radeonsi: implement nir_opt_frag_depth using kill_z instead of the NIR pass

This uses si_shader_info to store whether gl_FragDepth can be removed,
and it uses the kill_z epilog flag to do the removal without recompilation.

Reviewed-by: Qiang Yu <yuq825@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32713>

src/compiler/nir/nir.h

@@ -6890,6 +6890,7 @@ bool nir_opt_deref(nir_shader *shader);
 
 bool nir_opt_find_array_copies(nir_shader *shader);
 
+bool nir_def_is_frag_coord_z(nir_def *def);
 bool nir_opt_fragdepth(nir_shader *shader);
 
 bool nir_opt_gcm(nir_shader *shader, bool value_number);

src/compiler/nir/nir_opt_fragdepth.c

@@ -31,8 +31,8 @@
  * this value to gl_FragDepth, the store instruction is removed.
  */
 
-static bool
-ssa_def_is_source_depth(nir_def *def)
+bool
+nir_def_is_frag_coord_z(nir_def *def)
 {
    nir_scalar scalar = nir_scalar_resolved(def, 0);
    nir_instr *instr = scalar.def->parent_instr;
@@ -93,7 +93,7 @@ nir_opt_fragdepth(nir_shader *shader)
             /* This isn't the only write: give up on this optimization */
             goto end;
          } else {
-            if (ssa_def_is_source_depth(intrin->src[data_src].ssa)) {
+            if (nir_def_is_frag_coord_z(intrin->src[data_src].ssa)) {
                /* We're writing gl_FragCoord.z in gl_FragDepth: remember
                 * intrin so we can try to remove it later. */
                store_intrin = intrin;

src/gallium/drivers/radeonsi/si_shader.h

@@ -517,6 +517,9 @@ struct si_shader_info {
    bool reads_samplemask;   /**< does fragment shader read sample mask? */
    bool reads_tess_factors; /**< If TES reads TESSINNER or TESSOUTER */
    bool writes_z;           /**< does fragment shader write Z value? */
+   /* We need both because both can be present in different conditional blocks. */
+   bool output_z_equals_input_z; /**< gl_FragDepth == gl_FragCoord.z for any write */
+   bool output_z_is_not_input_z; /**< gl_FragDepth != gl_FragCoord.z for any write */
    bool writes_stencil;     /**< does fragment shader write stencil value? */
    bool writes_samplemask;  /**< does fragment shader write sample mask? */
    bool writes_edgeflag;    /**< vertex shader outputs edgeflag */

src/gallium/drivers/radeonsi/si_shader_info.c

@@ -279,6 +279,13 @@ static void scan_io_usage(const nir_shader *nir, struct si_shader_info *info,
          }
       }
    }
+
+   if (nir->info.stage == MESA_SHADER_FRAGMENT && !is_input && semantic == FRAG_RESULT_DEPTH) {
+      if (nir_def_is_frag_coord_z(intr->src[0].ssa))
+         info->output_z_equals_input_z = true;
+      else
+         info->output_z_is_not_input_z = true;
+   }
 }
 
 static bool is_bindless_handle_indirect(nir_instr *src)
@@ -649,6 +656,7 @@ void si_nir_scan_shader(struct si_screen *sscreen, const struct nir_shader *nir,
    }
 
    if (nir->info.stage == MESA_SHADER_FRAGMENT) {
+      info->output_z_equals_input_z &= !info->output_z_is_not_input_z;
       info->allow_flat_shading = !(info->uses_persp_center || info->uses_persp_centroid ||
                                    info->uses_persp_sample || info->uses_linear_center ||
                                    info->uses_linear_centroid || info->uses_linear_sample ||

src/gallium/drivers/radeonsi/si_state_shaders.cpp

@@ -2601,7 +2601,8 @@ void si_ps_key_update_framebuffer_blend_dsa_rasterizer(struct si_context *sctx)
 #endif
 
    key->ps.part.epilog.kill_z = sel->info.writes_z &&
-                                (!sctx->framebuffer.state.zsbuf || !dsa->depth_enabled);
+                                (!sctx->framebuffer.state.zsbuf || !dsa->depth_enabled ||
+                                 (sel->info.output_z_equals_input_z && !rs->multisample_enable));
    key->ps.part.epilog.kill_stencil = sel->info.writes_stencil &&
                                       (!sctx->framebuffer.has_stencil || !dsa->stencil_enabled);