[wasm-metadce] Add support for tags (#4250)

This adds support for tag-using instructions (`throw` and `catch`) to
wasm-metadce. We had to use a hacky workaround in
emscripten-core/emscripten#15266 because of the lack of this support;
after this lands we can remove it.

5 files changed, 58 insertions, 0 deletions

diff --git a/src/tools/wasm-metadce.cpp b/src/tools/wasm-metadce.cpp
index 23a2d0fcc..00fa87ce2 100644
--- a/src/tools/wasm-metadce.cpp
+++ b/src/tools/wasm-metadce.cpp
@@ -263,6 +263,12 @@ struct MetaDCEGraph {
       }
       void visitGlobalGet(GlobalGet* curr) { handleGlobal(curr->name); }
       void visitGlobalSet(GlobalSet* curr) { handleGlobal(curr->name); }
+      void visitThrow(Throw* curr) { handleTag(curr->tag); }
+      void visitTry(Try* curr) {
+        for (auto tag : curr->catchTags) {
+          handleTag(tag);
+        }
+      }
 
     private:
       MetaDCEGraph* parent;
@@ -282,6 +288,17 @@ struct MetaDCEGraph {
         parent->nodes[parent->functionToDCENode[getFunction()->name]]
           .reaches.push_back(dceName);
       }
+
+      void handleTag(Name name) {
+        Name dceName;
+        if (!getModule()->getTag(name)->imported()) {
+          dceName = parent->tagToDCENode[name];
+        } else {
+          dceName = parent->importIdToDCENode[parent->getTagImportId(name)];
+        }
+        parent->nodes[parent->functionToDCENode[getFunction()->name]]
+          .reaches.push_back(dceName);
+      }
     };
 
     PassRunner runner(&wasm);
diff --git a/test/metadce/tag.wast b/test/metadce/tag.wast
new file mode 100644
index 000000000..527e57d33
--- /dev/null
+++ b/test/metadce/tag.wast
@@ -0,0 +1,14 @@
+(module
+  (import "env" "imported_tag" (tag $t0))
+  (tag $t1)
+  (export "test" (func $test))
+
+  (func $test
+    (try
+      (do
+        (throw $t0)
+      )
+      (catch $t1)
+    )
+  )
+)
diff --git a/test/metadce/tag.wast.dced b/test/metadce/tag.wast.dced
new file mode 100644
index 000000000..78ae287f1
--- /dev/null
+++ b/test/metadce/tag.wast.dced
@@ -0,0 +1,16 @@
+(module
+ (type $none_=>_none (func))
+ (import "env" "imported_tag" (tag $t0 (param)))
+ (tag $t1 (param))
+ (export "test" (func $test))
+ (func $test
+  (try $try
+   (do
+    (throw $t0)
+   )
+   (catch $t1
+    (nop)
+   )
+  )
+ )
+)
diff --git a/test/metadce/tag.wast.dced.stdout b/test/metadce/tag.wast.dced.stdout
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/test/metadce/tag.wast.dced.stdout
diff --git a/test/metadce/tag.wast.graph.txt b/test/metadce/tag.wast.graph.txt
new file mode 100644
index 000000000..34a2d1fdf
--- /dev/null
+++ b/test/metadce/tag.wast.graph.txt
@@ -0,0 +1,11 @@
+[
+  {
+    "name": "tag_import",
+    "import": ["env", "imported_tag"]
+  },
+  {
+    "name": "func_export",
+    "root": true,
+    "export": "test"
+  }
+]