glsl: move to compiler/

Signed-off-by: Emil Velikov <[email protected]>
Acked-by: Matt Turner <[email protected]>
Acked-by: Jose Fonseca <[email protected]>

1 files changed, 1022 insertions, 0 deletions

diff --git a/src/compiler/glsl/lower_jumps.cpp b/src/compiler/glsl/lower_jumps.cpp
new file mode 100644
index 00000000000..3cfa2e00ae8
--- /dev/null
+++ b/src/compiler/glsl/lower_jumps.cpp
@@ -0,0 +1,1022 @@
+/*
+ * Copyright © 2010 Luca Barbieri
+ *
+ * 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_jumps.cpp
+ *
+ * This pass lowers jumps (break, continue, and return) to if/else structures.
+ *
+ * It can be asked to:
+ * 1. Pull jumps out of ifs where possible
+ * 2. Remove all "continue"s, replacing them with an "execute flag"
+ * 3. Replace all "break" with a single conditional one at the end of the loop
+ * 4. Replace all "return"s with a single return at the end of the function,
+ *    for the main function and/or other functions
+ *
+ * Applying this pass gives several benefits:
+ * 1. All functions can be inlined.
+ * 2. nv40 and other pre-DX10 chips without "continue" can be supported
+ * 3. nv30 and other pre-DX10 chips with no control flow at all are better
+ *    supported
+ *
+ * Continues are lowered by adding a per-loop "execute flag", initialized to
+ * true, that when cleared inhibits all execution until the end of the loop.
+ *
+ * Breaks are lowered to continues, plus setting a "break flag" that is checked
+ * at the end of the loop, and trigger the unique "break".
+ *
+ * Returns are lowered to breaks/continues, plus adding a "return flag" that
+ * causes loops to break again out of their enclosing loops until all the
+ * loops are exited: then the "execute flag" logic will ignore everything
+ * until the end of the function.
+ *
+ * Note that "continue" and "return" can also be implemented by adding
+ * a dummy loop and using break.
+ * However, this is bad for hardware with limited nesting depth, and
+ * prevents further optimization, and thus is not currently performed.
+ */
+
+#include "compiler/glsl_types.h"
+#include <string.h>
+#include "ir.h"
+
+/**
+ * Enum recording the result of analyzing how control flow might exit
+ * an IR node.
+ *
+ * Each possible value of jump_strength indicates a strictly stronger
+ * guarantee on control flow than the previous value.
+ *
+ * The ordering of strengths roughly reflects the way jumps are
+ * lowered: jumps with higher strength tend to be lowered to jumps of
+ * lower strength.  Accordingly, strength is used as a heuristic to
+ * determine which lowering to perform first.
+ *
+ * This enum is also used by get_jump_strength() to categorize
+ * instructions as either break, continue, return, or other.  When
+ * used in this fashion, strength_always_clears_execute_flag is not
+ * used.
+ *
+ * The control flow analysis made by this optimization pass makes two
+ * simplifying assumptions:
+ *
+ * - It ignores discard instructions, since they are lowered by a
+ *   separate pass (lower_discard.cpp).
+ *
+ * - It assumes it is always possible for control to flow from a loop
+ *   to the instruction immediately following it.  Technically, this
+ *   is not true (since all execution paths through the loop might
+ *   jump back to the top, or return from the function).
+ *
+ * Both of these simplifying assumtions are safe, since they can never
+ * cause reachable code to be incorrectly classified as unreachable;
+ * they can only do the opposite.
+ */
+enum jump_strength
+{
+   /**
+    * Analysis has produced no guarantee on how control flow might
+    * exit this IR node.  It might fall out the bottom (with or
+    * without clearing the execute flag, if present), or it might
+    * continue to the top of the innermost enclosing loop, break out
+    * of it, or return from the function.
+    */
+   strength_none,
+
+   /**
+    * The only way control can fall out the bottom of this node is
+    * through a code path that clears the execute flag.  It might also
+    * continue to the top of the innermost enclosing loop, break out
+    * of it, or return from the function.
+    */
+   strength_always_clears_execute_flag,
+
+   /**
+    * Control cannot fall out the bottom of this node.  It might
+    * continue to the top of the innermost enclosing loop, break out
+    * of it, or return from the function.
+    */
+   strength_continue,
+
+   /**
+    * Control cannot fall out the bottom of this node, or continue the
+    * top of the innermost enclosing loop.  It can only break out of
+    * it or return from the function.
+    */
+   strength_break,
+
+   /**
+    * Control cannot fall out the bottom of this node, continue to the
+    * top of the innermost enclosing loop, or break out of it.  It can
+    * only return from the function.
