[Mesa-dev] [PATCH 1/5] gallium: add SQRT shader opcode

[Christoph Bumiller]

On 01.02.2013 19:29, Brian Paul wrote:
> The glsl-to-tgsi translater will emit SQRT to implement GLSL's sqrt()
> and distance() functions if the PIPE_SHADER_CAP_TGSI_SQRT_SUPPORTED
> query says it's supported by the driver.
>
> Otherwise, sqrt(x) is implemented with x*rsq(x).  The problem with
> this is sqrt(0) must be handled specially because rsq(0) might be
> Inf/NaN/undefined (and then 0*rsq(0) is Inf/Nan/undefined).  In the
That's why we do rcp(rsq(x)), that works correctly.
I'm not sure we really need a cap for this though ... except to avoid
modifying drivers ;)

I'll advertise the cap anyway, I prefer to be able to handle it internally.
But I like this change, lowering SQRT (or not) is device specific and
shouldn't be done unconditionally just because the API can't represent it.

> glsl-to-tgsi code we use an extra CMP to check if x is zero and then
> replace the result of x*rsq(x) with zero.
>
> In the end, this makes sqrt() generate much more reasonable code for
> drivers that can do square roots.
>
> Note that many of piglit's generated shader tests use the GLSL
> distance() function.
> ---
>  src/gallium/docs/source/tgsi.rst           |    9 +++++++++
>  src/gallium/include/pipe/p_defines.h       |    3 ++-
>  src/gallium/include/pipe/p_shader_tokens.h |    2 +-
>  3 files changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/src/gallium/docs/source/tgsi.rst b/src/gallium/docs/source/tgsi.rst
> index 548a9a3..5f03f32 100644
> --- a/src/gallium/docs/source/tgsi.rst
> +++ b/src/gallium/docs/source/tgsi.rst
> @@ -89,6 +89,15 @@ This instruction replicates its result.
>    dst = \frac{1}{\sqrt{|src.x|}}
>  
>  
> +.. opcode:: SQRT - Square Root
> +
> +This instruction replicates its result.
> +
> +.. math::
> +
> +  dst = {\sqrt{src.x}}
> +
> +
>  .. opcode:: EXP - Approximate Exponential Base 2
>  
>  .. math::
> diff --git a/src/gallium/include/pipe/p_defines.h b/src/gallium/include/pipe/p_defines.h
> index d0db5e4..fdf6e7f 100644
> --- a/src/gallium/include/pipe/p_defines.h
> +++ b/src/gallium/include/pipe/p_defines.h
> @@ -542,7 +542,8 @@ enum pipe_shader_cap
>     PIPE_SHADER_CAP_SUBROUTINES = 16, /* BGNSUB, ENDSUB, CAL, RET */
>     PIPE_SHADER_CAP_INTEGERS = 17,
>     PIPE_SHADER_CAP_MAX_TEXTURE_SAMPLERS = 18,
> -   PIPE_SHADER_CAP_PREFERRED_IR = 19
> +   PIPE_SHADER_CAP_PREFERRED_IR = 19,
> +   PIPE_SHADER_CAP_TGSI_SQRT_SUPPORTED = 20
>  };
>  
>  /**
> diff --git a/src/gallium/include/pipe/p_shader_tokens.h b/src/gallium/include/pipe/p_shader_tokens.h
> index 3fb12fb..a9fb6aa 100644
> --- a/src/gallium/include/pipe/p_shader_tokens.h
> +++ b/src/gallium/include/pipe/p_shader_tokens.h
> @@ -275,7 +275,7 @@ struct tgsi_property_data {
>  #define TGSI_OPCODE_SUB                 17
>  #define TGSI_OPCODE_LRP                 18
>  #define TGSI_OPCODE_CND                 19
> -                                /* gap */
> +#define TGSI_OPCODE_SQRT                20
>  #define TGSI_OPCODE_DP2A                21
>                                  /* gap */
>  #define TGSI_OPCODE_FRC                 24