spirv: Don't remove variables used by resource indexing intrinsics

In Vulkan, for some variable modes, the generated NIR will have derefs
pointing to resource index intrinsics instead of the variable.  This
was letting nir_remove_dead_variables pass remove those variables,
which would lose information relevant for later passes after
spirv2nir.

Add a set to keep track of such variables and prevent them to be
removed when producing the NIR output.

Issue reported by Rhys.

Fixes: c4c9c780b1 ("spirv: Remove more dead variables")
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8706>

src/compiler/spirv/spirv_to_nir.c

@@ -5721,6 +5721,9 @@ vtn_create_builder(const uint32_t *words, size_t word_count,
    b->value_id_bound = value_id_bound;
    b->values = rzalloc_array(b, struct vtn_value, value_id_bound);
 
+   if (b->options->environment == NIR_SPIRV_VULKAN)
+      b->vars_used_indirectly = _mesa_pointer_set_create(b);
+
    return b;
  fail:
    ralloc_free(b);
@@ -5798,6 +5801,13 @@ vtn_emit_kernel_entry_point_wrapper(struct vtn_builder *b,
    return main_entry_point;
 }
 
+static bool
+can_remove(nir_variable *var, void *data)
+{
+   const struct set *vars_used_indirectly = data;
+   return !_mesa_set_search(vars_used_indirectly, var);
+}
+
 nir_shader *
 spirv_to_nir(const uint32_t *words, size_t word_count,
              struct nir_spirv_specialization *spec, unsigned num_spec,
@@ -5946,7 +5956,12 @@ spirv_to_nir(const uint32_t *words, size_t word_count,
     */
    nir_lower_variable_initializers(b->shader, nir_var_shader_out |
                                               nir_var_system_value);
-   nir_remove_dead_variables(b->shader, ~nir_var_function_temp, NULL);
+   const nir_remove_dead_variables_options dead_opts = {
+      .can_remove_var = can_remove,
+      .can_remove_var_data = b->vars_used_indirectly,
+   };
+   nir_remove_dead_variables(b->shader, ~nir_var_function_temp,
+                             b->vars_used_indirectly ? &dead_opts : NULL);
 
    /* We sometimes generate bogus derefs that, while never used, give the
     * validator a bit of heartburn.  Run dead code to get rid of them.

src/compiler/spirv/vtn_private.h

@@ -674,6 +674,13 @@ struct vtn_builder {
     */
    struct hash_table *phi_table;
 
+   /* In Vulkan, when lowering some modes variable access, the derefs of the
+    * variables are replaced with a resource index intrinsics, leaving the
+    * variable hanging.  This set keeps track of them so they can be filtered
+    * (and not removed) in nir_remove_dead_variables.
+    */
+   struct set *vars_used_indirectly;
+
    unsigned num_specializations;
    struct nir_spirv_specialization *specializations;
 

src/compiler/spirv/vtn_variables.c

@@ -237,6 +237,11 @@ vtn_variable_resource_index(struct vtn_builder *b, struct vtn_variable *var,
       desc_array_index = nir_imm_int(&b->nb, 0);
    }
 
+   if (b->vars_used_indirectly) {
+      vtn_assert(var->var);
+      _mesa_set_add(b->vars_used_indirectly, var->var);
+   }
+
    nir_intrinsic_instr *instr =
       nir_intrinsic_instr_create(b->nb.shader,
                                  nir_intrinsic_vulkan_resource_index);