[Mesa-dev] [PATCH 3/3] glsl: relax loop unrolling restrictions

[Samuel Pitoiset]

Here's a shader-db report:

https://pastebin.com/raw/QBMnF2pv

This doesn't sound like a total win actually...

For now, patches 1&2 are:

Reviewed-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>

On 06/21/2017 12:12 PM, Timothy Arceri wrote:
> The main reason these restriction exist is because glsl the loop
> unrolling pass is super slow with large loops.
> 
> be5f27a84d0d fixed things so that expression trees were counted
> against the limit, however it left the limit as
> max_iterations * 5 which is actually fine for most shaders but
> probably over conservative.
> 
> This change relaxes the limit to allow more loops to unroll in the
> Unigine Superposition benchmark.
> 
> Results from Unigine Superposition @ 1920x1080 - High - Fullscreen
> On radeonsi (RX480)
> 
> Before:
> 
> Average: 28.20 Frames Per Second
> 
> After:
> 
> Average: 28.60 Frames Per Second
> ---
> 
>   The increase is small but it seems to be consistent, I would be
>   interested in the results if others were interested in testing it.
> 
>   src/compiler/glsl/loop_unroll.cpp | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/compiler/glsl/loop_unroll.cpp b/src/compiler/glsl/loop_unroll.cpp
> index bc377df..64ebf0f 100644
> --- a/src/compiler/glsl/loop_unroll.cpp
> +++ b/src/compiler/glsl/loop_unroll.cpp
> @@ -350,21 +350,21 @@ loop_unroll_visitor::visit_leave(ir_loop *ir)
>      /* Don't try to unroll loops that have zillions of iterations either.
>       */
>      if (iterations > max_iterations)
>         return visit_continue;
>   
>      /* Don't try to unroll nested loops and loops with a huge body.
>       */
>      loop_unroll_count count(&ir->body_instructions, ls, options);
>   
>      bool loop_too_large =
> -      count.nested_loop || count.nodes * iterations > max_iterations * 5;
> +      count.nested_loop || count.nodes * iterations > max_iterations * 10;
>   
>      if (loop_too_large && !count.unsupported_variable_indexing &&
>          !count.array_indexed_by_induction_var_with_exact_iterations)
>         return visit_continue;
>   
>      /* Note: the limiting terminator contributes 1 to ls->num_loop_jumps.
>       * We'll be removing the limiting terminator before we unroll.
>       */
>      assert(ls->num_loop_jumps > 0);
>      unsigned predicted_num_loop_jumps = ls->num_loop_jumps - 1;
>