+    */
+   strength_return
+};
+
+namespace {
+
+struct block_record
+{
+   /* minimum jump strength (of lowered IR, not pre-lowering IR)
+    *
+    * If the block ends with a jump, must be the strength of the jump.
+    * Otherwise, the jump would be dead and have been deleted before)
+    *
+    * If the block doesn't end with a jump, it can be different than strength_none if all paths before it lead to some jump
+    * (e.g. an if with a return in one branch, and a break in the other, while not lowering them)
+    * Note that identical jumps are usually unified though.
+    */
+   jump_strength min_strength;
+
+   /* can anything clear the execute flag? */
+   bool may_clear_execute_flag;
+
+   block_record()
+   {
+      this->min_strength = strength_none;
+      this->may_clear_execute_flag = false;
+   }
+};
+
+struct loop_record
+{
+   ir_function_signature* signature;
+   ir_loop* loop;
+
+   /* used to avoid lowering the break used to represent lowered breaks */
+   unsigned nesting_depth;
+   bool in_if_at_the_end_of_the_loop;
+
+   bool may_set_return_flag;
+
+   ir_variable* break_flag;
+   ir_variable* execute_flag; /* cleared to emulate continue */
+
+   loop_record(ir_function_signature* p_signature = 0, ir_loop* p_loop = 0)
+   {
+      this->signature = p_signature;
+      this->loop = p_loop;
+      this->nesting_depth = 0;
+      this->in_if_at_the_end_of_the_loop = false;
+      this->may_set_return_flag = false;
+      this->break_flag = 0;
+      this->execute_flag = 0;
+   }
+
+   ir_variable* get_execute_flag()
+   {
+      /* also supported for the "function loop" */
+      if(!this->execute_flag) {
+         exec_list& list = this->loop ? this->loop->body_instructions : signature->body;
+         this->execute_flag = new(this->signature) ir_variable(glsl_type::bool_type, "execute_flag", ir_var_temporary);
+         list.push_head(new(this->signature) ir_assignment(new(this->signature) ir_dereference_variable(execute_flag), new(this->signature) ir_constant(true), 0));
+         list.push_head(this->execute_flag);
+      }
+      return this->execute_flag;
+   }
+
+   ir_variable* get_break_flag()
+   {
+      assert(this->loop);
+      if(!this->break_flag) {
+         this->break_flag = new(this->signature) ir_variable(glsl_type::bool_type, "break_flag", ir_var_temporary);
+         this->loop->insert_before(this->break_flag);
+         this->loop->insert_before(new(this->signature) ir_assignment(new(this->signature) ir_dereference_variable(break_flag), new(this->signature) ir_constant(false), 0));
+      }
+      return this->break_flag;
+   }
+};
+
+struct function_record
+{
+   ir_function_signature* signature;
+   ir_variable* return_flag; /* used to break out of all loops and then jump to the return instruction */
+   ir_variable* return_value;
+   bool lower_return;
+   unsigned nesting_depth;
+
+   function_record(ir_function_signature* p_signature = 0,
+                   bool lower_return = false)
+   {
+      this->signature = p_signature;
+      this->return_flag = 0;
+      this->return_value = 0;
+      this->nesting_depth = 0;
+      this->lower_return = lower_return;
+   }
+
+   ir_variable* get_return_flag()
+   {
+      if(!this->return_flag) {
+         this->return_flag = new(this->signature) ir_variable(glsl_type::bool_type, "return_flag", ir_var_temporary);
+         this->signature->body.push_head(new(this->signature) ir_assignment(new(this->signature) ir_dereference_variable(return_flag), new(this->signature) ir_constant(false), 0));
+         this->signature->body.push_head(this->return_flag);
+      }
+      return this->return_flag;
+   }
+
+   ir_variable* get_return_value()
+   {
+      if(!this->return_value) {
+         assert(!this->signature->return_type->is_void());
+         return_value = new(this->signature) ir_variable(this->signature->return_type, "return_value", ir_var_temporary);
+         this->signature->body.push_head(this->return_value);
+      }
+      return this->return_value;
+   }
+};
+
+struct ir_lower_jumps_visitor : public ir_control_flow_visitor {
+   /* Postconditions: on exit of any visit() function:
+    *
+    * ANALYSIS: this->block.min_strength,
+    * this->block.may_clear_execute_flag, and
+    * this->loop.may_set_return_flag are updated to reflect the
+    * characteristics of the visited statement.
