[Mesa-dev] [PATCH] glsl: remove rsq+rcp -> sqrt transformation to fix the game Risen

[Ian Romanick]

On 05/31/2016 03:46 AM, Marek Olšák wrote:
> From: Marek Olšák <marek.olsak at amd.com>
> 
> Using sqrt here breaks rendering.

That seems really weird.  Any idea why?  Does it just get a different
answer for certain boundary values?  0? -0?

> Apitrace: In the bugzilla.
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94627
> ---
>  src/compiler/glsl/opt_algebraic.cpp | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/src/compiler/glsl/opt_algebraic.cpp b/src/compiler/glsl/opt_algebraic.cpp
> index f5858c8..f5fec9d 100644
> --- a/src/compiler/glsl/opt_algebraic.cpp
> +++ b/src/compiler/glsl/opt_algebraic.cpp
> @@ -898,11 +898,7 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
>                                             neg(op_expr[0]->operands[0]));
>        }
>  
> -      /* While ir_to_mesa.cpp will lower sqrt(x) to rcp(rsq(x)), it does so at
> -       * its IR level, so we can always apply this transformation.
> -       */
> -      if (op_expr[0] && op_expr[0]->operation == ir_unop_rsq)
> -         return sqrt(op_expr[0]->operands[0]);
> +      /* DO NOT transform rsq+rcp into sqrt. It breaks the game Risen. */
>  
>        /* As far as we know, all backends are OK with rsq. */
>        if (op_expr[0] && op_expr[0]->operation == ir_unop_sqrt) {