drop completely unused locals in ReorderLocals

2 files changed, 10 insertions, 1 deletions

diff --git a/src/passes/ReorderLocals.cpp b/src/passes/ReorderLocals.cpp
index e378408b6..7f64c00f3 100644
--- a/src/passes/ReorderLocals.cpp
+++ b/src/passes/ReorderLocals.cpp
@@ -31,12 +31,17 @@ struct ReorderLocals : public WalkerPass<PostWalker<ReorderLocals>> {
   std::map<Name, uint32_t> counts;
 
   void visitFunction(Function *curr) {
-    sort(curr->locals.begin(), curr->locals.end(), [this](NameType a, NameType b) -> bool {
+    auto& locals = curr->locals;
+    sort(locals.begin(), locals.end(), [this](NameType a, NameType b) -> bool {
       if (this->counts[a.name] == this->counts[b.name]) {
         return strcmp(a.name.str, b.name.str) > 0;
       }
       return this->counts[a.name] > this->counts[b.name];
     });
+    // drop completely unused locals
+    while (locals.size() > 0 && counts[locals.back().name] == 0) {
+      locals.pop_back();
+    }
     counts.clear();
   }
 
diff --git a/src/passes/SimplifyLocals.cpp b/src/passes/SimplifyLocals.cpp
index faf6e3a6e..408fea7ab 100644
--- a/src/passes/SimplifyLocals.cpp
+++ b/src/passes/SimplifyLocals.cpp
@@ -20,6 +20,10 @@
 // This "sinks" set_locals, pushing them to the next get_local where possible,
 // and removing the set if there are no gets remaining (the latter is
 // particularly useful in ssa mode, but not only).
+//
+// After this pass, some locals may be completely unused. reorder-locals
+// can get rid of those (the operation is trivial there after it sorts by use
+// frequency).
 
 #include <wasm.h>
 #include <wasm-traversal.h>