optimize loop endings in RemoveUnusedBrs

 * rotate an if near the end of a loop as it can let a break out flow naturally and be removable
 * turn a br_if into an if it allows such an optimization in cases where it helps remove other structures

1 files changed, 26 insertions, 1 deletions

diff --git a/src/ast_utils.h b/src/ast_utils.h
index 5deaa6a2c..6b76dd61d 100644
--- a/src/ast_utils.h
+++ b/src/ast_utils.h
@@ -27,7 +27,7 @@ namespace wasm {
 
 struct BreakSeeker : public PostWalker<BreakSeeker, Visitor<BreakSeeker>> {
   Name target; // look for this one XXX looking by name may fall prey to duplicate names
-  size_t found;
+  Index found;
 
   BreakSeeker(Name target) : target(target), found(false) {}
 
@@ -47,6 +47,12 @@ struct BreakSeeker : public PostWalker<BreakSeeker, Visitor<BreakSeeker>> {
     breakSeeker.walk(tree);
     return breakSeeker.found > 0;
   }
+
+  static Index count(Expression* tree, Name target) {
+    BreakSeeker breakSeeker(target);
+    breakSeeker.walk(tree);
+    return breakSeeker.found;
+  }
 };
 
 // Finds all functions that are reachable via direct calls.
@@ -391,6 +397,25 @@ struct ExpressionAnalyzer {
     return func->result != none;
   }
 
+  // Checks if a break is a simple - no condition, no value, just a plain branching
+  static bool isSimple(Break* curr) {
+    return !curr->condition && !curr->value;
+  }
+
+  // Checks if an expression ends with a simple break,
+  // and returns a pointer to it if so.
+  // (It might also have other internal branches.)
+  static Expression* getEndingSimpleBreak(Expression* curr) {
+    if (auto* br = curr->dynCast<Break>()) {
+      if (isSimple(br)) return br;
+      return nullptr;
+    }
+    if (auto* block = curr->dynCast<Block>()) {
+      if (block->list.size() > 0) return getEndingSimpleBreak(block->list.back());
+    }
+    return nullptr;
+  }
+
   template<typename T>
   static bool flexibleEqual(Expression* left, Expression* right, T& comparer) {
     std::vector<Name> nameStack;