diff --git a/js/src/ion/Ion.cpp b/js/src/ion/Ion.cpp
index 308c8d86e58..488830e2621 100644
--- a/js/src/ion/Ion.cpp
+++ b/js/src/ion/Ion.cpp
@@ -1855,12 +1855,11 @@ ion::Invalidate(types::TypeCompartment &types, FreeOp *fop,
     // until its last invalidated frame is destroyed.
     for (size_t i = 0; i < invalid.length(); i++) {
         types::CompilerOutput &co = *invalid[i].compilerOutput(types);
-        ExecutionMode executionMode;
+        ExecutionMode executionMode = SequentialExecution;
         switch (co.kind()) {
           case types::CompilerOutput::MethodJIT:
             continue;
           case types::CompilerOutput::Ion:
-            executionMode = SequentialExecution;
             break;
           case types::CompilerOutput::ParallelIon:
             executionMode = ParallelExecution;
diff --git a/js/src/ion/shared/CodeGenerator-x86-shared.cpp b/js/src/ion/shared/CodeGenerator-x86-shared.cpp
index e2f654bdc92..b5d9682c684 100644
--- a/js/src/ion/shared/CodeGenerator-x86-shared.cpp
+++ b/js/src/ion/shared/CodeGenerator-x86-shared.cpp
@@ -575,7 +575,7 @@ CodeGeneratorX86Shared::visitOutOfLineUndoALUOperation(OutOfLineUndoALUOperation
     LInstruction *ins = ool->ins();
     Register reg = ToRegister(ins->getDef(0));
 
-    LAllocation *lhs = ins->getOperand(0);
+    mozilla::DebugOnly<LAllocation *> lhs = ins->getOperand(0);
     LAllocation *rhs = ins->getOperand(1);
 
     JS_ASSERT(reg == ToRegister(lhs));
diff --git a/js/src/jsclone.cpp b/js/src/jsclone.cpp
index 2d94b293b37..c8505e4c8cc 100644
--- a/js/src/jsclone.cpp
+++ b/js/src/jsclone.cpp
@@ -207,8 +207,10 @@ SCInput::SCInput(JSContext *cx, uint64_t *data, size_t nbytes)
 bool
 SCInput::read(uint64_t *p)
 {
-    if (point == end)
+    if (point == end) {
+        *p = 0;  /* initialize to shut GCC up */
         return eof();
+    }
     *p = SwapBytes(*point++);
     return true;
 }
@@ -216,7 +218,7 @@ SCInput::read(uint64_t *p)
 bool
 SCInput::readPair(uint32_t *tagp, uint32_t *datap)
 {
-    uint64_t u = 0;     /* initialize to shut GCC up */
+    uint64_t u;
     bool ok = read(&u);
     if (ok) {
         *tagp = uint32_t(u >> 32);
diff --git a/js/src/jsinfer.cpp b/js/src/jsinfer.cpp
index 4c2a0729b19..118ff72a5a8 100644
--- a/js/src/jsinfer.cpp
+++ b/js/src/jsinfer.cpp
@@ -6149,8 +6149,7 @@ TypeCompartment::sweep(FreeOp *fop)
     if (arrayTypeTable) {
         for (ArrayTypeTable::Enum e(*arrayTypeTable); !e.empty(); e.popFront()) {
             const ArrayTableKey &key = e.front().key;
-            TypeObject *obj = e.front().value;
-            JS_ASSERT(obj->proto == key.proto);
+            JS_ASSERT(e.front().value->proto == key.proto);
             JS_ASSERT(!key.type.isSingleObject());
 
             bool remove = false;
diff --git a/js/src/jsobjinlines.h b/js/src/jsobjinlines.h
index 9715bf8f8e9..39e7d7d85eb 100644
--- a/js/src/jsobjinlines.h
+++ b/js/src/jsobjinlines.h
@@ -928,8 +928,10 @@ JSObject::hasProperty(JSContext *cx, js::HandleObject obj,
     js::RootedObject pobj(cx);
     js::RootedShape prop(cx);
     JSAutoResolveFlags rf(cx, flags);
-    if (!lookupGeneric(cx, obj, id, &pobj, &prop))
+    if (!lookupGeneric(cx, obj, id, &pobj, &prop)) {
+        *foundp = false;  /* initialize to shut GCC up */
         return false;
+    }
     *foundp = !!prop;
     return true;
 }
diff --git a/js/src/jsworkers.cpp b/js/src/jsworkers.cpp
index d1fd28a6ec4..83659e558c9 100644
--- a/js/src/jsworkers.cpp
+++ b/js/src/jsworkers.cpp
@@ -315,7 +315,7 @@ WorkerThread::threadLoop()
 
         ionBuilder = state.ionWorklist.popCopy();
 
-        ion::ExecutionMode executionMode = ionBuilder->info().executionMode();
+        DebugOnly<ion::ExecutionMode> executionMode = ionBuilder->info().executionMode();
         JS_ASSERT(GetIonScript(ionBuilder->script().unsafeGet(), executionMode) == ION_COMPILING_SCRIPT);
 
         state.unlock();