mesa/src/compiler/glsl/opt_dead_code_local.cpp

/*
 * 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_dead_code_local.cpp
 *
 * Eliminates local dead assignments from the code.
 *
 * This operates on basic blocks, tracking assignments and finding if
 * they're used before the variable is completely reassigned.
 *
 * Compare this to ir_dead_code.cpp, which operates globally looking
 * for assignments to variables that are never read.
 */

#include "ir.h"
#include "ir_basic_block.h"
#include "ir_optimization.h"
#include "compiler/glsl_types.h"

static bool debug = false;

namespace {

class assignment_entry : public exec_node
{
public:
   /* override operator new from exec_node */
   DECLARE_LINEAR_ZALLOC_CXX_OPERATORS(assignment_entry)

   assignment_entry(ir_variable *lhs, ir_assignment *ir)
   {
      assert(lhs);
      assert(ir);
      this->lhs = lhs;
      this->ir = ir;
      this->unused = ir->write_mask;
   }

   ir_variable *lhs;
   ir_assignment *ir;

   /* bitmask of xyzw channels written that haven't been used so far. */
   int unused;
};

class kill_for_derefs_visitor : public ir_hierarchical_visitor {
public:
   using ir_hierarchical_visitor::visit;

   kill_for_derefs_visitor(exec_list *assignments)
   {
      this->assignments = assignments;
   }

   void use_channels(ir_variable *const var, int used)
   {
      foreach_in_list_safe(assignment_entry, entry, this->assignments) {
	 if (entry->lhs == var) {
	    if (var->type->is_scalar() || var->type->is_vector()) {
	       if (debug)
		  printf("used %s (0x%01x - 0x%01x)\n", entry->lhs->name,
			 entry->unused, used & 0xf);
	       entry->unused &= ~used;
	       if (!entry->unused)
		  entry->remove();
	    } else {
	       if (debug)
		  printf("used %s\n", entry->lhs->name);
	       entry->remove();
	    }
	 }
      }
   }

   virtual ir_visitor_status visit(ir_dereference_variable *ir)
   {
      use_channels(ir->var, ~0);

      return visit_continue;
   }

   virtual ir_visitor_status visit(ir_swizzle *ir)
   {
      ir_dereference_variable *deref = ir->val->as_dereference_variable();
      if (!deref)
	 return visit_continue;

      int used = 0;
      used |= 1 << ir->mask.x;
      if (ir->mask.num_components > 1)
         used |= 1 << ir->mask.y;
      if (ir->mask.num_components > 2)
         used |= 1 << ir->mask.z;
      if (ir->mask.num_components > 3)
         used |= 1 << ir->mask.w;

      use_channels(deref->var, used);

      return visit_continue_with_parent;
   }

   virtual ir_visitor_status visit_leave(ir_emit_vertex *)
   {
      /* For the purpose of dead code elimination, emitting a vertex counts as
       * "reading" all of the currently assigned output variables.
       */
      foreach_in_list_safe(assignment_entry, entry, this->assignments) {
         if (entry->lhs->data.mode == ir_var_shader_out) {
            if (debug)
               printf("kill %s\n", entry->lhs->name);
            entry->remove();
         }
      }

      return visit_continue;
   }

private:
   exec_list *assignments;
};

class array_index_visit : public ir_hierarchical_visitor {
public:
   array_index_visit(ir_hierarchical_visitor *v)
   {
      this->visitor = v;
   }

   virtual ir_visitor_status visit_enter(class ir_dereference_array *ir)
   {
      ir->array_index->accept(visitor);
      return visit_continue;
   }

   static void run(ir_instruction *ir, ir_hierarchical_visitor *v)
   {
      array_index_visit top_visit(v);
      ir->accept(& top_visit);
   }

   ir_hierarchical_visitor *visitor;
};

} /* unnamed namespace */

/**
 * Adds an entry to the available copy list if it's a plain assignment
 * of a variable to a variable.
 */
static bool
process_assignment(void *lin_ctx, ir_assignment *ir, exec_list *assignments)
{
   ir_variable *var = NULL;
   bool progress = false;
   kill_for_derefs_visitor v(assignments);

   if (ir->condition == NULL) {
      /* If this is an assignment of the form "foo = foo;", remove the whole
       * instruction and be done with it.
       */
      const ir_variable *const lhs_var = ir->whole_variable_written();
      if (lhs_var != NULL && lhs_var == ir->rhs->whole_variable_referenced()) {
         ir->remove();
         return true;
      }
   }

   /* Kill assignment entries for things used to produce this assignment. */
   ir->rhs->accept(&v);
   if (ir->condition) {
      ir->condition->accept(&v);
   }

