[Mesa-dev] [PATCH 7/8] glsl: Implement IEEE-compliant handling of atan2(±∞, ±∞).

[Juan A. Suarez Romero]

Reviewed-by: Juan A. Suarez Romero <jasuarez at igalia.com>


On Tue, 2017-01-24 at 15:26 -0800, Francisco Jerez wrote:
> ---
>  src/compiler/glsl/builtin_functions.cpp | 22 +++++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/src/compiler/glsl/builtin_functions.cpp b/src/compiler/glsl/builtin_functions.cpp
> index fd59381..9d6ab80 100644
> --- a/src/compiler/glsl/builtin_functions.cpp
> +++ b/src/compiler/glsl/builtin_functions.cpp
> @@ -3590,11 +3590,31 @@ builtin_builder::_atan2(const glsl_type *type)
>     body.emit(assign(rcp_scaled_t, rcp(mul(t, scale))));
>     ir_expression *s_over_t = mul(mul(s, scale), rcp_scaled_t);
>  
> +   /* For |x| = |y| assume tan = 1 even if infinite (i.e. pretend momentarily
> +    * that ∞/∞ = 1) in order to comply with the rather artificial rules
> +    * inherited from IEEE 754-2008, namely:
> +    *
> +    *  "atan2(±∞, −∞) is ±3π/4
> +    *   atan2(±∞, +∞) is ±π/4"
> +    *
> +    * Note that this is inconsistent with the rules for the neighborhood of
> +    * zero that are based on iterated limits:
> +    *
> +    *  "atan2(±0, −0) is ±π
> +    *   atan2(±0, +0) is ±0"
> +    *
> +    * but GLSL specifically allows implementations to deviate from IEEE rules
> +    * at (0,0), so we take that license (i.e. pretend that 0/0 = 1 here as
> +    * well).
> +    */
> +   ir_expression *tan = csel(equal(abs(x), abs(y)),
> +                             imm(1.0f, n), abs(s_over_t));
> +
>     /* Calculate the arctangent and fix up the result if we had flipped the
>      * coordinate system.
>      */
>     ir_variable *arc = body.make_temp(type, "arc");
> -   do_atan(body, type, arc, abs(s_over_t));
> +   do_atan(body, type, arc, tan);
>     body.emit(assign(arc, add(arc, mul(b2f(flip), imm(M_PI_2f)))));
>  
>     /* Rather convoluted calculation of the sign of the result.  When x < 0 we