nir/cf: have nir_remove_after_cf_node remove phis at the start too

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Backport-to: 25.1
Reviewed-by: Georg Lehmann <dadschoorse@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35975>
(cherry picked from commit f45026751f)

.pick_status.json

@@ -3204,7 +3204,7 @@
         "description": "nir/cf: have nir_remove_after_cf_node remove phis at the start too",
         "nominated": true,
         "nomination_type": 4,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null,
         "notes": null

src/compiler/nir/nir_control_flow.c

@@ -861,6 +861,28 @@ nir_cf_delete(nir_cf_list *cf_list)
    }
 }
 
+void
+nir_remove_after_cf_node(nir_cf_node *node)
+{
+   nir_cf_node *end = node;
+   while (!nir_cf_node_is_last(end))
+      end = nir_cf_node_next(end);
+
+   nir_cursor begin = nir_after_cf_node(node);
+   if (begin.option == nir_cursor_before_block) {
+      /* nir_cf_extract() would ignore these phis */
+      nir_function_impl *impl = nir_cf_node_get_function(node);
+      nir_foreach_phi_safe(phi, begin.block) {
+         replace_ssa_def_uses(&phi->def, impl);
+         nir_instr_remove_v(&phi->instr);
+      }
+   }
+
+   nir_cf_list list;
+   nir_cf_extract(&list, begin, nir_after_cf_node(end));
+   nir_cf_delete(&list);
+}
+
 struct block_index {
    nir_block *block;
    uint32_t index;

src/compiler/nir/nir_control_flow.h

@@ -171,17 +171,8 @@ nir_cf_node_remove(nir_cf_node *node)
    nir_cf_delete(&list);
 }
 
-static inline void
-nir_remove_after_cf_node(nir_cf_node *node)
-{
-   nir_cf_node *end = node;
-   while (!nir_cf_node_is_last(end))
-      end = nir_cf_node_next(end);
-
-   nir_cf_list list;
-   nir_cf_extract(&list, nir_after_cf_node(node), nir_after_cf_node(end));
-   nir_cf_delete(&list);
-}
+/** removes instructions after a control flow node, also removing any phis immediately after it */
+void nir_remove_after_cf_node(nir_cf_node *node);
 
 /** inserts undef phi sources from predcessor into phis of the block */
 void nir_insert_phi_undef(nir_block *block, nir_block *pred);

src/compiler/nir/tests/dce_tests.cpp

@@ -137,3 +137,49 @@ TEST_F(nir_opt_dead_cf_test, jump_before_constant_if)
 
    nir_validate_shader(b->shader, NULL);
 }
+
+TEST_F(nir_opt_dead_cf_test, unreachable_phi)
+{
+   /*
+    * Test that nir_opt_dead_cf/nir_remove_after_cf_node removes phis at the start of dead code.
+    *
+    * block b0:  // preds:
+    * 32    %0 = deref_var &cond (shader_in bool)
+    * 1     %1 = @load_deref (%0) (access=none)
+    *            // succs: b1 b2
+    * if %1 {
+    *     block b1:// preds: b0
+    *     return
+    *     // succs: b4
+    * } else {
+    *     block b2:// preds: b0
+    *     return
+    *     // succs: b4
+    * }
+    * block b3:  // preds:
+    * 32    %2 = phi
+    * 32    %3 = deref_var &out (shader_out int)
+    *            @store_deref (%3, %2) (wrmask=x, access=none)
+    *            // succs: b4
+    * block b4:
+    */
+   nir_variable *cond = nir_variable_create(b->shader, nir_var_shader_in, glsl_bool_type(), "cond");
+   nir_variable *var = nir_variable_create(b->shader, nir_var_shader_out, glsl_int_type(), "out");
+
+   nir_push_if(b, nir_load_var(b, cond));
+   nir_jump(b, nir_jump_return);
+   nir_push_else(b, NULL);
+   nir_jump(b, nir_jump_return);
+   nir_pop_if(b, NULL);
+
+   nir_phi_instr *phi = nir_phi_instr_create(b->shader);
+   nir_def_init(&phi->instr, &phi->def, 1, 32);
+   nir_builder_instr_insert(b, &phi->instr);
+   nir_store_var(b, var, &phi->def, 0x1);
+
+   ASSERT_TRUE(nir_opt_dead_cf(b->shader));
+   nir_validate_shader(b->shader, NULL);
+
+   ASSERT_TRUE(exec_list_is_empty(&nir_impl_last_block(b->impl)->instr_list));
+   ASSERT_FALSE(nir_opt_dead_cf(b->shader));
+}