Reject atomic arithmetic on non-integer types

Fixes #181

src/intrinsic.rs

@@ -126,6 +126,9 @@ impl<'a, 'mir, 'tcx> EvalContextExt<'tcx> for super::MiriEvalContext<'a, 'mir, '
             "atomic_xsub_acqrel" |
             "atomic_xsub_relaxed" => {
                 let ptr = self.ref_to_mplace(self.read_value(args[0])?)?;
+                if !ptr.layout.ty.is_integral() {
+                    return err!(Unimplemented(format!("Atomic arithmetic operations only work on integer types")));
+                }
                 let rhs = self.read_value(args[1])?;
                 let old = self.read_value(ptr.into())?;
                 self.write_value(*old, dest)?; // old value is returned

tests/compile-fail/atomic_non_integer_arithmetic.rs

@@ -0,0 +1,9 @@
+#![feature(core_intrinsics)]
+
+pub fn main() {
+    let mut z: f64 = 1.0;
+    unsafe {
+        ::std::intrinsics::atomic_xadd(&mut z, 2.0);
+        //~^ ERROR: Atomic arithmetic operations only work on integer types
+    }
+}