Bug 1326179: wasm: Disable parallel compilation on machines with 1 CPU; r=luke

MozReview-Commit-ID: EEGWa2u9U2X

--HG--
extra : rebase_source : b953c4c87bc2702929c474a55082781ff65201e3
extra : amend_source : aef6268c43ef4e2d9452315d459094779cd465c1

js/src/wasm/WasmGenerator.cpp

@@ -836,7 +836,10 @@ ModuleGenerator::startFuncDefs()
     MOZ_ASSERT(threads.threadCount > 1);
 
     uint32_t numTasks;
-    if (CanUseExtraThreads() && threads.wasmCompilationInProgress.compareExchange(false, true)) {
+    if (CanUseExtraThreads() &&
+        threads.cpuCount > 1 &&
+        threads.wasmCompilationInProgress.compareExchange(false, true))
+    {
 #ifdef DEBUG
         {
             AutoLockHelperThreadState lock;