Mesa (main): ac/nir: Reuse the repacked output positions of culling shaders.

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Module: Mesa
Branch: main
Commit: faf766b864d089c0eafc00018174f1732da4776e
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=faf766b864d089c0eafc00018174f1732da4776e

Author: Timur Kristóf <timur.kristof at gmail.com>
Date:   Tue May  4 11:36:02 2021 +0200

ac/nir: Reuse the repacked output positions of culling shaders.

The position outputs are stored into LDS and reloaded after
repacking, therefore the repacked position values can be
reused in the bottom part of the shader.

Fossil DB results on Sienna Cichlid (with NGG culling on):

Totals from 9016 (7.01% of 128647) affected shaders:
VGPRs: 372472 -> 347560 (-6.69%); split: -6.82%, +0.13%
SpillSGPRs: 437 -> 87 (-80.09%)
CodeSize: 32359340 -> 30441692 (-5.93%); split: -5.93%, +0.00%
MaxWaves: 222030 -> 238970 (+7.63%); split: +7.83%, -0.20%
Instrs: 6207833 -> 5834149 (-6.02%); split: -6.02%, +0.00%
Latency: 27626263 -> 27890632 (+0.96%); split: -5.34%, +6.29%
InvThroughput: 4792958 -> 4361336 (-9.01%); split: -9.01%, +0.00%
VClause: 144385 -> 139586 (-3.32%); split: -9.29%, +5.97%
SClause: 141350 -> 129875 (-8.12%); split: -8.57%, +0.45%
Copies: 580017 -> 568916 (-1.91%); split: -3.60%, +1.68%
Branches: 209067 -> 209154 (+0.04%); split: -0.24%, +0.28%
PreSGPRs: 281320 -> 277814 (-1.25%)
PreVGPRs: 290040 -> 273861 (-5.58%)

Signed-off-by: Timur Kristóf <timur.kristof at gmail.com>
Reviewed-by: Daniel Schürmann <daniel at schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10525>

---

 src/amd/common/ac_nir_lower_ngg.c | 67 ++++++++++++++++++++++++++++++++++++++-
 1 file changed, 66 insertions(+), 1 deletion(-)

diff --git a/src/amd/common/ac_nir_lower_ngg.c b/src/amd/common/ac_nir_lower_ngg.c
index 90dba07bb03..d21e9cb099c 100644
--- a/src/amd/common/ac_nir_lower_ngg.c
+++ b/src/amd/common/ac_nir_lower_ngg.c
@@ -409,6 +409,33 @@ remove_culling_shader_outputs(nir_shader *culling_shader, lower_ngg_nogs_state *
    } while (progress);
 }
 
+static void
+rewrite_uses_to_var(nir_builder *b, nir_ssa_def *old_def, nir_variable *replacement_var, unsigned replacement_var_channel)
+{
+   if (old_def->parent_instr->type == nir_instr_type_load_const)
+      return;
+
+   b->cursor = nir_after_instr(old_def->parent_instr);
+   if (b->cursor.instr->type == nir_instr_type_phi)
+      b->cursor = nir_after_phis(old_def->parent_instr->block);
+
+   nir_ssa_def *pos_val_rep = nir_load_var(b, replacement_var);
+   nir_ssa_def *replacement = nir_channel(b, pos_val_rep, replacement_var_channel);
+
+   if (old_def->num_components > 1) {
+      /* old_def uses a swizzled vector component.
+       * There is no way to replace the uses of just a single vector component,
+       * so instead create a new vector and replace all uses of the old vector.
+       */
+      nir_ssa_def *old_def_elements[NIR_MAX_VEC_COMPONENTS] = {0};
+      for (unsigned j = 0; j < old_def->num_components; ++j)
+         old_def_elements[j] = nir_channel(b, old_def, j);
+      replacement = nir_vec(b, old_def_elements, old_def->num_components);
+   }
+
+   nir_ssa_def_rewrite_uses_after(old_def, replacement, replacement->parent_instr);
+}
+
 static bool
 remove_extra_pos_output(nir_builder *b, nir_instr *instr, void *state)
 {
@@ -433,9 +460,47 @@ remove_extra_pos_output(nir_builder *b, nir_instr *instr, void *state)
 
    b->cursor = nir_before_instr(instr);
 
-   /* TODO: in case other outputs use what we calculated for pos, rewrite the usages of the store components here */
+   /* In case other outputs use what we calculated for pos,
+    * try to avoid calculating it again by rewriting the usages
+    * of the store components here.
+    */
+   nir_ssa_def *store_val = intrin->src[0].ssa;
+   unsigned store_pos_component = nir_intrinsic_component(intrin);
 
    nir_instr_remove(instr);
+
+   if (store_val->parent_instr->type == nir_instr_type_alu) {
+      nir_alu_instr *alu = nir_instr_as_alu(store_val->parent_instr);
+      if (nir_op_is_vec(alu->op)) {
+         /* Output store uses a vector, we can easily rewrite uses of each vector element. */
+
+         unsigned num_vec_src = 0;
+         if (alu->op == nir_op_mov)
+            num_vec_src = 1;
+         else if (alu->op == nir_op_vec2)
+            num_vec_src = 2;
+         else if (alu->op == nir_op_vec3)
+            num_vec_src = 3;
+         else if (alu->op == nir_op_vec4)
+            num_vec_src = 4;
+         assert(num_vec_src);
+
+         /* Remember the current components whose uses we wish to replace.
+          * This is needed because rewriting one source can affect the others too.
+          */
+         nir_ssa_def *vec_comps[NIR_MAX_VEC_COMPONENTS] = {0};
+         for (unsigned i = 0; i < num_vec_src; i++)
+            vec_comps[i] = alu->src[i].src.ssa;
+
+         for (unsigned i = 0; i < num_vec_src; i++)
+            rewrite_uses_to_var(b, vec_comps[i], s->pos_value_replacement, store_pos_component + i);
+      } else {
+         rewrite_uses_to_var(b, store_val, s->pos_value_replacement, store_pos_component);
+      }
+   } else {
+      rewrite_uses_to_var(b, store_val, s->pos_value_replacement, store_pos_component);
+   }
+
    return true;
 }