+    *
+    * DEAD_CODE_ELIMINATION: If this->block.min_strength is not
+    * strength_none, the visited node is at the end of its exec_list.
+    * In other words, any unreachable statements that follow the
+    * visited statement in its exec_list have been removed.
+    *
+    * CONTAINED_JUMPS_LOWERED: If the visited statement contains other
+    * statements, then should_lower_jump() is false for all of the
+    * return, break, or continue statements it contains.
+    *
+    * Note that visiting a jump does not lower it.  That is the
+    * responsibility of the statement (or function signature) that
+    * contains the jump.
+    */
+
+   bool progress;
+
+   struct function_record function;
+   struct loop_record loop;
+   struct block_record block;
+
+   bool pull_out_jumps;
+   bool lower_continue;
+   bool lower_break;
+   bool lower_sub_return;
+   bool lower_main_return;
+
+   ir_lower_jumps_visitor()
+      : progress(false),
+        pull_out_jumps(false),
+        lower_continue(false),
+        lower_break(false),
+        lower_sub_return(false),
+        lower_main_return(false)
+   {
+   }
+
+   void truncate_after_instruction(exec_node *ir)
+   {
+      if (!ir)
+         return;
+
+      while (!ir->get_next()->is_tail_sentinel()) {
+         ((ir_instruction *)ir->get_next())->remove();
+         this->progress = true;
+      }
+   }
+
+   void move_outer_block_inside(ir_instruction *ir, exec_list *inner_block)
+   {
+      while (!ir->get_next()->is_tail_sentinel()) {
+         ir_instruction *move_ir = (ir_instruction *)ir->get_next();
+
+         move_ir->remove();
+         inner_block->push_tail(move_ir);
+      }
+   }
+
+   /**
+    * Insert the instructions necessary to lower a return statement,
+    * before the given return instruction.
+    */
+   void insert_lowered_return(ir_return *ir)
+   {
+      ir_variable* return_flag = this->function.get_return_flag();
+      if(!this->function.signature->return_type->is_void()) {
+         ir_variable* return_value = this->function.get_return_value();
+         ir->insert_before(
+            new(ir) ir_assignment(
+               new (ir) ir_dereference_variable(return_value),
+               ir->value));
+      }
+      ir->insert_before(
+         new(ir) ir_assignment(
+            new (ir) ir_dereference_variable(return_flag),
+            new (ir) ir_constant(true)));
+      this->loop.may_set_return_flag = true;
+   }
+
+   /**
+    * If the given instruction is a return, lower it to instructions
+    * that store the return value (if there is one), set the return
+    * flag, and then break.
+    *
+    * It is safe to pass NULL to this function.
+    */
+   void lower_return_unconditionally(ir_instruction *ir)
+   {
+      if (get_jump_strength(ir) != strength_return) {
+         return;
+      }
+      insert_lowered_return((ir_return*)ir);
+      ir->replace_with(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
+   }
+
+   /**
+    * Create the necessary instruction to replace a break instruction.
+    */
+   ir_instruction *create_lowered_break()
+   {
+      void *ctx = this->function.signature;
+      return new(ctx) ir_assignment(
+          new(ctx) ir_dereference_variable(this->loop.get_break_flag()),
+          new(ctx) ir_constant(true),
+          0);
+   }
+
+   /**
+    * If the given instruction is a break, lower it to an instruction
+    * that sets the break flag, without consulting
+    * should_lower_jump().
+    *
+    * It is safe to pass NULL to this function.
+    */
+   void lower_break_unconditionally(ir_instruction *ir)
+   {
+      if (get_jump_strength(ir) != strength_break) {
+         return;
+      }
+      ir->replace_with(create_lowered_break());
+   }
+
+   /**
+    * If the block ends in a conditional or unconditional break, lower
+    * it, even though should_lower_jump() says it needn't be lowered.
+    */
+   void lower_final_breaks(exec_list *block)
+   {
+      ir_instruction *ir = (ir_instruction *) block->get_tail();
+      lower_break_unconditionally(ir);
+      ir_if *ir_if = ir->as_if();
+      if (ir_if) {
+          lower_break_unconditionally(
+              (ir_instruction *) ir_if->then_instructions.get_tail());
+          lower_break_unconditionally(
+              (ir_instruction *) ir_if->else_instructions.get_tail());
+      }
+   }
+
+   virtual void visit(class ir_loop_jump * ir)
+   {
+      /* Eliminate all instructions after each one, since they are
+       * unreachable.  This satisfies the DEAD_CODE_ELIMINATION
+       * postcondition.
