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/Makefile.sources

@@ -208,6 +208,7 @@ NIR_FILES = \
 	nir/nir_control_flow.h \
 	nir/nir_control_flow_private.h \
 	nir/nir_deref.c \
+	nir/nir_deref.h \
 	nir/nir_dominance.c \
 	nir/nir_format_convert.h \
 	nir/nir_from_ssa.c \

src/compiler/nir/meson.build

@@ -93,6 +93,7 @@ files_libnir = files(
   'nir_control_flow.h',
   'nir_control_flow_private.h',
   'nir_deref.c',
+  'nir_deref.h',
   'nir_dominance.c',
   'nir_format_convert.h',
   'nir_from_ssa.c',

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

src/compiler/nir/nir_deref.h

@@ -0,0 +1,55 @@
+/*
+ * Copyright © 2018 Intel Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+#ifndef NIR_DEREF_H
+#define NIR_DEREF_H
+
+#include "nir.h"
+#include "nir_builder.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct {
+   /** Short path so we can keep it on the stack most of the time. */
+   nir_deref_instr *_short_path[7];
+
+   /** A null-terminated array view of a deref chain
+    *
+    * The first element of this array will be the variable dereference
+    * followed by every deref_instr on the path to the final one.  The last
+    * element in the array is a NULL pointer which acts as a terminator.
+    */
+   nir_deref_instr **path;
+} nir_deref_path;
+
+void nir_deref_path_init(nir_deref_path *path,
+                         nir_deref_instr *deref, void *mem_ctx);
+void nir_deref_path_finish(nir_deref_path *path);
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#endif /* NIR_DEREF_H */