nir: Consider pointer initializers in nir_remove_dead_variables

Between the creation of a shader (from GLSL or SPIRV frontends) and
nir_lower_variable_initializers is called, variables may refer to
other variables for initialization.  Those referred variables need to
be kept alive, so consider that in the pass.

Fixes: 7acc81056f7 ("compiler/nir: Add support for variable initialization from a pointer")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8133>
(cherry picked from commit acce4ce04ec1b591b47afeda512a9b554dac14f3)

3 files changed, 63 insertions, 8 deletions

diff --git a/.pick_status.json b/.pick_status.json
index f3353e615d8..85c13b49fb5 100644
--- a/.pick_status.json
+++ b/.pick_status.json
@@ -31,7 +31,7 @@
         "description": "nir: Consider pointer initializers in nir_remove_dead_variables",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "master_sha": null,
         "because_sha": "7acc81056f7ac6a869ef6403573b2572c77dbecf"
     },
diff --git a/src/compiler/nir/nir_remove_dead_variables.c b/src/compiler/nir/nir_remove_dead_variables.c
index 01ea18cc37e..5555ca3f811 100644
--- a/src/compiler/nir/nir_remove_dead_variables.c
+++ b/src/compiler/nir/nir_remove_dead_variables.c
@@ -67,14 +67,21 @@ add_var_use_deref(nir_deref_instr *deref, struct set *live)
    if (deref->deref_type != nir_deref_type_var)
       return;
 
-   /* If it's not a local that never escapes the shader, then any access at
-    * all means we need to keep it alive.
+   /* Since these local variables don't escape the shader, writing doesn't
+    * make them live.  Only keep them if they are used by some intrinsic.
     */
-   if (!(deref->var->data.mode & (nir_var_function_temp |
-                                  nir_var_shader_temp |
-                                  nir_var_mem_shared)) ||
-       deref_used_for_not_store(deref))
-      _mesa_set_add(live, deref->var);
+   if ((deref->var->data.mode & (nir_var_function_temp |
+                                 nir_var_shader_temp |
+                                 nir_var_mem_shared)) &&
+       !deref_used_for_not_store(deref))
+      return;
+
+   nir_variable *var = deref->var;
+   do {
+      _mesa_set_add(live, var);
+      /* Also mark the chain of variables used to initialize it. */
+      var = var->pointer_initializer;
+   } while (var);
 }
 
 static void
diff --git a/src/compiler/nir/tests/vars_tests.cpp b/src/compiler/nir/tests/vars_tests.cpp
index 6d27830cd55..593f183ef9f 100644
--- a/src/compiler/nir/tests/vars_tests.cpp
+++ b/src/compiler/nir/tests/vars_tests.cpp
@@ -197,6 +197,7 @@ class nir_copy_prop_vars_test : public nir_vars_test {};
 class nir_dead_write_vars_test : public nir_vars_test {};
 class nir_combine_stores_test : public nir_vars_test {};
 class nir_split_vars_test : public nir_vars_test {};
+class nir_remove_dead_variables_test : public nir_vars_test {};
 
 } // namespace
 
@@ -2185,3 +2186,50 @@ TEST_F(nir_split_vars_test, split_wildcard_copy)
    ASSERT_EQ(count_function_temp_vars(), 8);
    ASSERT_EQ(count_intrinsics(nir_intrinsic_copy_deref), 4);
 }
+
+TEST_F(nir_remove_dead_variables_test, pointer_initializer_used)
+{
+   nir_variable *x = create_int(nir_var_shader_temp, "x");
+   nir_variable *y = create_int(nir_var_shader_temp, "y");
+   y->pointer_initializer = x;
+   nir_variable *out = create_int(nir_var_shader_out, "out");
+
+   nir_validate_shader(b->shader, NULL);
+
+   nir_copy_var(b, out, y);
+
+   bool progress = nir_remove_dead_variables(b->shader, nir_var_all, NULL);
+   EXPECT_FALSE(progress);
+
+   nir_validate_shader(b->shader, NULL);
+
+   unsigned count = 0;
+   nir_foreach_variable_in_shader(var, b->shader)
+      count++;
+
+   ASSERT_EQ(count, 3);
+}
+
+TEST_F(nir_remove_dead_variables_test, pointer_initializer_dead)
+{
+   nir_variable *x = create_int(nir_var_shader_temp, "x");
+   nir_variable *y = create_int(nir_var_shader_temp, "y");
+   nir_variable *z = create_int(nir_var_shader_temp, "z");
+   y->pointer_initializer = x;
+   z->pointer_initializer = y;
+
+   nir_validate_shader(b->shader, NULL);
+
+   bool progress = nir_remove_dead_variables(b->shader, nir_var_all, NULL);
+   EXPECT_TRUE(progress);
+
+   nir_validate_shader(b->shader, NULL);
+
+   unsigned count = 0;
+   nir_foreach_variable_in_shader(var, b->shader)
+      count++;
+
+   ASSERT_EQ(count, 0);
+}
+
+