+       */
+      truncate_after_instruction(ir);
+
+      /* Set this->block.min_strength based on this instruction.  This
+       * satisfies the ANALYSIS postcondition.  It is not necessary to
+       * update this->block.may_clear_execute_flag or
+       * this->loop.may_set_return_flag, because an unlowered jump
+       * instruction can't change any flags.
+       */
+      this->block.min_strength = ir->is_break() ? strength_break : strength_continue;
+
+      /* The CONTAINED_JUMPS_LOWERED postcondition is already
+       * satisfied, because jump statements can't contain other
+       * statements.
+       */
+   }
+
+   virtual void visit(class ir_return * ir)
+   {
+      /* Eliminate all instructions after each one, since they are
+       * unreachable.  This satisfies the DEAD_CODE_ELIMINATION
+       * postcondition.
+       */
+      truncate_after_instruction(ir);
+
+      /* Set this->block.min_strength based on this instruction.  This
+       * satisfies the ANALYSIS postcondition.  It is not necessary to
+       * update this->block.may_clear_execute_flag or
+       * this->loop.may_set_return_flag, because an unlowered return
+       * instruction can't change any flags.
+       */
+      this->block.min_strength = strength_return;
+
+      /* The CONTAINED_JUMPS_LOWERED postcondition is already
+       * satisfied, because jump statements can't contain other
+       * statements.
+       */
+   }
+
+   virtual void visit(class ir_discard * ir)
+   {
+      /* Nothing needs to be done.  The ANALYSIS and
+       * DEAD_CODE_ELIMINATION postconditions are already satisfied,
+       * because discard statements are ignored by this optimization
+       * pass.  The CONTAINED_JUMPS_LOWERED postcondition is already
+       * satisfied, because discard statements can't contain other
+       * statements.
+       */
+      (void) ir;
+   }
+
+   enum jump_strength get_jump_strength(ir_instruction* ir)
+   {
+      if(!ir)
+         return strength_none;
+      else if(ir->ir_type == ir_type_loop_jump) {
+         if(((ir_loop_jump*)ir)->is_break())
+            return strength_break;
+         else
+            return strength_continue;
+      } else if(ir->ir_type == ir_type_return)
+         return strength_return;
+      else
+         return strength_none;
+   }
+
+   bool should_lower_jump(ir_jump* ir)
+   {
+      unsigned strength = get_jump_strength(ir);
+      bool lower;
+      switch(strength)
+      {
+      case strength_none:
+         lower = false; /* don't change this, code relies on it */
+         break;
+      case strength_continue:
+         lower = lower_continue;
+         break;
+      case strength_break:
+         assert(this->loop.loop);
+         /* never lower "canonical break" */
+         if(ir->get_next()->is_tail_sentinel() && (this->loop.nesting_depth == 0
+               || (this->loop.nesting_depth == 1 && this->loop.in_if_at_the_end_of_the_loop)))
+            lower = false;
+         else
+            lower = lower_break;
+         break;
+      case strength_return:
+         /* never lower return at the end of a this->function */
+         if(this->function.nesting_depth == 0 && ir->get_next()->is_tail_sentinel())
+            lower = false;
+         else
+            lower = this->function.lower_return;
+         break;
+      }
+      return lower;
+   }
+
+   block_record visit_block(exec_list* list)
+   {
+      /* Note: since visiting a node may change that node's next
+       * pointer, we can't use visit_exec_list(), because
+       * visit_exec_list() caches the node's next pointer before
+       * visiting it.  So we use foreach_in_list() instead.
+       *
+       * foreach_in_list() isn't safe if the node being visited gets
+       * removed, but fortunately this visitor doesn't do that.
+       */
+
+      block_record saved_block = this->block;
+      this->block = block_record();
+      foreach_in_list(ir_instruction, node, list) {
+         node->accept(this);
+      }
+      block_record ret = this->block;
+      this->block = saved_block;
+      return ret;
+   }
+
+   virtual void visit(ir_if *ir)
+   {
+      if(this->loop.nesting_depth == 0 && ir->get_next()->is_tail_sentinel())
+         this->loop.in_if_at_the_end_of_the_loop = true;
+
+      ++this->function.nesting_depth;
+      ++this->loop.nesting_depth;
+
+      block_record block_records[2];
+      ir_jump* jumps[2];
+
+      /* Recursively lower nested jumps.  This satisfies the
+       * CONTAINED_JUMPS_LOWERED postcondition, except in the case of
+       * unconditional jumps at the end of ir->then_instructions and
+       * ir->else_instructions, which are handled below.
