[Mesa-dev] [PATCH] radeonsi: compute the absolute value before RSQ

[Marek Olšák]

Shouldn't we also use abs for SQRT? For example, this adds abs for
both RSQ and SQRT:

https://cgit.freedesktop.org/~mareko/mesa/commit/?id=5e0fb661a8e6ac5f7b2245dd31595155128e0664

Marek

On Thu, Jan 5, 2017 at 5:47 PM, Samuel Pitoiset
<samuel.pitoiset at gmail.com> wrote:
> As explained by Nicolai, it seems like D3D always compute the
> absolute value while GLSL says that the result of inversesqrt()
> is undefined if x <= 0. Using the absolute value looks like safer
> especially when the game has been ported from D3D to GL.
>
> This gets rid of the NaN values in the "Spec Ops: The Line" game
> as well as the black squares.
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97338
>
> Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
> ---
>
> Nouveau also computes the absolute value before emitting RSQ.
>
>  src/gallium/drivers/radeonsi/si_shader_tgsi_alu.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/src/gallium/drivers/radeonsi/si_shader_tgsi_alu.c b/src/gallium/drivers/radeonsi/si_shader_tgsi_alu.c
> index 1966752cc0..ec6d6b0534 100644
> --- a/src/gallium/drivers/radeonsi/si_shader_tgsi_alu.c
> +++ b/src/gallium/drivers/radeonsi/si_shader_tgsi_alu.c
> @@ -754,9 +754,12 @@ static void emit_rsq(const struct lp_build_tgsi_action *action,
>                      struct lp_build_tgsi_context *bld_base,
>                      struct lp_build_emit_data *emit_data)
>  {
> -       LLVMValueRef sqrt =
> -               lp_build_emit_llvm_unary(bld_base, TGSI_OPCODE_SQRT,
> -                                        emit_data->args[0]);
> +       LLVMValueRef abs, sqrt;
> +
> +       abs = lp_build_emit_llvm_unary(bld_base, TGSI_OPCODE_ABS,
> +                                      emit_data->args[0]);
> +
> +       sqrt = lp_build_emit_llvm_unary(bld_base, TGSI_OPCODE_SQRT, abs);
>
>         emit_data->output[emit_data->chan] =
>                 lp_build_emit_llvm_binary(bld_base, TGSI_OPCODE_DIV,
> --
> 2.11.0
>
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