[Mesa-dev] [PATCH] glsl: always do sqrt(abs()) and inversesqrt(abs())

[Roland Scheidegger]

Am 06.01.2017 um 10:42 schrieb Samuel Pitoiset:
> D3D always computes the absolute value while GLSL says that the
That should probably say "d3d9" - it is completely wrong for d3d10 and
later (which have it to be defined as a guaranteed NaN).
(Otherwise, I'm still not quite convinced it's the right thing to do
this always, but meh...)

Roland


> result of inversesqrt() is undefined if x <= 0 (and undefined if
> x < 0 for sqrt()). But some apps rely on this specific behaviour
> which is not clearly defined by OpenGL.
> 
> Computing the absolute value before sqrt()/inversesqrt() will
> prevent that, especially for apps which have been ported from D3D.
> Note that closed drivers seem to also use that quirk.
> 
> This gets rid of the NaN values in the "Spec Ops: The Line" game
> as well as the black squares with radeonsi. Note that Nouveau is
> not affected by this bug because we already take the absolute value
> when translating from TGSI to nv50/ir.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97338
> 
> Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
> ---
>  src/compiler/glsl/builtin_functions.cpp | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/src/compiler/glsl/builtin_functions.cpp b/src/compiler/glsl/builtin_functions.cpp
> index 797af08b6c..f816f2ff7d 100644
> --- a/src/compiler/glsl/builtin_functions.cpp
> +++ b/src/compiler/glsl/builtin_functions.cpp
> @@ -3623,12 +3623,30 @@ builtin_builder::_pow(const glsl_type *type)
>     return binop(always_available, ir_binop_pow, type, type, type);
>  }
>  
> +ir_function_signature *
> +builtin_builder::_sqrt(builtin_available_predicate avail,
> +                       const glsl_type *type)
> +{
> +   ir_variable *x = in_var(type, "x");
> +   MAKE_SIG(type, avail, 1, x);
> +   body.emit(ret(expr(ir_unop_sqrt, abs(x))));
> +   return sig;
> +}
> +
> +ir_function_signature *
> +builtin_builder::_inversesqrt(builtin_available_predicate avail,
> +                              const glsl_type *type)
> +{
> +   ir_variable *x = in_var(type, "x");
> +   MAKE_SIG(type, avail, 1, x);
> +   body.emit(ret(expr(ir_unop_rsq, abs(x))));
> +   return sig;
> +}
> +
>  UNOP(exp,         ir_unop_exp,  always_available)
>  UNOP(log,         ir_unop_log,  always_available)
>  UNOP(exp2,        ir_unop_exp2, always_available)
>  UNOP(log2,        ir_unop_log2, always_available)
> -UNOPA(sqrt,        ir_unop_sqrt)
> -UNOPA(inversesqrt, ir_unop_rsq)
>  
>  /** @} */
>  
>