+       */
+      block_records[0] = visit_block(&ir->then_instructions);
+      block_records[1] = visit_block(&ir->else_instructions);
+
+retry: /* we get here if we put code after the if inside a branch */
+
+      /* Determine which of ir->then_instructions and
+       * ir->else_instructions end with an unconditional jump.
+       */
+      for(unsigned i = 0; i < 2; ++i) {
+         exec_list& list = i ? ir->else_instructions : ir->then_instructions;
+         jumps[i] = 0;
+         if(!list.is_empty() && get_jump_strength((ir_instruction*)list.get_tail()))
+            jumps[i] = (ir_jump*)list.get_tail();
+      }
+
+      /* Loop until we have satisfied the CONTAINED_JUMPS_LOWERED
+       * postcondition by lowering jumps in both then_instructions and
+       * else_instructions.
+       */
+      for(;;) {
+         /* Determine the types of the jumps that terminate
+          * ir->then_instructions and ir->else_instructions.
+          */
+         jump_strength jump_strengths[2];
+
+         for(unsigned i = 0; i < 2; ++i) {
+            if(jumps[i]) {
+               jump_strengths[i] = block_records[i].min_strength;
+               assert(jump_strengths[i] == get_jump_strength(jumps[i]));
+            } else
+               jump_strengths[i] = strength_none;
+         }
+
+         /* If both code paths end in a jump, and the jumps are the
+          * same, and we are pulling out jumps, replace them with a
+          * single jump that comes after the if instruction.  The new
+          * jump will be visited next, and it will be lowered if
+          * necessary by the loop or conditional that encloses it.
+          */
+         if(pull_out_jumps && jump_strengths[0] == jump_strengths[1]) {
+            bool unify = true;
+            if(jump_strengths[0] == strength_continue)
+               ir->insert_after(new(ir) ir_loop_jump(ir_loop_jump::jump_continue));
+            else if(jump_strengths[0] == strength_break)
+               ir->insert_after(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
+            /* FINISHME: unify returns with identical expressions */
+            else if(jump_strengths[0] == strength_return && this->function.signature->return_type->is_void())
+               ir->insert_after(new(ir) ir_return(NULL));
+	    else
+	       unify = false;
+
+            if(unify) {
+               jumps[0]->remove();
+               jumps[1]->remove();
+               this->progress = true;
+
+               /* Update jumps[] to reflect the fact that the jumps
+                * are gone, and update block_records[] to reflect the
+                * fact that control can now flow to the next
+                * instruction.
+                */
+               jumps[0] = 0;
+               jumps[1] = 0;
+               block_records[0].min_strength = strength_none;
+               block_records[1].min_strength = strength_none;
+
+               /* The CONTAINED_JUMPS_LOWERED postcondition is now
+                * satisfied, so we can break out of the loop.
+                */
+               break;
+            }
+         }
+
+         /* lower a jump: if both need to lowered, start with the strongest one, so that
+          * we might later unify the lowered version with the other one
+          */
+         bool should_lower[2];
+         for(unsigned i = 0; i < 2; ++i)
+            should_lower[i] = should_lower_jump(jumps[i]);
+
+         int lower;
+         if(should_lower[1] && should_lower[0])
+            lower = jump_strengths[1] > jump_strengths[0];
+         else if(should_lower[0])
+            lower = 0;
+         else if(should_lower[1])
+            lower = 1;
+         else
+            /* Neither code path ends in a jump that needs to be
+             * lowered, so the CONTAINED_JUMPS_LOWERED postcondition
+             * is satisfied and we can break out of the loop.
+             */
+            break;
+
+         if(jump_strengths[lower] == strength_return) {
+            /* To lower a return, we create a return flag (if the
+             * function doesn't have one already) and add instructions
+             * that: 1. store the return value (if this function has a
+             * non-void return) and 2. set the return flag
+             */
+            insert_lowered_return((ir_return*)jumps[lower]);
+            if(this->loop.loop) {
+               /* If we are in a loop, replace the return instruction
+                * with a break instruction, and then loop so that the
+                * break instruction can be lowered if necessary.
+                */
+               ir_loop_jump* lowered = 0;
+               lowered = new(ir) ir_loop_jump(ir_loop_jump::jump_break);
+               /* Note: we must update block_records and jumps to
+                * reflect the fact that the control path has been
+                * altered from a return to a break.
+                */
+               block_records[lower].min_strength = strength_break;
+               jumps[lower]->replace_with(lowered);
+               jumps[lower] = lowered;
+            } else {
+               /* If we are not in a loop, we then proceed as we would
+                * for a continue statement (set the execute flag to
+                * false to prevent the rest of the function from
+                * executing).
