Mesa (master): glsl: Modify strategy for accumulating conditions when lowering if-statements
Ian Romanick
idr at kemper.freedesktop.org
Mon Aug 15 18:44:59 UTC 2011
Module: Mesa
Branch: master
Commit: a352e2d08e0a141298275e77f25541218a97afb7
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=a352e2d08e0a141298275e77f25541218a97afb7
Author: Ian Romanick <ian.d.romanick at intel.com>
Date: Mon Aug 1 14:13:10 2011 -0700
glsl: Modify strategy for accumulating conditions when lowering if-statements
Previously if-statements were lowered from inner-most to outer-most
(i.e., bottom-up). All assignments within an if-statement would have
the condition of the if-statement appended to its existing condition.
As a result the assignments from a deeply nested if-statement would
have a very long and complex condition.
Several shaders in the OpenGL ES2 conformance test suite contain
non-constant array indexing that has been lowered by the shader
writer. These tests usually look something like:
if (i == 0) {
value = array[0];
} else if (i == 1) {
value = array[1];
} else ...
The IR for the last assignment ends up as:
(assign (expression bool && (expression bool ! (var_ref if_to_cond_assign_condition) ) (expression bool && (expression bool ! (var_ref if_to_cond_assign_condition at 20) ) (expression bool && (expression bool ! (var_ref if_to_cond_assign_condition at 22) ) (expression bool && (expression bool ! (var_ref if_to_cond_assign_condition at 24) ) (var_ref if_to_cond_assign_condition at 26) ) ) ) ) (x) (var_ref value) (array_ref (var_ref array) (constant int (5)))
The Mesa IR that is generated from this is just as awesome as you
might expect.
Three changes are made to the way if-statements are lowered.
1. Two condition variables, if_to_cond_assign_then and
if_to_cond_assign_else, are created for each if-then-else structure.
The former contains the "positive" condition, and the later contains
the "negative" condtion. This change was implemented in the previous
patch.
2. Each condition variable is added to a hash-table when it is created.
3. When lowering an if-statement, assignments to existing condtion
variables get the current condition anded. This ensures that nested
condition variables are only set to true when the condition variable
for all outer if-statements is also true.
Changes #1 and #3 combine to ensure the correctness of the resulting
code.
4. When a condition assignment is encountered with a condition that is
a dereference of a previously added condition variable, the condition
is not modified.
Change #4 prevents the continuous accumulation of conditions on
assignments.
If the original if-statements were:
if (x) {
if (a && b && c && d && e) {
...
} else {
...
}
} else {
if (g && h && i && j && k) {
...
} else {
...
}
}
The lowered code will be
if_to_cond_assign_then at 1 = x;
if_to_cond_assign_then at 2 = a && b && c && d && e
&& if_to_cond_assign_then at 1;
...
if_to_cond_assign_else at 2 = !if_to_cond_assign_then
&& if_to_cond_assign_then at 1;
...
if_to_cond_assign_else at 1 = !if_to_cond_assign_then at 1;
if_to_cond_assign_then at 3 = g && h && i && j;
&& if_to_cond_assign_else at 1;
...
if_to_cond_assign_else at 3 = !if_to_cond_assign_then
&& if_to_cond_assign_else at 1;
...
Depending on how instructions are emitted, there may be an extra
instruction due to the duplication of the '&&
if_to_cond_assign_{then,else}@1' on the nested else conditions. In
addition, this may cause some unnecessary register pressure since in
the simple case (where the nested conditions are not complex) the
nested then-condition variables are live longer than strictly
necessary.
Before this change, one of the shaders in the OpenGL ES2 conformance
test suite's acos_float_frag_xvary generated 348 Mesa IR instructions.
After this change it only generates 124. Many, but not all, of these
instructions would have also been eliminated by CSE.
