[Mesa-dev] [PATCH 3/7] glsl/lower_if: don't lower branches touching tess control outputs

[Nicolai Hähnle]

On 29.10.2016 01:13, Marek Olšák wrote:
> From: Marek Olšák <marek.olsak at amd.com>
>
> ---
>  src/compiler/glsl/ir_optimization.h           |  3 ++-
>  src/compiler/glsl/lower_if_to_cond_assign.cpp | 23 ++++++++++++++++++++---
>  src/compiler/glsl/test_optpass.cpp            |  2 +-
>  src/mesa/drivers/dri/i965/brw_link.cpp        |  2 +-
>  src/mesa/program/ir_to_mesa.cpp               |  3 ++-
>  src/mesa/state_tracker/st_glsl_to_tgsi.cpp    |  3 ++-
>  6 files changed, 28 insertions(+), 8 deletions(-)
>
[snip]
> @@ -105,20 +109,33 @@ check_ir_node(ir_instruction *ir, void *data)
>     case ir_type_call:
>     case ir_type_discard:
>     case ir_type_loop:
>     case ir_type_loop_jump:
>     case ir_type_return:
>     case ir_type_emit_vertex:
>     case ir_type_end_primitive:
>     case ir_type_barrier:
>        v->found_unsupported_op = true;
>        break;
> +
> +   case ir_type_dereference_variable: {
> +      ir_variable *var = ir->as_dereference_variable()->variable_referenced();
> +
> +      /* Tess control shader outputs are like shared memory with complex
> +       * side effects, so treat it that way.
> +       */
> +      if (v->stage == MESA_SHADER_TESS_CTRL &&
> +          var->data.mode == ir_var_shader_out)
> +         v->found_unsupported_op = true;
> +      break;
> +   }
> +

I don't understand this rationale. What side effects are there, and can 
you give an example where the transformation would be wrong?

Thanks,
Nicolai