+                */
+               goto lower_continue;
+            }
+            this->progress = true;
+         } else if(jump_strengths[lower] == strength_break) {
+            /* To lower a break, we create a break flag (if the loop
+             * doesn't have one already) and add an instruction that
+             * sets it.
+             *
+             * Then we proceed as we would for a continue statement
+             * (set the execute flag to false to prevent the rest of
+             * the loop body from executing).
+             *
+             * The visit() function for the loop will ensure that the
+             * break flag is checked after executing the loop body.
+             */
+            jumps[lower]->insert_before(create_lowered_break());
+            goto lower_continue;
+         } else if(jump_strengths[lower] == strength_continue) {
+lower_continue:
+            /* To lower a continue, we create an execute flag (if the
+             * loop doesn't have one already) and replace the continue
+             * with an instruction that clears it.
+             *
+             * Note that this code path gets exercised when lowering
+             * return statements that are not inside a loop, so
+             * this->loop must be initialized even outside of loops.
+             */
+            ir_variable* execute_flag = this->loop.get_execute_flag();
+            jumps[lower]->replace_with(new(ir) ir_assignment(new (ir) ir_dereference_variable(execute_flag), new (ir) ir_constant(false), 0));
+            /* Note: we must update block_records and jumps to reflect
+             * the fact that the control path has been altered to an
+             * instruction that clears the execute flag.
+             */
+            jumps[lower] = 0;
+            block_records[lower].min_strength = strength_always_clears_execute_flag;
+            block_records[lower].may_clear_execute_flag = true;
+            this->progress = true;
+
+            /* Let the loop run again, in case the other branch of the
+             * if needs to be lowered too.
+             */
+         }
+      }
+
+      /* move out a jump out if possible */
+      if(pull_out_jumps) {
+         /* If one of the branches ends in a jump, and control cannot
+          * fall out the bottom of the other branch, then we can move
+          * the jump after the if.
+          *
+          * Set move_out to the branch we are moving a jump out of.
+          */
+         int move_out = -1;
+         if(jumps[0] && block_records[1].min_strength >= strength_continue)
+            move_out = 0;
+         else if(jumps[1] && block_records[0].min_strength >= strength_continue)
+            move_out = 1;
+
+         if(move_out >= 0)
+         {
+            jumps[move_out]->remove();
+            ir->insert_after(jumps[move_out]);
+            /* Note: we must update block_records and jumps to reflect
+             * the fact that the jump has been moved out of the if.
+             */
+            jumps[move_out] = 0;
+            block_records[move_out].min_strength = strength_none;
+            this->progress = true;
+         }
+      }
+
+      /* Now satisfy the ANALYSIS postcondition by setting
+       * this->block.min_strength and
+       * this->block.may_clear_execute_flag based on the
+       * characteristics of the two branches.
+       */
+      if(block_records[0].min_strength < block_records[1].min_strength)
+         this->block.min_strength = block_records[0].min_strength;
+      else
+         this->block.min_strength = block_records[1].min_strength;
+      this->block.may_clear_execute_flag = this->block.may_clear_execute_flag || block_records[0].may_clear_execute_flag || block_records[1].may_clear_execute_flag;
+
+      /* Now we need to clean up the instructions that follow the
+       * if.
+       *
+       * If those instructions are unreachable, then satisfy the
+       * DEAD_CODE_ELIMINATION postcondition by eliminating them.
+       * Otherwise that postcondition is already satisfied.
+       */
+      if(this->block.min_strength)
+         truncate_after_instruction(ir);
+      else if(this->block.may_clear_execute_flag)
+      {
+         /* If the "if" instruction might clear the execute flag, then
+          * we need to guard any instructions that follow so that they
+          * are only executed if the execute flag is set.
+          *
+          * If one of the branches of the "if" always clears the
+          * execute flag, and the other branch never clears it, then
+          * this is easy: just move all the instructions following the
+          * "if" into the branch that never clears it.
+          */
+         int move_into = -1;
+         if(block_records[0].min_strength && !block_records[1].may_clear_execute_flag)
+            move_into = 1;
+         else if(block_records[1].min_strength && !block_records[0].may_clear_execute_flag)
+            move_into = 0;
+
+         if(move_into >= 0) {
+            assert(!block_records[move_into].min_strength && !block_records[move_into].may_clear_execute_flag); /* otherwise, we just truncated */
+
+            exec_list* list = move_into ? &ir->else_instructions : &ir->then_instructions;
+            exec_node* next = ir->get_next();
+            if(!next->is_tail_sentinel()) {
+               move_outer_block_inside(ir, list);
+
+               /* If any instructions moved, then we need to visit
+                * them (since they are now inside the "if").  Since
+                * block_records[move_into] is in its default state
+                * (see assertion above), we can safely replace
+                * block_records[move_into] with the result of this
+                * analysis.
