glsl: Rebalance expression trees that are reduction operations.

The intention of this pass was to give us better instruction scheduling
opportunities, but it unexpectedly reduced some instruction counts as
well:

total instructions in shared programs: 1666639 -> 1666073 (-0.03%)
instructions in affected programs:     54612 -> 54046 (-1.04%)
(and trades 4 SIMD16 programs in SS3)

src/glsl/Makefile.sources

@@ -96,6 +96,7 @@ LIBGLSL_FILES = \
 	$(GLSL_SRCDIR)/opt_function_inlining.cpp \
 	$(GLSL_SRCDIR)/opt_if_simplification.cpp \
 	$(GLSL_SRCDIR)/opt_noop_swizzle.cpp \
+	$(GLSL_SRCDIR)/opt_rebalance_tree.cpp \
 	$(GLSL_SRCDIR)/opt_redundant_jumps.cpp \
 	$(GLSL_SRCDIR)/opt_structure_splitting.cpp \
 	$(GLSL_SRCDIR)/opt_swizzle_swizzle.cpp \

src/glsl/glsl_parser_extras.cpp

@@ -1569,6 +1569,7 @@ do_common_optimization(exec_list *ir, bool linked,
       progress = do_constant_variable_unlinked(ir) || progress;
    progress = do_constant_folding(ir) || progress;
    progress = do_cse(ir) || progress;
+   progress = do_rebalance_tree(ir) || progress;
    progress = do_algebraic(ir, native_integers) || progress;
    progress = do_lower_jumps(ir) || progress;
    progress = do_vec_index_to_swizzle(ir) || progress;

src/glsl/ir_optimization.h

@@ -71,6 +71,7 @@ bool do_common_optimization(exec_list *ir, bool linked,
                             const struct gl_shader_compiler_options *options,
                             bool native_integers);
 
+bool do_rebalance_tree(exec_list *instructions);
 bool do_algebraic(exec_list *instructions, bool native_integers);
 bool do_constant_folding(exec_list *instructions);
 bool do_constant_variable(exec_list *instructions);

