nir/opt_dce: add shader_info::assert_inputs_not_dead

Reviewed-by: Georg Lehmann <dadschoorse@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41166>

src/compiler/nir/nir_opt_dce.c

@@ -114,6 +114,17 @@ struct loop_state {
 static void
 remove_instr(nir_instr *instr, struct exec_list *dead_instrs)
 {
+#ifndef NDEBUG
+   /* Fail an assertion if an input load is dead. This is a debug option. */
+   if (instr->type == nir_instr_type_intrinsic) {
+      nir_shader *nir =
+         nir_cf_node_get_function(&instr->block->cf_node)->function->shader;
+
+      if (nir->info.assert_inputs_not_dead)
+         assert(!nir_is_input_load(nir_instr_as_intrinsic(instr)));
+   }
+#endif
+
    nir_instr_remove(instr);
    exec_list_push_tail(dead_instrs, &instr->node);
 }

src/compiler/shader_info.h

@@ -258,6 +258,17 @@ typedef struct shader_info {
     */
    bool io_lowered:1;
 
+   /* If true, fail an assertion in nir_opt_dce if a dead input is eliminated.
+    *
+    * This is a debug aid to easily identify passes that cause shader inputs
+    * to become dead after nir_opt_varyings when it's preferrable that dead
+    * shader inputs are identified before nir_opt_varyings.
+    *
+    * It shouldn't be enabled by default because inputs can become dead late
+    * for all sorts of reasons.
+    */
+   bool assert_inputs_not_dead : 1;
+
    /** Has nir_lower_var_copies called. To avoid calling any
     * lowering/optimization that would introduce any copy_deref later.
     */