Merge pull request #1007 from WebAssembly/brs

Misc minor remove-unused-brs improvements

Author: kripken

src/passes/RemoveUnusedBrs.cpp

@@ -122,6 +122,8 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
     } else if (curr->is<Nop>()) {
       // ignore (could be result of a previous cycle)
       self->valueCanFlow = false;
+    } else if (curr->is<Loop>()) {
+      // do nothing - it's ok for values to flow out
     } else {
       // anything else stops the flow
       flows.clear();
@@ -323,10 +325,7 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
       // map of all value-less breaks going to a block (and not a loop)
       std::map<Block*, std::vector<Break*>> breaksToBlock;
 
-      // number of definitions of each name - when a name is defined more than once, it is not trivially safe to do this
-      std::map<Name, Index> numDefs;
-
-      // the names to update, when we can (just one def)
+      // the names to update
       std::map<Break*, Name> newNames;
 
       void visitBreak(Break* curr) {
@@ -338,8 +337,6 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
       }
       // TODO: Switch?
       void visitBlock(Block* curr) {
-        if (curr->name.is()) numDefs[curr->name]++;
-
         auto& list = curr->list;
         if (list.size() == 1 && curr->name.is()) {
           // if this block has just one child, a sub-block, then jumps to the former are jumps to us, really
@@ -372,17 +369,12 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
           }
         }
       }
-      void visitLoop(Loop* curr) {
-        if (curr->name.is()) numDefs[curr->name]++;
-      }
 
       void finish() {
         for (auto& iter : newNames) {
           auto* br = iter.first;
           auto name = iter.second;
-          if (numDefs[name] == 1) {
-            br->name = name;
-          }
+          br->name = name;
         }
       }
     };

src/passes/pass.cpp

@@ -135,8 +135,9 @@ void PassRunner::addDefaultFunctionOptimizationPasses() {
   add("simplify-locals");
   add("vacuum"); // previous pass creates garbage
   add("reorder-locals");
+  add("merge-blocks"); // makes remove-unused-brs more effective
   add("remove-unused-brs"); // coalesce-locals opens opportunities for optimizations
-  add("merge-blocks");
+  add("merge-blocks"); // clean up remove-unused-brs new blocks
   add("optimize-instructions");
   add("precompute");
   if (options.shrinkLevel >= 2) {