src/glsl/opt_rebalance_tree.cpp

@@ -0,0 +1,300 @@
+/*
+ * Copyright © 2014 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_rebalance_tree.cpp
+ *
+ * Rebalances a reduction expression tree.
+ *
+ * For reduction operations (e.g., x + y + z + w) we generate an expression
+ * tree like
+ *
+ *        +
+ *       / \
+ *      +   w
+ *     / \
+ *    +   z
+ *   / \
+ *  x   y
+ *
+ * which we can rebalance into
+ *
+ *       +
+ *      / \
+ *     /   \
+ *    +     +
+ *   / \   / \
+ *  x   y z   w
+ *
+ * to get a better instruction scheduling.
+ *
+ * See "Tree Rebalancing in Optimal Editor Time and Space" by Quentin F. Stout
+ * and Bette L. Warren.
+ *
+ * Also see http://penguin.ewu.edu/~trolfe/DSWpaper/ for a very readable
+ * explanation of the of the tree_to_vine() (rightward rotation) and
+ * vine_to_tree() (leftward rotation) algorithms.
+ */
+
+#include "ir.h"
+#include "ir_visitor.h"
+#include "ir_rvalue_visitor.h"
+#include "ir_optimization.h"
+
+/* The DSW algorithm generates a degenerate tree (really, a linked list) in
+ * tree_to_vine(). We'd rather not leave a binary expression with only one
+ * operand, so trivial modifications (the ternary operators below) are needed
+ * to ensure that we only rotate around the ir_expression nodes of the tree.
+ */
+static unsigned
+tree_to_vine(ir_expression *root)
+{
+   unsigned size = 0;
+   ir_rvalue *vine_tail = root;
+   ir_rvalue *remainder = root->operands[1];
+
+   while (remainder != NULL) {
+      ir_expression *remainder_temp = remainder->as_expression();
+      ir_expression *remainder_left = remainder_temp ?
+         remainder_temp->operands[0]->as_expression() : NULL;
+
+      if (remainder_left == NULL) {
+         /* move vine_tail down one */
+         vine_tail = remainder;
+         remainder = remainder->as_expression() ?
+            ((ir_expression *)remainder)->operands[1] : NULL;
+         size++;
+      } else {
+         /* rotate */
+         ir_expression *tempptr = remainder_left;
+         ((ir_expression *)remainder)->operands[0] = tempptr->operands[1];
+         tempptr->operands[1] = remainder;
+         remainder = tempptr;
+         ((ir_expression *)vine_tail)->operands[1] = tempptr;
+      }
+   }
+
+   return size;
+}
+
+static void
+compression(ir_expression *root, unsigned count)
+{
+   ir_expression *scanner = root;
+
+   for (unsigned i = 0; i < count; i++) {
+      ir_expression *child = (ir_expression *)scanner->operands[1];
+      scanner->operands[1] = child->operands[1];
+      scanner = (ir_expression *)scanner->operands[1];
+      child->operands[1] = scanner->operands[0];
+      scanner->operands[0] = child;
+   }
+}
+
+static void
+vine_to_tree(ir_expression *root, unsigned size)
+{
+   int n = size - 1;
+   for (int m = n / 2; m > 0; m = n / 2) {
+      compression(root, m);
+      n -= m + 1;
+   }
+}
+
+namespace {
+
+class ir_rebalance_visitor : public ir_rvalue_enter_visitor {
+public:
+   ir_rebalance_visitor()
+   {
+      progress = false;
+   }
+
+   void handle_rvalue(ir_rvalue **rvalue);
+
+   bool progress;
+};
+
+struct is_reduction_data {
+   ir_expression_operation operation;
+   const glsl_type *type;
+   unsigned num_expr;
+   bool is_reduction;
+   bool contains_constant;
+};
+
+} /* anonymous namespace */
+
+static bool
+is_reduction_operation(ir_expression_operation operation)
+{
+   switch (operation) {
+   case ir_binop_add:
+   case ir_binop_mul:
+   case ir_binop_bit_and:
+   case ir_binop_bit_xor:
+   case ir_binop_bit_or:
+   case ir_binop_logic_and:
+   case ir_binop_logic_xor:
+   case ir_binop_logic_or:
+   case ir_binop_min:
+   case ir_binop_max:
+      return true;
+   default:
+      return false;
+   }
+}
+
+/* Note that this function does not attempt to recognize that reduction trees
+ * are already balanced.
+ *
+ * We return false from this function for a number of reasons other than an
+ * expression tree not being a mathematical reduction. Namely,
+ *
+ *    - if the tree contains multiple constants that we may be able to combine.
+ *    - if the tree contains matrices:
+ *       - they might contain vec4's with many constant components that we can
+ *         simplify after splitting.
+ *       - applying the matrix chain ordering optimization is more than just
+ *         balancing an expression tree.
+ *    - if the tree contains operations on multiple types.
+ *    - if the tree contains ir_dereference_{array,record}, since foo[a+b] + c
+ *      would trick the visiting pass.
+ */
+static void
+is_reduction(ir_instruction *ir, void *data)
+{
+   struct is_reduction_data *ird = (struct is_reduction_data *)data;
+   if (!ird->is_reduction)
+      return;
+
+   /* We don't want to balance a tree that contains multiple constants, since
+    * we'll be able to constant fold them if they're not in separate subtrees.
+    */
+   if (ir->as_constant()) {
+      if (ird->contains_constant) {
+         ird->is_reduction = false;
+      }
+      ird->contains_constant = true;
+      return;
+   }
+
+   /* Array/record dereferences have subtrees that are not part of the expr
+    * tree we're balancing. Skip trees containing them.
+    */
+   if (ir->ir_type == ir_type_dereference_array ||
+       ir->ir_type == ir_type_dereference_record) {
+      ird->is_reduction = false;
+      return;
+   }
+
+   ir_expression *expr = ir->as_expression();
+   if (!expr)
+      return;
+
+   /* Non-constant matrices might still contain constant vec4 that we can
+    * constant fold once split up. Handling matrices will need some more
+    * work.
+    */
+   if (expr->type->is_matrix()) {
+      ird->is_reduction = false;
+      return;
+   }
+
+   if (ird->type != NULL && ird->type != expr->type) {
+      ird->is_reduction = false;
+      return;
+   }
+   ird->type = expr->type;
+
+   ird->num_expr++;
+   if (is_reduction_operation(expr->operation)) {
+      if (ird->operation != 0 && ird->operation != expr->operation)
+         ird->is_reduction = false;
+      ird->operation = expr->operation;
+   } else {
+      ird->is_reduction = false;
+   }
+}
+
+static ir_rvalue *
+handle_expression(ir_expression *expr)
+{
+   struct is_reduction_data ird;
+   ird.operation = (ir_expression_operation)0;
+   ird.type = NULL;
+   ird.num_expr = 0;
+   ird.is_reduction = true;
+   ird.contains_constant = false;
+
+   visit_tree(expr, is_reduction, (void *)&ird);
+
+   if (ird.is_reduction && ird.num_expr > 2) {
+      ir_constant z = ir_constant(0.0f);
+      ir_expression pseudo_root = ir_expression(ir_binop_add, &z, expr);
+
+      unsigned size = tree_to_vine(&pseudo_root);
+      vine_to_tree(&pseudo_root, size);
+
+      expr = (ir_expression *)pseudo_root.operands[1];
+   }
+   return expr;
+}
+
+void
+ir_rebalance_visitor::handle_rvalue(ir_rvalue **rvalue)
+{
+   if (!*rvalue)
+      return;
+
+   ir_expression *expr = (*rvalue)->as_expression();
+   if (!expr || !is_reduction_operation(expr->operation))
+      return;
+
+   ir_rvalue *new_rvalue = handle_expression(expr);
+
+   /* If we failed to rebalance the tree (e.g., because it wasn't a reduction,
+    * or some other set of cases) new_rvalue will point to the same root as
+    * before.
+    *
+    * Similarly, if the tree rooted at *rvalue was a reduction and was already
+    * balanced, the algorithm will rearrange the tree but will ultimately
+    * return an identical tree, so this check will handle that as well and
+    * will not set progress = true.
+    */
+   if (new_rvalue == *rvalue)
+      return;
+
+   *rvalue = new_rvalue;
+   this->progress = true;
+}
+
+bool
+do_rebalance_tree(exec_list *instructions)
+{
+   ir_rebalance_visitor v;
+
+   v.run(instructions);
+
+   return v.progress;
+}