression.cpp
 
 LIBS = \

src/glsl/Makefile.am

@@ -63,7 +63,8 @@ glsl_SOURCES = \
 	ir_validate.cpp \
 	ir_vec_index_to_swizzle.cpp \
 	linker.cpp \
-	loop_analysis.cpp
+	loop_analysis.cpp \
+	loop_controls.cpp
 
 BUILT_SOURCES = glsl_parser.h glsl_parser.cpp glsl_lexer.cpp
 CLEANFILES = $(BUILT_SOURCES)

src/glsl/loop_analysis.h

@@ -36,6 +36,26 @@ extern class loop_state *
 analyze_loop_variables(exec_list *instructions);
 
 
+/**
+ * Fill in loop control fields
+ *
+ * Based on analysis of loop variables, this function tries to remove sequences
+ * in the loop of the form
+ *
+ *  (if (expression bool ...) (break))
+ *
+ * and fill in the \c ir_loop::from, \c ir_loop::to, and \c ir_loop::counter
+ * fields of the \c ir_loop.
+ *
+ * In this process, some conditional break-statements may be eliminated
+ * altogether.  For example, if it is provable that one loop exit condition will
+ * always be satisfied before another, the unnecessary exit condition will be
+ * removed.
+ */
+extern bool
+set_loop_controls(exec_list *instructions, loop_state *ls);
+
+
 /**
  * Tracking for all variables used in a loop
  */