Reviewed-by: Eric Anholt <eric at anholt.net>
---
src/glsl/lower_if_to_cond_assign.cpp | 66 ++++++++++++++++++++++++++++------
1 files changed, 55 insertions(+), 11 deletions(-)
diff --git a/src/glsl/lower_if_to_cond_assign.cpp b/src/glsl/lower_if_to_cond_assign.cpp
index a709755..94b24db 100644
--- a/src/glsl/lower_if_to_cond_assign.cpp
+++ b/src/glsl/lower_if_to_cond_assign.cpp
@@ -47,6 +47,7 @@
#include "glsl_types.h"
#include "ir.h"
+#include "program/hash_table.h"
class ir_if_to_cond_assign_visitor : public ir_hierarchical_visitor {
public:
@@ -55,6 +56,14 @@ public:
this->progress = false;
this->max_depth = max_depth;
this->depth = 0;
+
+ this->condition_variables = hash_table_ctor(0, hash_table_pointer_hash,
+ hash_table_pointer_compare);
+ }
+
+ ~ir_if_to_cond_assign_visitor()
+ {
+ hash_table_dtor(this->condition_variables);
}
ir_visitor_status visit_enter(ir_if *);
@@ -63,6 +72,8 @@ public:
bool progress;
unsigned max_depth;
unsigned depth;
+
+ struct hash_table *condition_variables;
};
bool
@@ -95,7 +106,8 @@ check_control_flow(ir_instruction *ir, void *data)
void
move_block_to_cond_assign(void *mem_ctx,
ir_if *if_ir, ir_rvalue *cond_expr,
- exec_list *instructions)
+ exec_list *instructions,
+ struct hash_table *ht)
{
foreach_list_safe(node, instructions) {
ir_instruction *ir = (ir_instruction *) node;
@@ -103,14 +115,33 @@ move_block_to_cond_assign(void *mem_ctx,
if (ir->ir_type == ir_type_assignment) {
ir_assignment *assign = (ir_assignment *)ir;
- if (!assign->condition) {
- assign->condition = cond_expr->clone(mem_ctx, NULL);
- } else {
- assign->condition =
- new(mem_ctx) ir_expression(ir_binop_logic_and,
- glsl_type::bool_type,
- cond_expr->clone(mem_ctx, NULL),
- assign->condition);
+ if (hash_table_find(ht, assign) == NULL) {
+ hash_table_insert(ht, assign, assign);
+
+ /* If the LHS of the assignment is a condition variable that was
+ * previously added, insert an additional assignment of false to
+ * the variable.
+ */
+ const bool assign_to_cv =
+ hash_table_find(ht, assign->lhs->variable_referenced()) != NULL;
+
+ if (!assign->condition) {
+ if (assign_to_cv) {
+ assign->rhs =
+ new(mem_ctx) ir_expression(ir_binop_logic_and,
+ glsl_type::bool_type,
+ cond_expr->clone(mem_ctx, NULL),
+ assign->rhs);
+ } else {
+ assign->condition = cond_expr->clone(mem_ctx, NULL);
+ }
+ } else {
+ assign->condition =
+ new(mem_ctx) ir_expression(ir_binop_logic_and,
+ glsl_type::bool_type,
+ cond_expr->clone(mem_ctx, NULL),
+ assign->condition);
+ }
}
}
@@ -125,6 +156,7 @@ ir_if_to_cond_assign_visitor::visit_enter(ir_if *ir)
{
(void) ir;
this->depth++;
+
return visit_continue;
}
@@ -170,7 +202,13 @@ ir_if_to_cond_assign_visitor::visit_leave(ir_if *ir)
ir->insert_before(assign);
move_block_to_cond_assign(mem_ctx, ir, then_cond,
- &ir->then_instructions);
+ &ir->then_instructions,
+ this->condition_variables);
+
+ /* Add the new condition variable to the hash table. This allows us to
+ * find this variable when lowering other (enclosing) if-statements.
+ */
+ hash_table_insert(this->condition_variables, then_var, then_var);
/* If there are instructions in the else-clause, store the inverse of the
* condition to a variable. Move all of the instructions from the
@@ -195,7 +233,13 @@ ir_if_to_cond_assign_visitor::visit_leave(ir_if *ir)
ir->insert_before(assign);
move_block_to_cond_assign(mem_ctx, ir, else_cond,
- &ir->else_instructions);
+ &ir->else_instructions,
+ this->condition_variables);
+
+ /* Add the new condition variable to the hash table. This allows us to
+ * find this variable when lowering other (enclosing) if-statements.
+ */
+ hash_table_insert(this->condition_variables, else_var, else_var);
}
ir->remove();
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