diff --git a/src/bootstrap/bin/rustc.rs b/src/bootstrap/bin/rustc.rs
index 631c9f72f3500..30afd52f44824 100644
--- a/src/bootstrap/bin/rustc.rs
+++ b/src/bootstrap/bin/rustc.rs
@@ -246,6 +246,9 @@ fn main() {
         // When running miri tests, we need to generate MIR for all libraries
         if env::var("TEST_MIRI").ok().map_or(false, |val| val == "true") {
             cmd.arg("-Zalways-encode-mir");
+            if stage != "0" {
+                cmd.arg("-Zmiri");
+            }
             cmd.arg("-Zmir-emit-validate=1");
         }
 
diff --git a/src/bootstrap/check.rs b/src/bootstrap/check.rs
index ee780d1245ed8..eee403dcbe3e7 100644
--- a/src/bootstrap/check.rs
+++ b/src/bootstrap/check.rs
@@ -769,6 +769,7 @@ impl Step for Compiletest {
         if build.config.rust_debuginfo_tests {
             flags.push("-g".to_string());
         }
+        flags.push("-Zmiri -Zunstable-options".to_string());
 
         if let Some(linker) = build.linker(target) {
             cmd.arg("--linker").arg(linker);
diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index 0dcd3e8081080..e6138b34c8084 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -1141,6 +1141,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
           "print some statistics about MIR"),
     always_encode_mir: bool = (false, parse_bool, [TRACKED],
           "encode MIR of all functions into the crate metadata"),
+    miri: bool = (false, parse_bool, [TRACKED],
+          "check the miri const evaluator against the old ctfe"),
     osx_rpath_install_name: bool = (false, parse_bool, [TRACKED],
           "pass `-install_name @rpath/...` to the macOS linker"),
     sanitizer: Option<Sanitizer> = (None, parse_sanitizer, [TRACKED],
diff --git a/src/librustc_const_eval/eval.rs b/src/librustc_const_eval/eval.rs
index f400380536ea3..95b6dc80b14a0 100644
--- a/src/librustc_const_eval/eval.rs
+++ b/src/librustc_const_eval/eval.rs
@@ -729,27 +729,31 @@ pub(crate) fn const_eval<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     trace!("running old const eval");
     let old_result = ConstContext::new(tcx, key.param_env.and(substs), tables).eval(&body.value);
     trace!("old const eval produced {:?}", old_result);
-    let instance = ty::Instance::new(def_id, substs);
-    trace!("const eval instance: {:?}, {:?}", instance, key.param_env);
-    let miri_result = ::rustc::mir::interpret::eval_body(tcx, instance, key.param_env);
-    match (miri_result, old_result) {
-        ((Err(err), ecx), Ok(ok)) => {
-            trace!("miri failed, ctfe returned {:?}", ok);
-            tcx.sess.span_warn(
-                tcx.def_span(key.value.0),
-                "miri failed to eval, while ctfe succeeded",
-            );
-            let () = unwrap_miri(&ecx, Err(err));
-            Ok(ok)
-        },
-        ((Ok(_), _), Err(err)) => {
-            Err(err)
-        },
-        ((Err(_), _), Err(err)) => Err(err),
-        ((Ok((miri_val, miri_ty)), mut ecx), Ok(ctfe)) => {
-            check_ctfe_against_miri(&mut ecx, miri_val, miri_ty, ctfe.val);
-            Ok(ctfe)
+    if tcx.sess.opts.debugging_opts.miri {
+        let instance = ty::Instance::new(def_id, substs);
+        trace!("const eval instance: {:?}, {:?}", instance, key.param_env);
+        let miri_result = ::rustc::mir::interpret::eval_body(tcx, instance, key.param_env);
+        match (miri_result, old_result) {
+            ((Err(err), ecx), Ok(ok)) => {
+                trace!("miri failed, ctfe returned {:?}", ok);
+                tcx.sess.span_warn(
+                    tcx.def_span(key.value.0),
+                    "miri failed to eval, while ctfe succeeded",
+                );
+                let () = unwrap_miri(&ecx, Err(err));
+                Ok(ok)
+            },
+            ((Ok(_), _), Err(err)) => {
+                Err(err)
+            },
+            ((Err(_), _), Err(err)) => Err(err),
+            ((Ok((miri_val, miri_ty)), mut ecx), Ok(ctfe)) => {
+                check_ctfe_against_miri(&mut ecx, miri_val, miri_ty, ctfe.val);
+                Ok(ctfe)
+            }
         }
+    } else {
+        old_result
     }
 }