[Mesa-dev] [PATCH] spirv: Emit switch conditions on-the-fly

[Lionel Landwerlin]

On 12/01/2019 03:45, Jason Ekstrand wrote:
> Instead of emitting all of the conditions for the cases of a switch
> statement up-front, emit them on-the-fly as we emit the code for each
> case.  The original justification for this was that we were going to
> have to build a default case anyway which would need them all.  However,
> we can just trust CSE to clean up the mess in that case.  Emitting each
> condition right before the if statement that uses it reduces register
> pressure and, in one customer benchmark, gets rid of spilling and
> improves performance by about 2x.
> ---
>   src/compiler/spirv/vtn_cfg.c | 62 +++++++++++++++---------------------
>   1 file changed, 26 insertions(+), 36 deletions(-)
>
> diff --git a/src/compiler/spirv/vtn_cfg.c b/src/compiler/spirv/vtn_cfg.c
> index aef1b7e18fb..34a910accc6 100644
> --- a/src/compiler/spirv/vtn_cfg.c
> +++ b/src/compiler/spirv/vtn_cfg.c
> @@ -852,6 +852,30 @@ vtn_emit_branch(struct vtn_builder *b, enum vtn_branch_type branch_type,
>      }
>   }
>   
> +static nir_ssa_def *
> +vtn_switch_case_condition(struct vtn_builder *b, struct vtn_switch *swtch,
> +                          nir_ssa_def *sel, struct vtn_case *cse)
> +{
> +   if (cse->is_default) {
> +      nir_ssa_def *any = nir_imm_false(&b->nb);
> +      list_for_each_entry(struct vtn_case, other, &swtch->cases, link) {
> +         if (cse->is_default)
> +            continue;
> +
> +         any = nir_ior(&b->nb, any,
> +                       vtn_switch_case_condition(b, swtch, sel, other));
> +      }
> +      return any;


return nir_inot(&b->nb, any); here?


> +   } else {
> +      nir_ssa_def *cond = nir_imm_false(&b->nb);
> +      util_dynarray_foreach(&cse->values, uint64_t, val) {
> +         nir_ssa_def *imm = nir_imm_intN_t(&b->nb, *val, sel->bit_size);
> +         cond = nir_ior(&b->nb, cond, nir_ieq(&b->nb, sel, imm));
> +      }
> +      return cond;
> +   }
> +}
> +
>   static void
>   vtn_emit_cf_list(struct vtn_builder *b, struct list_head *cf_list,
>                    nir_variable *switch_fall_var, bool *has_switch_break,
> @@ -978,46 +1002,13 @@ vtn_emit_cf_list(struct vtn_builder *b, struct list_head *cf_list,
>               nir_local_variable_create(b->nb.impl, glsl_bool_type(), "fall");
>            nir_store_var(&b->nb, fall_var, nir_imm_false(&b->nb), 1);
>   
> -         /* Next, we gather up all of the conditions.  We have to do this
> -          * up-front because we also need to build an "any" condition so
> -          * that we can use !any for default.
> -          */
> -         const int num_cases = list_length(&vtn_switch->cases);
> -         NIR_VLA(nir_ssa_def *, conditions, num_cases);
> -
>            nir_ssa_def *sel = vtn_ssa_value(b, vtn_switch->selector)->def;
> -         /* An accumulation of all conditions.  Used for the default */
> -         nir_ssa_def *any = NULL;
> -
> -         int i = 0;
> -         list_for_each_entry(struct vtn_case, cse, &vtn_switch->cases, link) {
> -            if (cse->is_default) {
> -               conditions[i++] = NULL;
> -               continue;
> -            }
> -
> -            nir_ssa_def *cond = NULL;
> -            util_dynarray_foreach(&cse->values, uint64_t, val) {
> -               nir_ssa_def *imm = nir_imm_intN_t(&b->nb, *val, sel->bit_size);
> -               nir_ssa_def *is_val = nir_ieq(&b->nb, sel, imm);
> -
> -               cond = cond ? nir_ior(&b->nb, cond, is_val) : is_val;
> -            }
> -
> -            any = any ? nir_ior(&b->nb, any, cond) : cond;
> -            conditions[i++] = cond;
> -         }
> -         vtn_assert(i == num_cases);
>   
>            /* Now we can walk the list of cases and actually emit code */
> -         i = 0;
>            list_for_each_entry(struct vtn_case, cse, &vtn_switch->cases, link) {
>               /* Figure out the condition */
> -            nir_ssa_def *cond = conditions[i++];
> -            if (cse->is_default) {
> -               vtn_assert(cond == NULL);
> -               cond = nir_inot(&b->nb, any);
> -            }
> +            nir_ssa_def *cond =
> +               vtn_switch_case_condition(b, vtn_switch, sel, cse);
>               /* Take fallthrough into account */
>               cond = nir_ior(&b->nb, cond, nir_load_var(&b->nb, fall_var));
>   
> @@ -1030,7 +1021,6 @@ vtn_emit_cf_list(struct vtn_builder *b, struct list_head *cf_list,
>   
>               nir_pop_if(&b->nb, case_if);
>            }
> -         vtn_assert(i == num_cases);
>   
>            break;
>         }