nir: Add a deref path helper struct

This commit introduces a new nir_deref.h header for helpers that are
less common and really only needed by a few heavy-duty passes.  In this
header is a new struct for representing a full deref path which can be
walked in either direction.

v2 (Jason Ekstrand):
 - Assert that deref != NULL (Caio)
 - Fill _short_path with 0xdeadbeef in debug builds when not used (Caio)
 - Make nir_deref_path a typedef (Rob)

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/nir/nir_deref.c

@@ -23,6 +23,63 @@
 
 #include "nir.h"
 #include "nir_builder.h"
+#include "nir_deref.h"
+
+void
+nir_deref_path_init(nir_deref_path *path,
+                    nir_deref_instr *deref, void *mem_ctx)
+{
+   assert(deref != NULL);
+
+   /* The length of the short path is at most ARRAY_SIZE - 1 because we need
+    * room for the NULL terminator.
+    */
+   static const int max_short_path_len = ARRAY_SIZE(path->_short_path) - 1;
+
+   int count = 0;
+
+   nir_deref_instr **tail = &path->_short_path[max_short_path_len];
+   nir_deref_instr **head = tail;
+
+   *tail = NULL;
+   for (nir_deref_instr *d = deref; d; d = nir_deref_instr_parent(d)) {
+      count++;
+      if (count <= max_short_path_len)
+         *(--head) = d;
+   }
+
+   if (count <= max_short_path_len) {
+      /* If we're under max_short_path_len, just use the short path. */
+      path->path = head;
+      goto done;
+   }
+
+#ifndef NDEBUG
+   /* Just in case someone uses short_path by accident */
+   for (unsigned i = 0; i < ARRAY_SIZE(path->_short_path); i++)
+      path->_short_path[i] = (void *)0xdeadbeef;
+#endif
+
+   path->path = ralloc_array(mem_ctx, nir_deref_instr *, count + 1);
+   head = tail = path->path + count;
+   *tail = NULL;
+   for (nir_deref_instr *d = deref; d; d = nir_deref_instr_parent(d))
+      *(--head) = d;
+
+done:
+   assert(head == path->path);
+   assert(tail == head + count);
+   assert((*head)->deref_type == nir_deref_type_var);
+   assert(*tail == NULL);
+}
+
+void
+nir_deref_path_finish(nir_deref_path *path)
+{
+   if (path->path < &path->_short_path[0] ||
+       path->path > &path->_short_path[ARRAY_SIZE(path->_short_path) - 1])
+      ralloc_free(path->path);
+}
 
 /**
  * Recursively removes unused deref instructions