+                */
+               exec_list list;
+               list.head = next;
+               block_records[move_into] = visit_block(&list);
+
+               /*
+                * Then we need to re-start our jump lowering, since one
+                * of the instructions we moved might be a jump that
+                * needs to be lowered.
+                */
+               this->progress = true;
+               goto retry;
+            }
+         } else {
+            /* If we get here, then the simple case didn't apply; we
+             * need to actually guard the instructions that follow.
+             *
+             * To avoid creating unnecessarily-deep nesting, first
+             * look through the instructions that follow and unwrap
+             * any instructions that that are already wrapped in the
+             * appropriate guard.
+             */
+            ir_instruction* ir_after;
+            for(ir_after = (ir_instruction*)ir->get_next(); !ir_after->is_tail_sentinel();)
+            {
+               ir_if* ir_if = ir_after->as_if();
+               if(ir_if && ir_if->else_instructions.is_empty()) {
+                  ir_dereference_variable* ir_if_cond_deref = ir_if->condition->as_dereference_variable();
+                  if(ir_if_cond_deref && ir_if_cond_deref->var == this->loop.execute_flag) {
+                     ir_instruction* ir_next = (ir_instruction*)ir_after->get_next();
+                     ir_after->insert_before(&ir_if->then_instructions);
+                     ir_after->remove();
+                     ir_after = ir_next;
+                     continue;
+                  }
+               }
+               ir_after = (ir_instruction*)ir_after->get_next();
+
+               /* only set this if we find any unprotected instruction */
+               this->progress = true;
+            }
+
+            /* Then, wrap all the instructions that follow in a single
+             * guard.
+             */
+            if(!ir->get_next()->is_tail_sentinel()) {
+               assert(this->loop.execute_flag);
+               ir_if* if_execute = new(ir) ir_if(new(ir) ir_dereference_variable(this->loop.execute_flag));
+               move_outer_block_inside(ir, &if_execute->then_instructions);
+               ir->insert_after(if_execute);
+            }
+         }
+      }
+      --this->loop.nesting_depth;
+      --this->function.nesting_depth;
+   }
+
+   virtual void visit(ir_loop *ir)
+   {
+      /* Visit the body of the loop, with a fresh data structure in
+       * this->loop so that the analysis we do here won't bleed into
+       * enclosing loops.
+       *
+       * We assume that all code after a loop is reachable from the
+       * loop (see comments on enum jump_strength), so the
+       * DEAD_CODE_ELIMINATION postcondition is automatically
+       * satisfied, as is the block.min_strength portion of the
+       * ANALYSIS postcondition.
+       *
+       * The block.may_clear_execute_flag portion of the ANALYSIS
+       * postcondition is automatically satisfied because execute
+       * flags do not propagate outside of loops.
+       *
+       * The loop.may_set_return_flag portion of the ANALYSIS
+       * postcondition is handled below.
+       */
+      ++this->function.nesting_depth;
+      loop_record saved_loop = this->loop;
+      this->loop = loop_record(this->function.signature, ir);
+
+      /* Recursively lower nested jumps.  This satisfies the
+       * CONTAINED_JUMPS_LOWERED postcondition, except in the case of
+       * an unconditional continue or return at the bottom of the
+       * loop, which are handled below.
+       */
+      block_record body = visit_block(&ir->body_instructions);
+
+      /* If the loop ends in an unconditional continue, eliminate it
+       * because it is redundant.
+       */
+      ir_instruction *ir_last
+         = (ir_instruction *) ir->body_instructions.get_tail();
+      if (get_jump_strength(ir_last) == strength_continue) {
+         ir_last->remove();
+      }
+
+      /* If the loop ends in an unconditional return, and we are
+       * lowering returns, lower it.
+       */
+      if (this->function.lower_return)
+         lower_return_unconditionally(ir_last);
+
+      if(body.min_strength >= strength_break) {
+         /* FINISHME: If the min_strength of the loop body is
+          * strength_break or strength_return, that means that it
+          * isn't a loop at all, since control flow always leaves the
+          * body of the loop via break or return.  In principle the
+          * loop could be eliminated in this case.  This optimization
+          * is not implemented yet.
