[Mesa-dev] [PATCH] i965/nir: do int64 lowering before optimization

[Iago Toral]

This didn't get any reviews yet. Any takers?

On Fri, 2017-12-01 at 13:46 +0100, Iago Toral Quiroga wrote:
> Otherwise loop unrolling will fail to see the actual cost of
> the unrolling operations when the loop body contains 64-bit integer
> instructions, and very specially when the divmod64 lowering applies,
> since its lowering is quite expensive.
> 
> Without this change, some in-development CTS tests for int64
> get stuck forever trying to register allocate a shader with
> over 50K SSA values. The large number of SSA values is the result
> of NIR first unrolling multiple seemingly simple loops that involve
> int64 instructions, only to then lower these instructions to produce
> a massive pile of code (due to the divmod64 lowering in the unrolled
> instructions).
> 
> With this change, loop unrolling will see the loops with the int64
> code already lowered and will realize that it is too expensive to
> unroll.
> ---
>  src/intel/compiler/brw_nir.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/src/intel/compiler/brw_nir.c
> b/src/intel/compiler/brw_nir.c
> index 8f3f77f89a..ef12cdfff8 100644
> --- a/src/intel/compiler/brw_nir.c
> +++ b/src/intel/compiler/brw_nir.c
> @@ -636,6 +636,10 @@ brw_preprocess_nir(const struct brw_compiler
> *compiler, nir_shader *nir)
>  
>     OPT(nir_split_var_copies);
>  
> +   nir_lower_int64(nir, nir_lower_imul64 |
> +                        nir_lower_isign64 |
> +                        nir_lower_divmod64);
> +
>     nir = brw_nir_optimize(nir, compiler, is_scalar);
>  
>     if (is_scalar) {
> @@ -663,10 +667,6 @@ brw_preprocess_nir(const struct brw_compiler
> *compiler, nir_shader *nir)
>        brw_nir_no_indirect_mask(compiler, nir->info.stage);
>     nir_lower_indirect_derefs(nir, indirect_mask);
>  
> -   nir_lower_int64(nir, nir_lower_imul64 |
> -                        nir_lower_isign64 |
> -                        nir_lower_divmod64);
> -
>     /* Get rid of split copies */
>     nir = brw_nir_optimize(nir, compiler, is_scalar);
>