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

[Jason Ekstrand]

One trivial request:  If we do land this patch, please include a link the
the mailing list archives in the commit message so that we can easily track
down this discussion if we ever need to in the future.

On Fri, Jan 6, 2017 at 1:42 AM, Samuel Pitoiset <samuel.pitoiset at gmail.com>
wrote:

> D3D always computes the absolute value while GLSL says that the
> 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)
>
>  /** @} */
>
> --
> 2.11.0
>
> _______________________________________________
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> mesa-dev at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
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