Function parallelism (#343)

* allow traversals to mark themselves as function-parallel, in which case we run them using a thread pool. also mark some thread-safety risks (interned strings, arena allocators) with assertions they modify only on the main thread

1 files changed, 14 insertions, 10 deletions

diff --git a/src/emscripten-optimizer/istring.h b/src/emscripten-optimizer/istring.h
index 8e91d044d..149f77d23 100644
--- a/src/emscripten-optimizer/istring.h
+++ b/src/emscripten-optimizer/istring.h
@@ -29,6 +29,8 @@
 #include <stdio.h>
 #include <assert.h>
 
+#include "support/threads.h"
+
 namespace cashew {
 
 struct IString {
@@ -66,22 +68,24 @@ struct IString {
     typedef std::unordered_set<const char *, CStringHash, CStringEqual> StringSet;
     static StringSet* strings = new StringSet();
 
-    if (reuse) {
-      auto result = strings->insert(s); // if already present, does nothing
-      str = *(result.first);
-    } else {
-      auto existing = strings->find(s);
-      if (existing == strings->end()) {
+    auto existing = strings->find(s);
+
+    if (existing == strings->end()) {
+      // the StringSet cache is a global shared structure, which should
+      // not be modified by multiple threads at once.
+      assert(!wasm::ThreadPool::isRunning());
+      if (!reuse) {
         size_t len = strlen(s) + 1;
         char *copy = (char*)malloc(len); // XXX leaked
         strncpy(copy, s, len);
         s = copy;
-        strings->insert(s);
-      } else {
-        s = *existing;
       }
-      str = s;
+      strings->insert(s);
+    } else {
+      s = *existing;
     }
+
+    str = s;
   }
 
   void set(const IString &s) {