nir/deref: Support casts and ptr_as_array in comparisons

The code which constructs deref paths already gives you the path
starting at the nearest deref_cast or deref_var.  All we need to do for
casts is handle the case where the start of the path isn't a deref_var.
For ptr_as_array derefs, we just bail if we have any after the
divergence point between the two derefs.  We may be able to do better in
the future but this works for now.

Reviewed-by: Alejandro Piñeiro <[email protected]>
Reviewed-by: Caio Marcelo de Oliveira Filho <[email protected]>

1 files changed, 68 insertions, 7 deletions

diff --git a/src/compiler/nir/nir_deref.c b/src/compiler/nir/nir_deref.c
index 93f6f90234b..edd602a8f4a 100644
--- a/src/compiler/nir/nir_deref.c
+++ b/src/compiler/nir/nir_deref.c
@@ -70,7 +70,6 @@ nir_deref_path_init(nir_deref_path *path,
 done:
    assert(head == path->path);
    assert(tail == head + count);
-   assert((*head)->deref_type == nir_deref_type_var);
    assert(*tail == NULL);
 }
 
@@ -290,13 +289,51 @@ nir_fixup_deref_modes(nir_shader *shader)
    }
 }
 
+static bool
+modes_may_alias(nir_variable_mode a, nir_variable_mode b)
+{
+   /* Two pointers can only alias if they have the same mode.
+    *
+    * NOTE: In future, with things like OpenCL generic pointers, this may not
+    * be true and will have to be re-evaluated.  However, with graphics only,
+    * it should be safe.
+    */
+   return a == b;
+}
+
 nir_deref_compare_result
 nir_compare_deref_paths(nir_deref_path *a_path,
                         nir_deref_path *b_path)
 {
-   if (a_path->path[0]->var != b_path->path[0]->var)
+   if (!modes_may_alias(b_path->path[0]->mode, a_path->path[0]->mode))
       return nir_derefs_do_not_alias;
 
+   if (a_path->path[0]->deref_type != b_path->path[0]->deref_type)
+      return nir_derefs_may_alias_bit;
+
+   if (a_path->path[0]->deref_type == nir_deref_type_var) {
+      /* If we can chase the deref all the way back to the variable and
+       * they're not the same variable, we know they can't possibly alias.
+       */
+      if (a_path->path[0]->var != b_path->path[0]->var)
+         return nir_derefs_do_not_alias;
+   } else {
+      assert(a_path->path[0]->deref_type == nir_deref_type_cast);
+      /* If they're not exactly the same cast, it's hard to compare them so we
+       * just assume they alias.  Comparing casts is tricky as there are lots
+       * of things such as mode, type, etc. to make sure work out; for now, we
+       * just assume nit_opt_deref will combine them and compare the deref
+       * instructions.
+       *
+       * TODO: At some point in the future, we could be clever and understand
+       * that a float[] and int[] have the same layout and aliasing structure
+       * but double[] and vec3[] do not and we could potentially be a bit
+       * smarter here.
+       */
+      if (a_path->path[0] != b_path->path[0])
+         return nir_derefs_may_alias_bit;
+   }
+
    /* Start off assuming they fully compare.  We ignore equality for now.  In
     * the end, we'll determine that by containment.
     */
@@ -306,13 +343,35 @@ nir_compare_deref_paths(nir_deref_path *a_path,
 
    nir_deref_instr **a_p = &a_path->path[1];
    nir_deref_instr **b_p = &b_path->path[1];
+   while (*a_p != NULL && *a_p == *b_p) {
+      a_p++;
+      b_p++;
+   }
+
+   /* We're at either the tail or the divergence point between the two deref
+    * paths.  Look to see if either contains a ptr_as_array deref.  It it
+    * does we don't know how to safely make any inferences.  Hopefully,
+    * nir_opt_deref will clean most of these up and we can start inferring
+    * things again.
+    *
+    * In theory, we could do a bit better.  For instance, we could detect the
+    * case where we have exactly one ptr_as_array deref in the chain after the
+    * divergence point and it's matched in both chains and the two chains have
+    * different constant indices.
+    */
+   for (nir_deref_instr **t_p = a_p; *t_p; t_p++) {
+      if ((*t_p)->deref_type == nir_deref_type_ptr_as_array)
+         return nir_derefs_may_alias_bit;
+   }
+   for (nir_deref_instr **t_p = b_p; *t_p; t_p++) {
+      if ((*t_p)->deref_type == nir_deref_type_ptr_as_array)
+         return nir_derefs_may_alias_bit;
+   }
+
    while (*a_p != NULL && *b_p != NULL) {
       nir_deref_instr *a_tail = *(a_p++);
       nir_deref_instr *b_tail = *(b_p++);
 
-      if (a_tail == b_tail)
-         continue;
-
       switch (a_tail->deref_type) {
       case nir_deref_type_array:
       case nir_deref_type_array_wildcard: {
@@ -386,8 +445,10 @@ nir_compare_derefs(nir_deref_instr *a, nir_deref_instr *b)
    nir_deref_path a_path, b_path;
    nir_deref_path_init(&a_path, a, NULL);
    nir_deref_path_init(&b_path, b, NULL);
-   assert(a_path.path[0]->deref_type == nir_deref_type_var);
-   assert(b_path.path[0]->deref_type == nir_deref_type_var);
+   assert(a_path.path[0]->deref_type == nir_deref_type_var ||
+          a_path.path[0]->deref_type == nir_deref_type_cast);
+   assert(b_path.path[0]->deref_type == nir_deref_type_var ||
+          b_path.path[0]->deref_type == nir_deref_type_cast);
 
    nir_deref_compare_result result = nir_compare_deref_paths(&a_path, &b_path);