+          */
+      }
+
+      if(this->loop.break_flag) {
+         /* We only get here if we are lowering breaks */
+         assert (lower_break);
+
+         /* If a break flag was generated while visiting the body of
+          * the loop, then at least one break was lowered, so we need
+          * to generate an if statement at the end of the loop that
+          * does a "break" if the break flag is set.  The break we
+          * generate won't violate the CONTAINED_JUMPS_LOWERED
+          * postcondition, because should_lower_jump() always returns
+          * false for a break that happens at the end of a loop.
+          *
+          * However, if the loop already ends in a conditional or
+          * unconditional break, then we need to lower that break,
+          * because it won't be at the end of the loop anymore.
+          */
+         lower_final_breaks(&ir->body_instructions);
+
+         ir_if* break_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->loop.break_flag));
+         break_if->then_instructions.push_tail(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
+         ir->body_instructions.push_tail(break_if);
+      }
+
+      /* If the body of the loop may set the return flag, then at
+       * least one return was lowered to a break, so we need to ensure
+       * that the return flag is checked after the body of the loop is
+       * executed.
+       */
+      if(this->loop.may_set_return_flag) {
+         assert(this->function.return_flag);
+         /* Generate the if statement to check the return flag */
+         ir_if* return_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->function.return_flag));
+         /* Note: we also need to propagate the knowledge that the
+          * return flag may get set to the outer context.  This
+          * satisfies the loop.may_set_return_flag part of the
+          * ANALYSIS postcondition.
+          */
+         saved_loop.may_set_return_flag = true;
+         if(saved_loop.loop)
+            /* If this loop is nested inside another one, then the if
+             * statement that we generated should break out of that
+             * loop if the return flag is set.  Caller will lower that
+             * break statement if necessary.
+             */
+            return_if->then_instructions.push_tail(new(ir) ir_loop_jump(ir_loop_jump::jump_break));
+         else
+            /* Otherwise, all we need to do is ensure that the
+             * instructions that follow are only executed if the
+             * return flag is clear.  We can do that by moving those
+             * instructions into the else clause of the generated if
+             * statement.
+             */
+            move_outer_block_inside(ir, &return_if->else_instructions);
+         ir->insert_after(return_if);
+      }
+
+      this->loop = saved_loop;
+      --this->function.nesting_depth;
+   }
+
+   virtual void visit(ir_function_signature *ir)
+   {
+      /* these are not strictly necessary */
+      assert(!this->function.signature);
+      assert(!this->loop.loop);
+
+      bool lower_return;
+      if (strcmp(ir->function_name(), "main") == 0)
+         lower_return = lower_main_return;
+      else
+         lower_return = lower_sub_return;
+
+      function_record saved_function = this->function;
+      loop_record saved_loop = this->loop;
+      this->function = function_record(ir, lower_return);
+      this->loop = loop_record(ir);
+
+      assert(!this->loop.loop);
+
+      /* Visit the body of the function to lower any jumps that occur
+       * in it, except possibly an unconditional return statement at
+       * the end of it.
+       */
+      visit_block(&ir->body);
+
+      /* If the body ended in an unconditional return of non-void,
+       * then we don't need to lower it because it's the one canonical
+       * return.
+       *
+       * If the body ended in a return of void, eliminate it because
+       * it is redundant.
+       */
+      if (ir->return_type->is_void() &&
+          get_jump_strength((ir_instruction *) ir->body.get_tail())) {
+         ir_jump *jump = (ir_jump *) ir->body.get_tail();
+         assert (jump->ir_type == ir_type_return);
+         jump->remove();
+      }
+
+      if(this->function.return_value)
+         ir->body.push_tail(new(ir) ir_return(new (ir) ir_dereference_variable(this->function.return_value)));
+
+      this->loop = saved_loop;
+      this->function = saved_function;
+   }
+
+   virtual void visit(class ir_function * ir)
+   {
+      visit_block(&ir->signatures);
+   }
+};
+
+} /* anonymous namespace */
+
+bool
+do_lower_jumps(exec_list *instructions, bool pull_out_jumps, bool lower_sub_return, bool lower_main_return, bool lower_continue, bool lower_break)
+{
+   ir_lower_jumps_visitor v;
+   v.pull_out_jumps = pull_out_jumps;
+   v.lower_continue = lower_continue;
+   v.lower_break = lower_break;
+   v.lower_sub_return = lower_sub_return;
+   v.lower_main_return = lower_main_return;
+
+   bool progress_ever = false;
+   do {
+      v.progress = false;
+      visit_exec_list(instructions, &v);
+      progress_ever = v.progress || progress_ever;
+   } while (v.progress);
+
+   return progress_ever;
+}