[bug] Fix offline cache emit dependencies (taichi-dev#8510)

Issue: #

Fix SNodeTree storing in unordered container when generating kernel
offline cache key. The potential different traverse order of a unordered
container each time would generate different offline cache keys for same
kernel, which breaks the offline cache i.e., triggering re-compilation
each time.

### Brief Summary

copilot:summary

### Walkthrough

copilot:walkthrough

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

taichi/analysis/gen_offline_cache_key.cpp

@@ -532,7 +532,7 @@ class ASTSerializer : public IRVisitor, public ExpressionVisitor {
       emit(static_cast<std::size_t>(snode->get_snode_tree_id()));
       emit(static_cast<std::size_t>(snode->id));
       const auto *root = snode->get_root();
-      snode_tree_roots_.insert(root);
+      snode_tree_roots_.push_back(root);
     } else {
       emit(std::numeric_limits<std::size_t>::max());
       emit(std::numeric_limits<std::size_t>::max());
@@ -654,8 +654,8 @@ class ASTSerializer : public IRVisitor, public ExpressionVisitor {
 #undef DEFINE_EMIT_ENUM
 
   std::ostream *os_{nullptr};
-  std::unordered_set<const SNode *> snode_tree_roots_;
-  std::unordered_map<Function *, std::size_t> real_funcs_;
+  std::vector<const SNode *> snode_tree_roots_;
+  std::map<Function *, std::size_t> real_funcs_;
   std::vector<char> string_pool_;
 };
 

tests/python/test_offline_cache.py

@@ -384,6 +384,74 @@ def helper():
     assert added_files() == expected_num_cache_files(len(simple_kernels_to_test))
 
 
+@pytest.mark.parametrize("curr_arch", supported_archs_offline_cache)
+@_test_offline_cache_dec
+def test_offline_cache_with_different_snode_trees(curr_arch):
+    count_of_cache_file = cache_files_cnt()
+
+    def added_files():
+        return cache_files_cnt() - count_of_cache_file
+
+    def helper():
+        x = ti.field(float, shape=5)
+
+        @ti.kernel
+        def trigger_compile():
+            x[0] += 1
+
+        # This case is used for testing SNodeTree storeing order matters (i.e., use a ordered container such as vector instead of unordered_map or unordered_set) when generating kernel offline cache key
+        # The multiple `trigger_compile` equalivant to allocate each field to a different SNodeTree
+        # i.e.,
+        # x = ti.field(float)
+        # fb.dense(ti.i, 5).place(x)
+        # fb.finalize()
+
+        trigger_compile()
+        a = ti.field(float, shape=5)
+        trigger_compile()
+        b = ti.field(float, shape=10)
+        trigger_compile()
+        c = ti.field(float, shape=5)
+        trigger_compile()
+        d = ti.field(float, shape=10)
+        trigger_compile()
+        e = ti.field(float, shape=5)
+        trigger_compile()
+        f = ti.field(float, shape=10)
+        trigger_compile()
+        g = ti.field(float, shape=5)
+        trigger_compile()
+        h = ti.field(float, shape=10)
+
+        @ti.kernel
+        def kernel_forward():
+            for i in range(5):
+                a[i] += i
+                b[i] += i
+                c[i] += i
+                d[i] += i
+                e[i] += i
+                f[i] += i
+                g[i] += i
+                h[i] += i
+
+        kernel_forward()
+
+    assert added_files() == expected_num_cache_files(0)
+    ti.init(arch=curr_arch, enable_fallback=False, **current_thread_ext_options())
+    helper()
+
+    ti.init(arch=curr_arch, enable_fallback=False, **current_thread_ext_options())
+    assert added_files() == expected_num_cache_files(2)
+    helper()
+
+    # The number of cache file should not change
+    for _ in range(5):
+        ti.init(arch=curr_arch, enable_fallback=False, **current_thread_ext_options())
+        assert added_files() == expected_num_cache_files(2)
+        helper()
+
+
 @pytest.mark.parametrize("curr_arch", supported_archs_offline_cache)
 @_test_offline_cache_dec
 def test_offline_cache_with_changing_compile_config(curr_arch):