[Mesa-dev] [PATCH v3] nir: allow nir_lower_phis_to_scalar() on more src types

[Jason Ekstrand]

On Fri, Feb 22, 2019 at 5:21 PM Timothy Arceri <tarceri at itsqueeze.com>
wrote:

> Rather than only lowering if all srcs are scalarizable we instead
> check that at least one src is scalarizable.
>
> We remove undef type from the is_phi_src_scalarizable() as using
> this as a check just cause regressions when it is the only
> scalarizable src.
>
> total instructions in shared programs: 13219105 -> 13024547 (-1.47%)
> instructions in affected programs: 1153797 -> 959239 (-16.86%)
> helped: 581
> HURT: 74
>
> total cycles in shared programs: 333968972 -> 324807922 (-2.74%)
> cycles in affected programs: 129809402 -> 120648352 (-7.06%)
> helped: 571
> HURT: 131
>
> total spills in shared programs: 57947 -> 29130 (-49.73%)
> spills in affected programs: 53364 -> 24547 (-54.00%)
> helped: 351
> HURT: 0
>
> total fills in shared programs: 51310 -> 25468 (-50.36%)
> fills in affected programs: 44882 -> 19040 (-57.58%)
> helped: 351
> HURT: 0
> ---
>  src/compiler/nir/nir_lower_phis_to_scalar.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/src/compiler/nir/nir_lower_phis_to_scalar.c
> b/src/compiler/nir/nir_lower_phis_to_scalar.c
> index 16001f73685..b2ca89d3513 100644
> --- a/src/compiler/nir/nir_lower_phis_to_scalar.c
> +++ b/src/compiler/nir/nir_lower_phis_to_scalar.c
> @@ -75,7 +75,6 @@ is_phi_src_scalarizable(nir_phi_src *src,
>        return should_lower_phi(nir_instr_as_phi(src_instr), state);
>
>     case nir_instr_type_load_const:
> -   case nir_instr_type_ssa_undef:
>

Instead of removing it, would you mind handling it as

case nir_instr_type_ssa_undef:
    /* The caller of this function is going to OR the results and we don't
want undefs to count so we return false. */
   return false;

With that,

Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>


>        /* These are trivially scalarizable */
>        return true;
>
> @@ -150,11 +149,16 @@ should_lower_phi(nir_phi_instr *phi, struct
> lower_phis_to_scalar_state *state)
>      */
>     entry = _mesa_hash_table_insert(state->phi_table, phi, (void
> *)(intptr_t)1);
>
> -   bool scalarizable = true;
> +   bool scalarizable = false;
>
>     nir_foreach_phi_src(src, phi) {
> +      /* This loop ignores srcs that are not scalarizable because its
> likely
> +       * still worth copying to temps if another phi source is
> scalarizable.
> +       * This reduces register spilling by a huge amount in the i965
> driver for
> +       * Deus Ex: MD.
> +       */
>        scalarizable = is_phi_src_scalarizable(src, state);
> -      if (!scalarizable)
> +      if (scalarizable)
>           break;
>     }
>
> --
> 2.20.1
>
>
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