[Mesa-dev] [PATCH v2 18/19] i965/vs: Generalize computation of array strides in preparation for GS.

[Paul Berry]

Geometry shader inputs are arrays, but they use an unusual array
layout: instead of all array elements for a given geometry shader
input being stored consecutively, all geometry shader inputs are
interleaved into one giant array.  As a result, the array stride we
use to access geometry shader inputs must be equal to the size of the
input VUE, rather than the size of the array element.

This patch introduces a new virtual function,
vec4_visitor::compute_array_stride(), which will allow geometry shader
compilation to specialize the computation of array stride to account
for the unusual layout of geometry shader input arrays.  It also
renames the local variable that the ir_dereference_array visitor uses
to store the stride, to avoid confusion.

Reviewed-by: Jordan Justen <jordan.l.justen at intel.com>
---
 src/mesa/drivers/dri/i965/brw_vec4.h           |  1 +
 src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp | 19 +++++++++++++++----
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_vec4.h b/src/mesa/drivers/dri/i965/brw_vec4.h
index 98b496a..ddd6087 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4.h
+++ b/src/mesa/drivers/dri/i965/brw_vec4.h
@@ -488,6 +488,7 @@ protected:
    virtual void emit_thread_end() = 0;
    virtual void emit_urb_write_header(int mrf) = 0;
    virtual vec4_instruction *emit_urb_write_opcode(bool complete) = 0;
+   virtual int compute_array_stride(ir_dereference_array *ir);
 };
 
 class vec4_vs_visitor : public vec4_visitor
diff --git a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
index a98aae8..e77ca6e 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
@@ -1692,12 +1692,23 @@ vec4_visitor::visit(ir_dereference_variable *ir)
       this->result.swizzle = swizzle_for_size(type->vector_elements);
 }
 
+
+int
+vec4_visitor::compute_array_stride(ir_dereference_array *ir)
+{
+   /* Under normal circumstances array elements are stored consecutively, so
+    * the stride is equal to the size of the array element.
+    */
+   return type_size(ir->type);
+}
+
+
 void
 vec4_visitor::visit(ir_dereference_array *ir)
 {
    ir_constant *constant_index;
    src_reg src;
-   int element_size = type_size(ir->type);
+   int array_stride = compute_array_stride(ir);
 
    constant_index = ir->array_index->constant_expression_value();
 
@@ -1705,7 +1716,7 @@ vec4_visitor::visit(ir_dereference_array *ir)
    src = this->result;
 
    if (constant_index) {
-      src.reg_offset += constant_index->value.i[0] * element_size;
+      src.reg_offset += constant_index->value.i[0] * array_stride;
    } else {
       /* Variable index array dereference.  It eats the "vec4" of the
        * base of the array and an index that offsets the Mesa register
@@ -1715,12 +1726,12 @@ vec4_visitor::visit(ir_dereference_array *ir)
 
       src_reg index_reg;
 
-      if (element_size == 1) {
+      if (array_stride == 1) {
 	 index_reg = this->result;
       } else {
 	 index_reg = src_reg(this, glsl_type::int_type);
 
-	 emit(MUL(dst_reg(index_reg), this->result, src_reg(element_size)));
+	 emit(MUL(dst_reg(index_reg), this->result, src_reg(array_stride)));
       }
 
       if (src.reladdr) {
-- 
1.8.2.1