[Mesa-dev] [PATCH 6/6] glsl: Ignore loop-too-large heuristic if there's bad variable indexing.

[Kenneth Graunke]

Many shaders use a pattern such as:

for (int i = 0; i < NUM_LIGHTS; i++) {
   ...access a uniform array, or shader input/output array...
}

where NUM_LIGHTS is a small constant (such as 2, 4, or 8).

The expectation is that the compiler will unroll those loops, turning
the array access into constant indexing, which is more efficient, and
which may enable array splitting and other optimizations.

In many cases, our heuristic fails - either there's another tiny nested
loop inside, or the estimated number of instructions is just barely
beyond the threshold.  So, we fail to unroll the loop, leaving the
variable indexing in place.

Drivers which don't support the particular flavor of variable indexing
will call lower_variable_index_to_cond_assign(), which generates piles
and piles of immensely inefficient code.  We'd like to avoid generating
that.

This patch detects unsupported forms of variable-indexing in loops, where
the array index is a loop induction variable.  In that case, it bypasses
the loop-too-large heuristic and forces unrolling.

Improves performance in a PCF soft-shadow microbenchmark by 2x.
No changes in shader-db.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/glsl/loop_unroll.cpp | 61 +++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 58 insertions(+), 3 deletions(-)

diff --git a/src/glsl/loop_unroll.cpp b/src/glsl/loop_unroll.cpp
index 1ce4d58..baede92 100644
--- a/src/glsl/loop_unroll.cpp
+++ b/src/glsl/loop_unroll.cpp
@@ -63,13 +63,17 @@ is_break(ir_instruction *ir)
 class loop_unroll_count : public ir_hierarchical_visitor {
 public:
    int nodes;
+   bool unsupported_variable_indexing;
    /* If there are nested loops, the node count will be inaccurate. */
    bool nested_loop;
 
-   loop_unroll_count(exec_list *list)
+   loop_unroll_count(exec_list *list, loop_variable_state *ls,
+                     const struct gl_shader_compiler_options *options)
+      : ls(ls), options(options)
    {
       nodes = 0;
       nested_loop = false;
+      unsupported_variable_indexing = false;
 
       run(list);
    }
@@ -91,6 +95,54 @@ public:
       nested_loop = true;
       return visit_continue;
    }
+
+   virtual ir_visitor_status visit_enter(ir_dereference_array *ir)
+   {
+      /* Check for arrays variably-indexed by a loop induction variable.
+       * Unrolling the loop may convert that access into constant-indexing.
+       *
+       * Many drivers don't support particular kinds of variable indexing,
+       * and have to resort to using lower_variable_index_to_cond_assign to
+       * handle it.  This results in huge amounts of horrible code, so we'd
+       * like to avoid that if possible.  Here, we just note that it will
+       * happen.
+       */
+      if ((ir->type->is_array() || ir->type->is_matrix()) &&
+          !ir->array_index->as_constant()) {
+         ir_variable *array = ir->array->variable_referenced();
+         loop_variable *lv = ls->get(ir->array_index->variable_referenced());
+         if (lv && lv->is_induction_var()) {
+            switch (array->data.mode) {
+            case ir_var_auto:
+            case ir_var_temporary:
+            case ir_var_const_in:
+            case ir_var_function_in:
+            case ir_var_function_out:
+            case ir_var_function_inout:
+               if (options->EmitNoIndirectTemp)
+                  unsupported_variable_indexing = true;
+               break;
+            case ir_var_uniform:
+               if (options->EmitNoIndirectUniform)
+                  unsupported_variable_indexing = true;
+               break;
+            case ir_var_shader_in:
+               if (options->EmitNoIndirectInput)
+                  unsupported_variable_indexing = true;
+               break;
+            case ir_var_shader_out:
+               if (options->EmitNoIndirectOutput)
+                  unsupported_variable_indexing = true;
+               break;
+            }
+         }
+      }
+      return visit_continue;
+   }
+
+private:
+   loop_variable_state *ls;
+   const struct gl_shader_compiler_options *options;
 };
 
 
@@ -257,9 +309,12 @@ loop_unroll_visitor::visit_leave(ir_loop *ir)
 
    /* Don't try to unroll nested loops and loops with a huge body.
     */
-   loop_unroll_count count(&ir->body_instructions);
+   loop_unroll_count count(&ir->body_instructions, ls, options);
+
+   bool loop_too_large =
+      count.nested_loop || count.nodes * iterations > max_iterations * 5;
 
-   if (count.nested_loop || count.nodes * iterations > max_iterations * 5)
+   if (loop_too_large && !count.unsupported_variable_indexing)
       return visit_continue;
 
    /* Note: the limiting terminator contributes 1 to ls->num_loop_jumps.
-- 
1.9.1