   /* Kill assignment enties used as array indices.
    */
   array_index_visit::run(ir->lhs, &v);
   var = ir->lhs->variable_referenced();
   assert(var);

   /* Now, check if we did a whole-variable assignment. */
   if (!ir->condition) {
      ir_dereference_variable *deref_var = ir->lhs->as_dereference_variable();

      /* If it's a vector type, we can do per-channel elimination of
       * use of the RHS.
       */
      if (deref_var && (deref_var->var->type->is_scalar() ||
			deref_var->var->type->is_vector())) {

	 if (debug)
	    printf("looking for %s.0x%01x to remove\n", var->name,
		   ir->write_mask);

	 foreach_in_list_safe(assignment_entry, entry, assignments) {
	    if (entry->lhs != var)
	       continue;

            /* Skip if the assignment we're trying to eliminate isn't a plain
             * variable deref. */
            if (entry->ir->lhs->ir_type != ir_type_dereference_variable)
               continue;

	    int remove = entry->unused & ir->write_mask;
	    if (debug) {
	       printf("%s 0x%01x - 0x%01x = 0x%01x\n",
		      var->name,
		      entry->ir->write_mask,
		      remove, entry->ir->write_mask & ~remove);
	    }
	    if (remove) {
	       progress = true;

	       if (debug) {
		  printf("rewriting:\n  ");
		  entry->ir->print();
		  printf("\n");
	       }

	       entry->ir->write_mask &= ~remove;
	       entry->unused &= ~remove;
	       if (entry->ir->write_mask == 0) {
		  /* Delete the dead assignment. */
		  entry->ir->remove();
		  entry->remove();
	       } else {
		  void *mem_ctx = ralloc_parent(entry->ir);
		  /* Reswizzle the RHS arguments according to the new
		   * write_mask.
		   */
		  unsigned components[4];
		  unsigned channels = 0;
		  unsigned next = 0;

		  for (int i = 0; i < 4; i++) {
		     if ((entry->ir->write_mask | remove) & (1 << i)) {
			if (!(remove & (1 << i)))
			   components[channels++] = next;
			next++;
		     }
		  }

		  entry->ir->rhs = new(mem_ctx) ir_swizzle(entry->ir->rhs,
							   components,
							   channels);
		  if (debug) {
		     printf("to:\n  ");
		     entry->ir->print();
		     printf("\n");
		  }
	       }
	    }
	 }
      } else if (ir->whole_variable_written() != NULL) {
	 /* We did a whole-variable assignment.  So, any instruction in
	  * the assignment list with the same LHS is dead.
	  */
	 if (debug)
	    printf("looking for %s to remove\n", var->name);
	 foreach_in_list_safe(assignment_entry, entry, assignments) {
	    if (entry->lhs == var) {
	       if (debug)
		  printf("removing %s\n", var->name);
	       entry->ir->remove();
	       entry->remove();
	       progress = true;
	    }
	 }
      }
   }

   /* Add this instruction to the assignment list available to be removed. */
   assignment_entry *entry = new(lin_ctx) assignment_entry(var, ir);
   assignments->push_tail(entry);

   if (debug) {
      printf("add %s\n", var->name);

      printf("current entries\n");
      foreach_in_list(assignment_entry, entry, assignments) {
	 printf("    %s (0x%01x)\n", entry->lhs->name, entry->unused);
      }
   }

   return progress;
}

static void
dead_code_local_basic_block(ir_instruction *first,
			     ir_instruction *last,
			     void *data)
{
   ir_instruction *ir, *ir_next;
   /* List of avaialble_copy */
   exec_list assignments;
   bool *out_progress = (bool *)data;
   bool progress = false;

   void *ctx = ralloc_context(NULL);
   void *lin_ctx = linear_alloc_parent(ctx, 0);

   /* Safe looping, since process_assignment */
   for (ir = first, ir_next = (ir_instruction *)first->next;;
	ir = ir_next, ir_next = (ir_instruction *)ir->next) {
      ir_assignment *ir_assign = ir->as_assignment();

      if (debug) {
	 ir->print();
	 printf("\n");
      }

      if (ir_assign) {
	 progress = process_assignment(lin_ctx, ir_assign, &assignments) ||
                    progress;
      } else {
	 kill_for_derefs_visitor kill(&assignments);
	 ir->accept(&kill);
      }

      if (ir == last)
	 break;
   }
   *out_progress = progress;
   ralloc_free(ctx);
}

/**
 * Does a copy propagation pass on the code present in the instruction stream.
 */
bool
do_dead_code_local(exec_list *instructions)
{
   bool progress = false;

   call_for_basic_blocks(instructions, dead_code_local_basic_block, &progress);

   return progress;
}