glsl: Add explanatory comments to lower_jumps.cpp.

No functional change.

Reviewed-by: Kenneth Graunke <[email protected]>

1 files changed, 322 insertions, 14 deletions

diff --git a/src/glsl/lower_jumps.cpp b/src/glsl/lower_jumps.cpp
index dd2601d1aad..da85c6b49c0 100644
--- a/src/glsl/lower_jumps.cpp
+++ b/src/glsl/lower_jumps.cpp
@@ -60,12 +60,76 @@
 #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
 };
 
@@ -180,6 +244,27 @@ struct function_record
 };
 
 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;
@@ -220,18 +305,57 @@ struct ir_lower_jumps_visitor : public ir_control_flow_visitor {
 
    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.
+       */
    }
 
    enum jump_strength get_jump_strength(ir_instruction* ir)
@@ -304,18 +428,34 @@ struct ir_lower_jumps_visitor : public ir_control_flow_visitor {
       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 */
-   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();
-   }
 
+      /* 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) {
@@ -326,7 +466,12 @@ retry: /* we get here if we put code after the if inside a branch */
                jump_strengths[i] = strength_none;
          }
 
-         /* move both jumps out if possible */
+         /* 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)
@@ -344,10 +489,19 @@ retry: /* we get here if we put code after the if inside a branch */
                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;
             }
          }
@@ -367,9 +521,18 @@ retry: /* we get here if we put code after the if inside a branch */
          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
+             */
             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();
@@ -378,29 +541,58 @@ retry: /* we get here if we put code after the if inside a branch */
             jumps[lower]->insert_before(new(ir) ir_assignment(new (ir) ir_dereference_variable(return_flag), new (ir) ir_constant(true), NULL));
             this->loop.may_set_return_flag = true;
             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
+            } 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) {
-            /* We can't lower to an actual continue because that would execute the increment.
+            /* To lower a break, we create a break flag (if the loop
+             * doesn't have one already) and add an instruction that
+             * sets it.
              *
-             * In the lowered code, we instead put the break check between the this->loop body and the increment,
-             * which is impossible with a real continue as defined by the GLSL IR currently.
+             * 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).
              *
-             * Smarter options (such as undoing the increment) are possible but it's not worth implementing them,
-             * because if break is lowered, continue is almost surely lowered too.
+             * The visit() function for the loop will ensure that the
+             * break flag is checked after executing the loop body.
              */
             jumps[lower]->insert_before(new(ir) ir_assignment(new (ir) ir_dereference_variable(this->loop.get_break_flag()), new (ir) ir_constant(true), 0));
             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;
@@ -411,6 +603,12 @@ lower_continue:
 
       /* 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;
@@ -421,22 +619,46 @@ lower_continue:
          {
             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;
@@ -451,14 +673,34 @@ lower_continue:
             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();)
             {
@@ -479,6 +721,9 @@ lower_continue:
                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));
@@ -493,29 +738,87 @@ lower_continue:
 
    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.
+       */
       block_record body = visit_block(&ir->body_instructions);
 
       if(body.min_strength >= strength_break) {
-         /* FINISHME: turn the this->loop into an if, or replace it with its body */
+         /* 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) {
+         /* 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.
+          */
          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);
       }
@@ -536,6 +839,11 @@ lower_continue:
       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(this->function.return_value)