[NFC] Scan first in UniqueNameMapper (#5696)

We can scan far faster than the name mapping process itself, so do that
first, as in the common case nothing needs to be fixed up.

This together with #5697 significantly reduces the overhead in 
--inlining-optimizing of UniqueNameMapper, from something like
50% to 10%.

1 files changed, 43 insertions, 2 deletions

diff --git a/src/wasm/parsing.cpp b/src/wasm/parsing.cpp
index 46ec39865..c6134922a 100644
--- a/src/wasm/parsing.cpp
+++ b/src/wasm/parsing.cpp
@@ -16,6 +16,7 @@
 
 #include "parsing.h"
 #include "ir/branch-utils.h"
+#include "support/small_set.h"
 
 namespace wasm {
 
@@ -104,7 +105,48 @@ void UniqueNameMapper::clear() {
   reverseLabelMapping.clear();
 }
 
+namespace {
+
+struct DuplicateNameScanner
+  : public PostWalker<DuplicateNameScanner,
+                      UnifiedExpressionVisitor<DuplicateNameScanner>> {
+
+  // Whether things are ok. If not, we need to fix things up.
+  bool ok = true;
+
+  // It is rare to have many nested names in general, so track the seen names
+  // as we go in an efficient way.
+  SmallUnorderedSet<Name, 10> seen;
+
+  void visitExpression(Expression* curr) {
+    BranchUtils::operateOnScopeNameDefs(curr, [&](Name& name) {
+      if (!name.is()) {
+        return;
+      }
+      // TODO: This could be done in a single insert operation that checks
+      //       whether we actually inserted, if we improved
+      //       SmallSetBase::insert to return a value like std::set does.
+      if (seen.count(name)) {
+        // A name has been defined more than once; we'll need to fix that.
+        ok = false;
+      } else {
+        seen.insert(name);
+      }
+    });
+  }
+};
+
+} // anonymous namespace
+
 void UniqueNameMapper::uniquify(Expression* curr) {
+  // First, scan the code to see if anything needs to be fixed up, since in the
+  // common case nothing needs fixing, and we can verify that very quickly.
+  DuplicateNameScanner scanner;
+  scanner.walk(curr);
+  if (scanner.ok) {
+    return;
+  }
+
   struct Walker
     : public ControlFlowWalker<Walker, UnifiedExpressionVisitor<Walker>> {
     UniqueNameMapper mapper;
@@ -132,9 +174,8 @@ void UniqueNameMapper::uniquify(Expression* curr) {
         }
       });
     }
-  };
+  } walker;
 
-  Walker walker;
   walker.walk(curr);
 }