[Mesa-dev] [PATCH 1/2] nir: Add is_divergent_vector search helper

[Ilia Mirkin]

Divergence is generally when multiple parallel "lanes" go in different
directions -- a jump that some lanes take and others don't, which
requires the GPU to execute some lanes first, and then the rest,
separately.

IMO better names might be is_scalar_swizzle or something.

On Mon, May 6, 2019 at 11:00 PM Alyssa Rosenzweig <alyssa at rosenzweig.io> wrote:
>
> This allows algebraic optimizations to check if the argument accesses
> multiple distinct components of a vector. So a swizzle like "xyz" will
> return true, but "yyy" will return false, as will a scalar. This can be
> useful for optimizations on vector processors, where a convergent
> swizzle can be done in one clock (replicating as if a scalar) but a
> divergent one must be scalarized. In these cases, it is useful to
> optimize differently based on whether the swizzle diverges. (Use case is
> the "csel" condition on Midgard).
>
> Signed-off-by: Alyssa Rosenzweig <alyssa at rosenzweig.io>
> Cc: Jason Ekstrand <jason at jlekstrand.net>
> ---
>  src/compiler/nir/nir_search_helpers.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/src/compiler/nir/nir_search_helpers.h b/src/compiler/nir/nir_search_helpers.h
> index 1624508993d..46d7c300643 100644
> --- a/src/compiler/nir/nir_search_helpers.h
> +++ b/src/compiler/nir/nir_search_helpers.h
> @@ -143,6 +143,22 @@ is_not_const(nir_alu_instr *instr, unsigned src, UNUSED unsigned num_components,
>     return !nir_src_is_const(instr->src[src].src);
>  }
>
> +/* I.e. a vector that actually accesses multiple channels */
> +
> +static inline bool
> +is_divergent_vector(nir_alu_instr *instr, UNUSED unsigned src, unsigned num_components,
> +             const uint8_t *swizzle)
> +{
> +   unsigned first_component = swizzle[0];
> +
> +   for (unsigned i = 1; i < num_components; ++i) {
> +      if (swizzle[i] != first_component)
> +         return true;
> +   }

Can num_components be 1? If so, then this will return false, whereas
you probably wanted it to return true.

> +
> +   return false;
> +}
> +
>  static inline bool
>  is_used_more_than_once(nir_alu_instr *instr)
>  {
> --
> 2.20.1
>
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