Remove writes to globals that are never written to (#2741)

Since the global is never read, we know that any write operation
will be unobservable.

2 files changed, 40 insertions, 0 deletions

diff --git a/src/passes/SimplifyGlobals.cpp b/src/passes/SimplifyGlobals.cpp
index 5fa1686fc..71aafa491 100644
--- a/src/passes/SimplifyGlobals.cpp
+++ b/src/passes/SimplifyGlobals.cpp
@@ -51,6 +51,7 @@ struct GlobalInfo {
   bool imported = false;
   bool exported = false;
   std::atomic<bool> written;
+  std::atomic<bool> read;
 };
 
 using GlobalInfoMap = std::map<Name, GlobalInfo>;
@@ -64,6 +65,8 @@ struct GlobalUseScanner : public WalkerPass<PostWalker<GlobalUseScanner>> {
 
   void visitGlobalSet(GlobalSet* curr) { (*infos)[curr->name].written = true; }
 
+  void visitGlobalGet(GlobalGet* curr) { (*infos)[curr->name].read = true; }
+
 private:
   GlobalInfoMap* infos;
 };
@@ -163,6 +166,23 @@ private:
   std::map<Name, Literals> currConstantGlobals;
 };
 
+struct GlobalSetRemover : public WalkerPass<PostWalker<GlobalSetRemover>> {
+  GlobalSetRemover(NameSet* toRemove) : toRemove(toRemove) {}
+
+  bool isFunctionParallel() override { return true; }
+
+  GlobalSetRemover* create() override { return new GlobalSetRemover(toRemove); }
+
+  void visitGlobalSet(GlobalSet* curr) {
+    if (toRemove->count(curr->name) != 0) {
+      ExpressionManipulator::nop(curr);
+    }
+  }
+
+private:
+  NameSet* toRemove;
+};
+
 } // anonymous namespace
 
 struct SimplifyGlobals : public Pass {
@@ -180,6 +200,8 @@ struct SimplifyGlobals : public Pass {
 
     analyze();
 
+    removeWritesToUnreadGlobals();
+
     preferEarlierImports();
 
     propagateConstantsToGlobals();
@@ -211,6 +233,23 @@ struct SimplifyGlobals : public Pass {
     }
   }
 
+  void removeWritesToUnreadGlobals() {
+    // Globals that are not exports and not read from can be eliminated
+    // (even if they are written to).
+    NameSet unreadGlobals;
+    for (auto& global : module->globals) {
+      auto& info = map[global->name];
+      if (!info.imported && !info.exported && !info.read) {
+        unreadGlobals.insert(global->name);
+        // We can now mark this global as immutable, and un-written, since we
+        // are about to remove all the `.set` operations on it.
+        global->mutable_ = false;
+        info.written = false;
+      }
+    }
+    GlobalSetRemover(&unreadGlobals).run(runner, module);
+  }
+
   void preferEarlierImports() {
     // Optimize uses of immutable globals, prefer the earlier import when
     // there is a copy.
diff --git a/src/passes/pass.cpp b/src/passes/pass.cpp
index 733b61467..4da870ae9 100644
--- a/src/passes/pass.cpp
+++ b/src/passes/pass.cpp
@@ -483,6 +483,7 @@ void PassRunner::addDefaultGlobalOptimizationPostPasses() {
   } else {
     add("simplify-globals");
   }
+  add("vacuum"); // simplify-globals can generate extra nops
   add("remove-unused-module-elements");
   // may allow more inlining/dae/etc., need --converge for that
   add("directize");