From 7a41bf1e2f9b24325430d924f2e8c539c1987f79 Mon Sep 17 00:00:00 2001
From: Ben Clayton <bclayton@google.com>
Date: Thu, 12 Jan 2023 10:36:27 +0000
Subject: [PATCH] tint/transform: Skip Unshadow if possible

Change-Id: I73b7213aafe3ff25a5f6526e7002d2ec3dda57c3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/116871
Kokoro: Ben Clayton <bclayton@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
---
 src/tint/transform/unshadow.cc      | 36 ++++++++++++++++++-----------
 src/tint/transform/unshadow_test.cc | 11 ++-------
 2 files changed, 25 insertions(+), 22 deletions(-)

diff --git a/src/tint/transform/unshadow.cc b/src/tint/transform/unshadow.cc
index 3ec5f3053e..7c9f75cced 100644
--- a/src/tint/transform/unshadow.cc
+++ b/src/tint/transform/unshadow.cc
@@ -81,19 +81,29 @@ struct Unshadow::State {
                 });
         };
 
-        ctx.ReplaceAll([&](const ast::Variable* v) -> const ast::Variable* {
-            if (auto* local = sem.Get<sem::LocalVariable>(v)) {
-                if (local->Shadows()) {
-                    return rename(local);
-                }
-            }
-            if (auto* param = sem.Get<sem::Parameter>(v)) {
-                if (param->Shadows()) {
-                    return rename(param);
-                }
-            }
-            return nullptr;
-        });
+        bool made_changes = false;
+
+        for (auto* node : ctx.src->SemNodes().Objects()) {
+            Switch(
+                node,  //
+                [&](const sem::LocalVariable* local) {
+                    if (local->Shadows()) {
+                        ctx.Replace(local->Declaration(), [&, local] { return rename(local); });
+                        made_changes = true;
+                    }
+                },
+                [&](const sem::Parameter* param) {
+                    if (param->Shadows()) {
+                        ctx.Replace(param->Declaration(), [&, param] { return rename(param); });
+                        made_changes = true;
+                    }
+                });
+        }
+
+        if (!made_changes) {
+            return SkipTransform;
+        }
+
         ctx.ReplaceAll(
             [&](const ast::IdentifierExpression* ident) -> const tint::ast::IdentifierExpression* {
                 if (auto* sem_ident = sem.Get(ident)) {
diff --git a/src/tint/transform/unshadow_test.cc b/src/tint/transform/unshadow_test.cc
index f9e976b361..d2ee8a5c4e 100644
--- a/src/tint/transform/unshadow_test.cc
+++ b/src/tint/transform/unshadow_test.cc
@@ -23,11 +23,8 @@ using UnshadowTest = TransformTest;
 
 TEST_F(UnshadowTest, EmptyModule) {
     auto* src = "";
-    auto* expect = "";
 
-    auto got = Run<Unshadow>(src);
-
-    EXPECT_EQ(expect, str(got));
+    EXPECT_FALSE(ShouldRun<Unshadow>(src));
 }
 
 TEST_F(UnshadowTest, Noop) {
@@ -48,11 +45,7 @@ fn F(c : i32) {
 }
 )";
 
-    auto* expect = src;
-
-    auto got = Run<Unshadow>(src);
-
-    EXPECT_EQ(expect, str(got));
+    EXPECT_FALSE(ShouldRun<Unshadow>(src));
 }
 
 TEST_F(UnshadowTest, LocalShadowsAlias) {