nir: Rewrite lower_undef_to_zero

This both fixes its metadata handling (it was flagging dirty regardless
of progress) and reduces the entire pass to 21 LOC including whitespace
by making better use of helpers.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7565>

src/compiler/nir/nir_lower_undef_to_zero.c

@@ -40,45 +40,24 @@
 
 #include "nir_builder.h"
 
+static bool
+lower_undef_instr_to_zero(nir_builder *b, nir_instr *instr, UNUSED void *_state)
+{
+   if (instr->type != nir_instr_type_ssa_undef)
+      return false;
+
+   nir_ssa_undef_instr *und = nir_instr_as_ssa_undef(instr);
+   b->cursor = nir_instr_remove(&und->instr);
+   nir_ssa_def *zero = nir_imm_zero(b, und->def.num_components,
+                                       und->def.bit_size);
+   nir_ssa_def_rewrite_uses(&und->def, nir_src_for_ssa(zero));
+   return true;
+}
+
 bool
 nir_lower_undef_to_zero(nir_shader *shader)
 {
-   bool progress = false;
-
-   nir_foreach_function(function, shader) {
-      if (!function->impl) continue;
-
-      nir_builder b;
-      nir_builder_init(&b, function->impl);
-
-      nir_foreach_block(block, function->impl) {
-         nir_foreach_instr_safe(instr, block) {
-            if (instr->type != nir_instr_type_ssa_undef) continue;
-
-            nir_ssa_undef_instr *und = nir_instr_as_ssa_undef(instr);
-
-            /* Get the required size */
-            unsigned c = und->def.num_components;
-            unsigned s = und->def.bit_size;
-
-            nir_const_value v[NIR_MAX_VEC_COMPONENTS];
-            memset(v, 0, sizeof(v));
-
-            b.cursor = nir_before_instr(instr);
-            nir_ssa_def *zero = nir_build_imm(&b, c, s, v);
-            nir_src zerosrc = nir_src_for_ssa(zero);
-
-            nir_ssa_def_rewrite_uses(&und->def, zerosrc);
-
-            progress |= true;
-         }
-      }
-
-      nir_metadata_preserve(function->impl, nir_metadata_block_index | nir_metadata_dominance);
-
-   }
-
-   return progress;
+   return nir_shader_instructions_pass(shader, lower_undef_instr_to_zero,
+                                       nir_metadata_block_index |
+                                       nir_metadata_dominance, NULL);
 }
-
-