Add option to remove empty function exports

src/passes/RemoveUnusedModuleElements.cpp

@@ -35,6 +35,10 @@
 // reference forces us to keep something in the IR to be referred to, but only
 // a use actually makes us keep its contents as well.
 //
+// The remove-unused-module-elements-consider-empty-exports-unused pass
+// argument treats exports of empty functions as unused. This is useful for
+// runtimes where such exports are optional.
+//
 
 #include <memory>
 #include <vector>
@@ -887,6 +891,31 @@ struct RemoveUnusedModuleElements : public Pass {
   void run(Module* module) override {
     prepare(module);
 
+    // Some runtimes treat missing exports as optional. For those runtimes,
+    // allow exported functions that do nothing to be handled like other
+    // unused module elements. Remove the exports before finding roots so that
+    // an otherwise-unreferenced function can be removed as well.
+    if (!rootAllFunctions &&
+        hasArgument(
+          "remove-unused-module-elements-consider-empty-exports-unused")) {
+      module->removeExports([&](Export* curr) {
+        if (curr->kind != ExternalKind::Function) {
+          return false;
+        }
+        auto* func = module->getFunction(*curr->getInternalName());
+        if (func->imported()) {
+          return false;
+        }
+        if (func->body->is<Nop>()) {
+          return true;
+        }
+        if (auto* block = func->body->dynCast<Block>()) {
+          return block->list.empty();
+        }
+        return false;
+      });
+    }
+
     std::vector<ModuleElement> roots;
     // Module start is a root.
     if (module->start.is()) {

test/lit/passes/remove-unused-module-elements-empty-exports.wat

@@ -0,0 +1,36 @@
+;; RUN: wasm-opt %s --remove-unused-module-elements -S -o - | filecheck %s --check-prefix=DEFAULT
+;; RUN: wasm-opt %s --remove-unused-module-elements --pass-arg=remove-unused-module-elements-consider-empty-exports-unused -S -o - | filecheck %s --check-prefix=OPTION
+
+(module
+  (import "env" "noop" (func $imported))
+
+  (func $empty)
+  (func $empty-used)
+  (func $call-empty
+    (call $empty-used)
+  )
+  (func $nonempty
+    (drop
+      (i32.const 0)
+    )
+  )
+
+  (export "empty" (func $empty))
+  (export "empty-used" (func $empty-used))
+  (export "call-empty" (func $call-empty))
+  (export "nonempty" (func $nonempty))
+  (export "imported" (func $imported))
+)
+
+;; DEFAULT:      (export "empty" (func $empty))
+;; DEFAULT-NEXT: (export "empty-used" (func $empty-used))
+;; DEFAULT:      (func $empty
+;; DEFAULT:      (func $empty-used
+
+;; OPTION-NOT:   (export "empty"
+;; OPTION-NOT:   (export "empty-used"
+;; OPTION:       (export "call-empty" (func $call-empty))
+;; OPTION-NEXT:  (export "nonempty" (func $nonempty))
+;; OPTION-NEXT:  (export "imported" (func $imported))
+;; OPTION-NOT:   (func $empty{{[[:space:]]*$}}
+;; OPTION:       (func $empty-used