diff --git a/src/librand/reseeding.rs b/src/librand/reseeding.rs
index 06828911471..f4d3e975b75 100644
--- a/src/librand/reseeding.rs
+++ b/src/librand/reseeding.rs
@@ -134,7 +134,11 @@ pub trait Reseeder<R> {
 /// Reseed an RNG using a `Default` instance. This reseeds by
 /// replacing the RNG with the result of a `Default::default` call.
 #[derive(Copy)]
-pub struct ReseedWithDefault;
+pub struct ReseedWithDefault { __hack: [u8; 0] }
+// FIXME(#21721) used to be an unit struct but that can cause
+// certain LLVM versions to abort during optimizations.
+#[allow(non_upper_case_globals)]
+pub const ReseedWithDefault: ReseedWithDefault = ReseedWithDefault { __hack: [] };
 
 impl<R: Rng + Default> Reseeder<R> for ReseedWithDefault {
     fn reseed(&mut self, rng: &mut R) {
diff --git a/src/test/bench/task-perf-alloc-unwind.rs b/src/test/bench/task-perf-alloc-unwind.rs
index f09e64cb9c5..c45efe5f54b 100644
--- a/src/test/bench/task-perf-alloc-unwind.rs
+++ b/src/test/bench/task-perf-alloc-unwind.rs
@@ -40,7 +40,9 @@ fn run(repeat: int, depth: int) {
     }
 }
 
-type nillist = List<()>;
+// FIXME(#21721) used to be `List<()>` but that can cause
+// certain LLVM versions to abort during optimizations.
+type nillist = List<[u8; 0]>;
 
 // Filled with things that have to be unwound
 
@@ -81,11 +83,11 @@ fn recurse_or_panic(depth: int, st: Option<State>) {
             }
             Some(st) => {
                 let mut v = st.vec.clone();
-                v.push_all(&[box List::Cons((), st.vec.last().unwrap().clone())]);
+                v.push_all(&[box List::Cons([], st.vec.last().unwrap().clone())]);
                 State {
-                    unique: box List::Cons((), box *st.unique),
+                    unique: box List::Cons([], box *st.unique),
                     vec: v,
-                    res: r(box List::Cons((), st.res._l.clone())),
+                    res: r(box List::Cons([], st.res._l.clone())),
                 }
             }
         };