glsl: add continue/break/return unification/elimination pass (v2)

Changes in v2:
- Base class renamed to ir_control_flow_visitor
- Tried to comply with coding style

This is a new pass that supersedes ir_if_return and "lowers" jumps
to if/else structures.

Currently it causes no regressions on softpipe and nv40, but I'm not sure
whether the piglit glsl tests are thorough enough, so consider this
experimental.

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

This gives several great benefits:
1. All functions can be inlined after this pass
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

Note that for full effect we should also teach the unroller to unroll
loops with a fixed maximum number of iterations but with the canonical
conditional "break" that this pass will insert if asked to.

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.

1 files changed, 544 insertions, 0 deletions

diff --git a/src/glsl/ir_lower_jumps.cpp b/src/glsl/ir_lower_jumps.cpp
new file mode 100644
index 00000000000..79eb6f0939d
--- /dev/null
+++ b/src/glsl/ir_lower_jumps.cpp
@@ -0,0 +1,544 @@
+/*
+ * 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 ir_remove_loop_jumps.cpp
+ */
+
+#include "glsl_types.h"
+#include <string.h>
+#include "ir.h"
+
+enum jump_strength
+{
+   strength_none,
+   strength_always_clears_execute_flag,
+   strength_continue,
+   strength_break,
+   strength_return,
+   strength_discard
+};
+
+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 is_main;
+   unsigned nesting_depth;
+
+   function_record(ir_function_signature* p_signature = 0)
+   {
+      this->signature = p_signature;
+      this->return_flag = 0;
+      this->return_value = 0;
+      this->nesting_depth = 0;
+      this->is_main = this->signature && (strcmp(this->signature->function_name(), "main") == 0);
+   }
+
+   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 {
+   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()
+   {
+      this->progress = 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);
+      }
+   }
+
+   virtual void visit(class ir_loop_jump * ir)
+   {
+      truncate_after_instruction(ir);
+      this->block.min_strength = ir->is_break() ? strength_break : strength_continue;
+   }
+
+   virtual void visit(class ir_return * ir)
+   {
+      truncate_after_instruction(ir);
+      this->block.min_strength = strength_return;
+   }
+
+   virtual void visit(class ir_discard * ir)
+   {
+      truncate_after_instruction(ir);
+      this->block.min_strength = strength_discard;
+   }
+
+   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 if(ir->ir_type == ir_type_discard)
+         return strength_discard;
+      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 if (this->function.is_main)
+            lower = lower_main_return;
+         else
+            lower = lower_sub_return;
+         break;
+      case strength_discard:
+         lower = false; /* probably nothing needs this lowered */
+         break;
+      }
+      return lower;
+   }
+
+   block_record visit_block(exec_list* list)
+   {
+      block_record saved_block = this->block;
+      this->block = block_record();
+      visit_exec_list(list, 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];
+
+      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();
+   }
+
+      for(;;) {
+         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;
+         }
+
+         /* move both jumps out if possible */
+         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));
+            /* FINISHME: unify discards */
+            else
+               unify = false;
+
+            if(unify) {
+               jumps[0]->remove();
+               jumps[1]->remove();
+               this->progress = true;
+
+               jumps[0] = 0;
+               jumps[1] = 0;
+               block_records[0].min_strength = strength_none;
+               block_records[1].min_strength = strength_none;
+               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
+            break;
+
+         if(jump_strengths[lower] == strength_return) {
+            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();
+               jumps[lower]->insert_before(new(ir) ir_assignment(new (ir) ir_dereference_variable(return_value), ((ir_return*)jumps[lower])->value, NULL));
+            }
+            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) {
+               ir_loop_jump* lowered = 0;
+               lowered = new(ir) ir_loop_jump(ir_loop_jump::jump_break);
+               block_records[lower].min_strength = strength_break;
+               jumps[lower]->replace_with(lowered);
+               jumps[lower] = lowered;
+            } else
+               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.
+             *
+             * 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.
+             *
+             * 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.
+             */
+            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:
+            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));
+            jumps[lower] = 0;
+            block_records[lower].min_strength = strength_always_clears_execute_flag;
+            block_records[lower].may_clear_execute_flag = true;
+            this->progress = true;
+            break;
+         }
+      }
+
+      /* move out a jump out if possible */
+      if(pull_out_jumps) {
+         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]);
+            jumps[move_out] = 0;
+            block_records[move_out].min_strength = strength_none;
+            this->progress = true;
+         }
+      }
+
+      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;
+
+      if(this->block.min_strength)
+         truncate_after_instruction(ir);
+      else if(this->block.may_clear_execute_flag)
+      {
+         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);
+
+               exec_list list;
+               list.head = next;
+               block_records[move_into] = visit_block(&list);
+
+               this->progress = true;
+               goto retry;
+            }
+         } else {
+            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;
+            }
+
+            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)
+   {
+      ++this->function.nesting_depth;
+      loop_record saved_loop = this->loop;
+      this->loop = loop_record(this->function.signature, ir);
+
+      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 */
+      }
+
+      if(this->loop.break_flag) {
+         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(this->loop.may_set_return_flag) {
+         assert(this->function.return_flag);
+         ir_if* return_if = new(ir) ir_if(new(ir) ir_dereference_variable(this->function.return_flag));
+         return_if->then_instructions.push_tail(new(ir) ir_loop_jump(saved_loop.loop ? ir_loop_jump::jump_break : ir_loop_jump::jump_continue));
+         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);
+
+      function_record saved_function = this->function;
+      loop_record saved_loop = this->loop;
+      this->function = function_record(ir);
+      this->loop = loop_record(ir);
+
+      assert(!this->loop.loop);
+      visit_block(&ir->body);
+
+      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);
+   }
+};
+
+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;
+
+   do {
+      v.progress = false;
+      visit_exec_list(instructions, &v);
+   } while (v.progress);
+
+   return v.progress;
+}