[Mesa-dev] [PATCH v2 2/3] glsl: check if induction var incremented before use in terminator

[Michael Schellenberger Costa]

Out of curriosity what about decrement and other shenanigans?

--Michael


Am 21.09.2017 um 12:55 schrieb Timothy Arceri:
> do-while loops can increment the starting value before the
> condition is checked. e.g.
>
>    do {
>      ndx++;
>    } while (ndx < 3);
>
> This commit changes the code to detect this and reduces the
> iteration count by 1 if found.
>
> V2: fix terminator spelling
>
> Reviewed-by: Nicolai Hähnle <nicolai.haehnle at amd.com>
> Reviewed-by: Elie Tournier <elie.tournier at collabora.com>
> ---
>   src/compiler/glsl/loop_analysis.cpp | 38 +++++++++++++++++++++++++++++++++++++
>   1 file changed, 38 insertions(+)
>
> diff --git a/src/compiler/glsl/loop_analysis.cpp b/src/compiler/glsl/loop_analysis.cpp
> index 81a07f78f8..78279844dc 100644
> --- a/src/compiler/glsl/loop_analysis.cpp
> +++ b/src/compiler/glsl/loop_analysis.cpp
> @@ -164,20 +164,54 @@ calculate_iterations(ir_rvalue *from, ir_rvalue *to, ir_rvalue *increment,
>            iter_value += bias[i];
>            valid_loop = true;
>            break;
>         }
>      }
>   
>      ralloc_free(mem_ctx);
>      return (valid_loop) ? iter_value : -1;
>   }
>   
> +static bool
> +incremented_before_terminator(ir_loop *loop, ir_variable *var,
> +                              ir_if *terminator)
> +{
> +   for (exec_node *node = loop->body_instructions.get_head();
> +        !node->is_tail_sentinel();
> +        node = node->get_next()) {
> +      ir_instruction *ir = (ir_instruction *) node;
> +
> +      switch (ir->ir_type) {
> +      case ir_type_if:
> +         if (ir->as_if() == terminator)
> +            return false;
> +         break;
> +
> +      case ir_type_assignment: {
> +         ir_assignment *assign = ir->as_assignment();
> +         ir_variable *assignee = assign->lhs->whole_variable_referenced();
> +
> +         if (assignee == var) {
> +            assert(assign->condition == NULL);
> +            return true;
> +         }
> +
> +         break;
> +      }
> +
> +      default:
> +         break;
> +      }
> +   }
> +
> +   unreachable("Unable to find induction variable");
> +}
>   
>   /**
>    * Record the fact that the given loop variable was referenced inside the loop.
>    *
>    * \arg in_assignee is true if the reference was on the LHS of an assignment.
>    *
>    * \arg in_conditional_code_or_nested_loop is true if the reference occurred
>    * inside an if statement or a nested loop.
>    *
>    * \arg current_assignment is the ir_assignment node that the loop variable is
> @@ -575,20 +609,24 @@ loop_analysis::visit_leave(ir_loop *ir)
>   
>   	 ir_variable *var = counter->variable_referenced();
>   
>   	 ir_rvalue *init = find_initial_value(ir, var);
>   
>            loop_variable *lv = ls->get(var);
>            if (lv != NULL && lv->is_induction_var()) {
>               t->iterations = calculate_iterations(init, limit, lv->increment,
>                                                    cmp);
>   
> +            if (incremented_before_terminator(ir, var, t->ir)) {
> +               t->iterations--;
> +            }
> +
>               if (t->iterations >= 0 &&
>                   (ls->limiting_terminator == NULL ||
>                    t->iterations < ls->limiting_terminator->iterations)) {
>                  ls->limiting_terminator = t;
>               }
>            }
>            break;
>         }
>   
>         default: