4 files changed, 44 insertions, 2 deletions
diff --git a/src/ast_utils.h b/src/ast_utils.h
index f91cfab24..9336d2b3a 100644
--- a/src/ast_utils.h
+++ b/src/ast_utils.h
@@ -117,6 +117,37 @@ struct ExpressionManipulator {
   }
 };
 
+struct ExpressionAnalyzer {
+  // Given a stack of expressions, checks if the topmost is used as a result.
+  // For example, if the parent is a block and the node is before the last position,
+  // it is not used.
+  static bool isResultUsed(std::vector<Expression*> stack, Function* func) {
+    for (int i = int(stack.size()) - 2; i >= 0; i--) {
+      auto* curr = stack[i];
+      auto* above = stack[i + 1];
+      // only if and block can drop values
+      if (curr->is<Block>()) {
+        auto* block = curr->cast<Block>();
+        for (size_t j = 0; j < block->list.size() - 1; j++) {
+          if (block->list[j] == above) return false;
+        }
+        assert(block->list.back() == above);
+        // continue down
+      } else if (curr->is<If>()) {
+        auto* iff = curr->cast<If>();
+        if (!iff->ifFalse) return false;
+        if (above == iff->condition) return true;
+        assert(above == iff->ifTrue || above == iff->ifFalse);
+        // continue down
+      } else {
+        return true; // all other node types use the result
+      }
+    }
+    // The value might be used, so it depends on if the function returns
+    return func->result != none;
+  }
+};
+
 } // namespace wasm
 
 #endif // wasm_ast_utils_h
diff --git a/src/pass.cpp b/src/pass.cpp
index 1a8aaabc9..7095da71d 100644
--- a/src/pass.cpp
+++ b/src/pass.cpp
@@ -60,9 +60,9 @@ std::string PassRegistry::getPassDescription(std::string name) {
 void PassRunner::addDefaultOptimizationPasses() {
   add("remove-unused-brs");
   add("remove-unused-names");
-  add("merge-blocks");
   add("optimize-instructions");
   add("simplify-locals");
+  add("merge-blocks");
   add("reorder-locals");
   add("vacuum");
 }
diff --git a/src/passes/Print.cpp b/src/passes/Print.cpp
index ddeec3dce..59ade1c76 100644
--- a/src/passes/Print.cpp
+++ b/src/passes/Print.cpp
@@ -68,7 +68,10 @@ struct PrintSExpression : public Visitor<PrintSExpression> {
   }
 
   Name printableLocal(Index index) {
-    Name name = currFunction->tryLocalName(index);
+    Name name;
+    if (currFunction) {
+      name = currFunction->tryLocalName(index);
+    }
     if (!name.is()) {
       name = Name::fromInt(index);
     }
diff --git a/src/passes/Vacuum.cpp b/src/passes/Vacuum.cpp
index dfede6d25..863d78f14 100644
--- a/src/passes/Vacuum.cpp
+++ b/src/passes/Vacuum.cpp
@@ -20,6 +20,7 @@
 
 #include <wasm.h>
 #include <pass.h>
+#include <ast_utils.h>
 
 namespace wasm {
 
@@ -41,6 +42,13 @@ struct Vacuum : public WalkerPass<PostWalker<Vacuum, Visitor<Vacuum>>> {
     if (skip > 0) {
       list.resize(size - skip);
     }
+    if (!curr->name.is()) {
+      if (list.size() == 1) {
+        replaceCurrent(list[0]);
+      } else if (list.size() == 0) {
+        ExpressionManipulator::nop(curr);
+      }
+    }
   }
 };