tint/transform: Skip LocalizeStructArrayAssignment if possible

Change-Id: I148c576f97359923c0d6fd6ffe864ca84ccd5b6b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/116869
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/tint/transform/localize_struct_array_assignment.cc

@@ -47,21 +47,25 @@ struct LocalizeStructArrayAssignment::State {
             utils::Vector<const ast::Statement*, 4> insert_after_stmts;
         } s;
 
-        ctx.ReplaceAll([&](const ast::AssignmentStatement* assign_stmt) -> const ast::Statement* {
+        bool made_changes = false;
+
+        for (auto* node : ctx.src->ASTNodes().Objects()) {
+            if (auto* assign_stmt = node->As<ast::AssignmentStatement>()) {
                 // Process if it's an assignment statement to a dynamically indexed array
                 // within a struct on a function or private storage variable. This
                 // specific use-case is what FXC fails to compile with:
                 // error X3500: array reference cannot be used as an l-value; not natively
                 // addressable
                 if (!ContainsStructArrayIndex(assign_stmt->lhs)) {
-                return nullptr;
+                    continue;
                 }
                 auto og = GetOriginatingTypeAndAddressSpace(assign_stmt);
                 if (!(og.first->Is<sem::Struct>() && (og.second == ast::AddressSpace::kFunction ||
                                                       og.second == ast::AddressSpace::kPrivate))) {
-                return nullptr;
+                    continue;
                 }
 
+                ctx.Replace(assign_stmt, [&, assign_stmt] {
                     // Reset shared state for this assignment statement
                     s = Shared{};
 
@@ -85,6 +89,14 @@ struct LocalizeStructArrayAssignment::State {
                     return b.Block(std::move(stmts));
                 });
 
+                made_changes = true;
+            }
+        }
+
+        if (!made_changes) {
+            return SkipTransform;
+        }
+
         ctx.ReplaceAll(
             [&](const ast::IndexAccessorExpression* index_access) -> const ast::Expression* {
                 if (!s.process_nested_nodes) {

src/tint/transform/localize_struct_array_assignment_test.cc

@@ -25,9 +25,7 @@ using LocalizeStructArrayAssignmentTest = TransformTest;
 
 TEST_F(LocalizeStructArrayAssignmentTest, EmptyModule) {
     auto* src = R"()";
-    auto* expect = src;
+    EXPECT_FALSE(ShouldRun<LocalizeStructArrayAssignment>(src));
-    auto got = Run<Unshadow, SimplifyPointers, LocalizeStructArrayAssignment>(src);
-    EXPECT_EQ(expect, str(got));
 }
 
 TEST_F(LocalizeStructArrayAssignmentTest, StructArray) {
@@ -842,10 +840,7 @@ fn main() {
 
     // Transform does nothing here as we're not actually assigning to the array in
     // the struct.
-    auto* expect = src;
+    EXPECT_FALSE(ShouldRun<LocalizeStructArrayAssignment>(src));
-
-    auto got = Run<Unshadow, SimplifyPointers, LocalizeStructArrayAssignment>(src);
-    EXPECT_EQ(expect, str(got));
 }
 
 }  // namespace