src/glsl/loop_controls.cpp

@@ -0,0 +1,282 @@
+/*
+ * 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.
+ */
+
+#include <climits>
+#include "main/compiler.h"
+#include "glsl_types.h"
+#include "loop_analysis.h"
+#include "ir_hierarchical_visitor.h"
+
+/**
+ * Find an initializer of a variable outside a loop
+ *
+ * Works backwards from the loop to find the pre-loop value of the variable.
+ * This is used, for example, to find the initial value of loop induction
+ * variables.
+ *
+ * \param loop  Loop where \c var is an induction variable
+ * \param var   Variable whose initializer is to be found
+ *
+ * \return
+ * The \c ir_rvalue assigned to the variable outside the loop.  May return
+ * \c NULL if no initializer can be found.
+ */
+ir_rvalue *
+find_initial_value(ir_loop *loop, ir_variable *var)
+{
+   for (exec_node *node = loop->prev;
+	!node->is_head_sentinel();
+	node = node->prev) {
+      ir_instruction *ir = (ir_instruction *) node;
+
+      switch (ir->ir_type) {
+      case ir_type_call:
+      case ir_type_loop:
+      case ir_type_loop_jump:
+      case ir_type_return:
+      case ir_type_if:
+	 return NULL;
+
+      case ir_type_function:
+      case ir_type_function_signature:
+	 assert(!"Should not get here.");
+	 return NULL;
+
+      case ir_type_assignment: {
+	 ir_assignment *assign = ir->as_assignment();
+	 ir_variable *assignee = assign->lhs->whole_variable_referenced();
+
+	 if (assignee == var)
+	    return (assign->condition != NULL) ? NULL : assign->rhs;
+
+	 break;
+      }
+
+      default:
+	 break;
+      }
+   }
+
+   return NULL;
+}
+
+
+int
+calculate_iterations(ir_rvalue *from, ir_rvalue *to, ir_rvalue *increment,
+		     enum ir_expression_operation op)
+{
+   void *mem_ctx = talloc_init(__func__);
+
+   ir_expression *const sub =
+      new(mem_ctx) ir_expression(ir_binop_sub, from->type, to, from);
+
+   ir_expression *const div =
+      new(mem_ctx) ir_expression(ir_binop_div, sub->type, sub, increment);
+
+   ir_constant *iter = div->constant_expression_value();
+
+   if (iter == NULL)
+      return -1;
+
+   if (!iter->type->is_integer()) {
+      ir_rvalue *cast =
+	 new(mem_ctx) ir_expression(ir_unop_f2i, glsl_type::int_type, iter,
+				    NULL);
+
+      iter = cast->constant_expression_value();
+   }
+
+   int iter_value = iter->get_int_component(0);
+
+   /* Make sure that the calculated number of iterations satisfies the exit
+    * condition.  This is needed to catch off-by-one errors and some types of
+    * ill-formed loops.  For example, we need to detect that the following
+    * loop does not have a maximum iteration count.
+    *
+    *    for (float x = 0.0; x != 0.9; x += 0.2)
+    *        ;
+    */
+   const int bias[] = { -1, 0, 1 };
+   bool valid_loop = false;
+
+   for (unsigned i = 0; i < Elements(bias); i++) {
+      iter = (increment->type->is_integer())
+	 ? new(mem_ctx) ir_constant(iter_value + bias[i])
+	 : new(mem_ctx) ir_constant(float(iter_value + bias[i]));
+
+      ir_expression *const mul =
+	 new(mem_ctx) ir_expression(ir_binop_mul, increment->type, iter,
+				    increment);
+
+      ir_expression *const add =
+	 new(mem_ctx) ir_expression(ir_binop_add, mul->type, mul, from);
+
+      ir_expression *const cmp =
+	 new(mem_ctx) ir_expression(op, glsl_type::bool_type, add, to);
+
+      ir_constant *const cmp_result = cmp->constant_expression_value();
+
+      assert(cmp_result != NULL);
+      if (cmp_result->get_bool_component(0)) {
+	 iter_value += bias[i];
+	 valid_loop = true;
+	 break;
+      }
+   }
+
+   talloc_free(mem_ctx);
+   return (valid_loop) ? iter_value : -1;
+}
+
+
+class loop_control_visitor : public ir_hierarchical_visitor {
+public:
+   loop_control_visitor(loop_state *state)
+   {
+      this->state = state;
+      this->progress = false;
+   }
+
+   virtual ir_visitor_status visit_leave(ir_loop *ir);
+
+   loop_state *state;
+
+   bool progress;
+};
+
+
+ir_visitor_status
+loop_control_visitor::visit_leave(ir_loop *ir)
+{
+   loop_variable_state *const ls = this->state->get(ir);
+
+   /* If we've entered a loop that hasn't been analyzed, something really,
+    * really bad has happened.
+    */
+   if (ls == NULL) {
+      assert(ls != NULL);
+      return visit_continue;
+   }
+
+   /* Search the loop terminating conditions for one of the form 'i < c' where
+    * i is a loop induction variable, c is a constant, and < is any relative
+    * operator.
+    */
+   int max_iterations = INT_MAX;
+   foreach_list(node, &ls->terminators) {
+      loop_terminator *t = (loop_terminator *) node;
+      ir_if *if_stmt = t->ir;
+
+      /* If-statements can be either 'if (expr)' or 'if (deref)'.  We only care
+       * about the former here.
+       */
+      ir_expression *cond = if_stmt->condition->as_expression();
+      if (cond == NULL)
+	 continue;
+
+      switch (cond->operation) {
+      case ir_binop_less:
+      case ir_binop_greater:
+      case ir_binop_lequal:
+      case ir_binop_gequal: {
+	 /* The expressions that we care about will either be of the form
+	  * 'counter < limit' or 'limit < counter'.  Figure out which is
+	  * which.
+	  */
+	 ir_rvalue *counter = cond->operands[0]->as_dereference_variable();
+	 ir_constant *limit = cond->operands[1]->constant_expression_value();
+	 enum ir_expression_operation cmp = cond->operation;
+
+	 if (limit == NULL) {
+	    counter = cond->operands[1]->as_dereference_variable();
+	    limit = cond->operands[0]->constant_expression_value();
+
+	    switch (cmp) {
+	    case ir_binop_less:    cmp = ir_binop_gequal;  break;
+	    case ir_binop_greater: cmp = ir_binop_lequal;  break;
+	    case ir_binop_lequal:  cmp = ir_binop_greater; break;
+	    case ir_binop_gequal:  cmp = ir_binop_less;    break;
+	    default: assert(!"Should not get here.");
+	    }
+	 }
+
+	 if ((counter == NULL) || (limit == NULL))
+	    break;
+
+	 ir_variable *var = counter->variable_referenced();
+
+	 ir_rvalue *init = find_initial_value(ir, var);
+
+	 foreach_list(iv_node, &ls->induction_variables) {
+	    loop_variable *lv = (loop_variable *) iv_node;
+
+	    if (lv->var == var) {
+	       const int iterations = calculate_iterations(init, limit,
+							   lv->increment,
+							   cmp);
+	       if (iterations > 0) {
+		  /* If the new iteration count is lower than the previously
+		   * believed iteration count, update the loop control values.
+		   */
+		  if (iterations < max_iterations) {
+		     ir->from = init->clone(ir, NULL);
+		     ir->to = limit->clone(ir, NULL);
+		     ir->increment = lv->increment->clone(ir, NULL);
+		     ir->counter = lv->var;
+		     ir->cmp = cmp;
+
+		     max_iterations = iterations;
+		  }
+
+		  /* Remove the conditional break statement.  The loop
+		   * controls are now set such that the exit condition will be
+		   * satisfied.
+		   */
+		  if_stmt->remove();
+		  this->progress = true;
+	       }
+
+	       break;
+	    }
+	 }
+	 break;
+      }
+
+      default:
+	 break;
+      }
+   }
+
+   return visit_continue;
+}
+
+
+bool
+set_loop_controls(exec_list *instructions, loop_state *ls)
+{
+   loop_control_visitor v(ls);
+
+   v.run(instructions);
+
+   return v.progress;
+}