diff --git a/README.md b/README.md
index 3d7003af72e4..a1b978316684 100644
--- a/README.md
+++ b/README.md
@@ -193,6 +193,8 @@ Moreover, Miri recognizes some environment variables:
 * `MIRI_TEST_TARGET` (recognized by the test suite) indicates which target
   architecture to test against.  `miri` and `cargo miri` accept the `--target`
   flag for the same purpose.
+* `MIRI_TEST_FLAGS` (recognized by the test suite) defines extra flags to be
+  passed to Miri.
 
 ## Contributing and getting help
 
diff --git a/tests/compiletest.rs b/tests/compiletest.rs
index 9cead530b57c..f43ff8ca349d 100644
--- a/tests/compiletest.rs
+++ b/tests/compiletest.rs
@@ -24,9 +24,10 @@ fn rustc_lib_path() -> PathBuf {
     option_env!("RUSTC_LIB_PATH").unwrap().into()
 }
 
-fn run_tests(mode: &str, path: &str, target: &str, mut flags: Vec<String>) {
+fn run_tests(mode: &str, path: &str, target: &str) {
     let in_rustc_test_suite = rustc_test_suite().is_some();
     // Add some flags we always want.
+    let mut flags = Vec::new();
     flags.push("--edition 2018".to_owned());
     if in_rustc_test_suite {
         // Less aggressive warnings to make the rustc toolstate management less painful.
@@ -38,6 +39,12 @@ fn run_tests(mode: &str, path: &str, target: &str, mut flags: Vec<String>) {
     if let Ok(sysroot) = std::env::var("MIRI_SYSROOT") {
         flags.push(format!("--sysroot {}", sysroot));
     }
+    if let Ok(extra_flags) = std::env::var("MIRI_TEST_FLAGS") {
+        flags.push(extra_flags);
+    }
+
+    let flags = flags.join(" ");
+    eprintln!("   Compiler flags: {}", flags);
 
     // The rest of the configuration.
     let mut config = compiletest::Config::default().tempdir();
@@ -51,48 +58,36 @@ fn run_tests(mode: &str, path: &str, target: &str, mut flags: Vec<String>) {
     config.host = get_host();
     config.src_base = PathBuf::from(path);
     config.target = target.to_owned();
-    config.target_rustcflags = Some(flags.join(" "));
+    config.target_rustcflags = Some(flags);
     compiletest::run_tests(&config);
 }
 
-fn compile_fail(path: &str, target: &str, opt: bool) {
-    let opt_str = if opt { " with optimizations" } else { "" };
+fn compile_fail(path: &str, target: &str) {
     eprintln!(
         "{}",
         format!(
-            "## Running compile-fail tests in {} against miri for target {}{}",
-            path, target, opt_str
+            "## Running compile-fail tests in {} against miri for target {}",
+            path, target
         )
         .green()
         .bold()
     );
 
-    let mut flags = Vec::new();
-    if opt {
-        flags.push("-Zmir-opt-level=3".to_owned());
-    }
-
-    run_tests("compile-fail", path, target, flags);
+    run_tests("compile-fail", path, target);
 }
 
-fn miri_pass(path: &str, target: &str, opt: bool) {
-    let opt_str = if opt { " with optimizations" } else { "" };
+fn miri_pass(path: &str, target: &str) {
     eprintln!(
         "{}",
         format!(
-            "## Running run-pass tests in {} against miri for target {}{}",
-            path, target, opt_str
+            "## Running run-pass tests in {} against miri for target {}",
+            path, target
         )
         .green()
         .bold()
     );
 
-    let mut flags = Vec::new();
-    if opt {
-        flags.push("-Zmir-opt-level=3".to_owned());
-    }
-
-    run_tests("ui", path, target, flags);
+    run_tests("ui", path, target);
 }
 
 fn get_host() -> String {
@@ -112,21 +107,11 @@ fn get_target() -> String {
     std::env::var("MIRI_TEST_TARGET").unwrap_or_else(|_| get_host())
 }
 
-fn run_pass_miri(opt: bool) {
-    miri_pass("tests/run-pass", &get_target(), opt);
-}
-
-fn compile_fail_miri(opt: bool) {
-    compile_fail("tests/compile-fail", &get_target(), opt);
-}
-
 fn test_runner(_tests: &[&()]) {
-    // Add a test env var to do environment communication tests
+    // Add a test env var to do environment communication tests.
     std::env::set_var("MIRI_ENV_VAR_TEST", "0");
 
-    run_pass_miri(false);
-    run_pass_miri(true);
-
-    compile_fail_miri(false);
-    compile_fail_miri(true);
+    let target = get_target();
+    miri_pass("tests/run-pass", &target);
+    compile_fail("tests/compile-fail", &target);
 }
diff --git a/travis.sh b/travis.sh
index 3d4b8651fb83..fec9145ab0f7 100755
--- a/travis.sh
+++ b/travis.sh
@@ -17,6 +17,7 @@ echo
 # Test
 function run_tests {
     ./miri test --locked
+    MIRI_TEST_FLAGS="-Z mir-opt-level=3" ./miri test
     # "miri test" has built the sysroot for us, now this should pass without
     # any interactive questions.
     test-cargo-miri/run-test.py