diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index 8ec9c7f0109f3..31915b1169198 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -199,6 +199,10 @@ pub fn rustc<'a>(build: &'a Build, target: &str, compiler: &Compiler<'a>) {
     if !build.unstable_features {
         cargo.env("CFG_DISABLE_UNSTABLE_FEATURES", "1");
     }
+    // Flag that rust llvm is in use
+    if build.config.target_config.get(target).is_none() {
+        cargo.env("LLVM_RUSTLLVM", "1");
+    }
     cargo.env("LLVM_CONFIG", build.llvm_config(target));
     if build.config.llvm_static_stdcpp {
         cargo.env("LLVM_STATIC_STDCPP",
diff --git a/src/librustc_llvm/build.rs b/src/librustc_llvm/build.rs
index a2c808cbcb6b6..96b419d647ee3 100644
--- a/src/librustc_llvm/build.rs
+++ b/src/librustc_llvm/build.rs
@@ -112,6 +112,10 @@ fn main() {
         cfg.flag(&flag);
     }
 
+    if env::var_os("LLVM_RUSTLLVM").is_some() {
+        cfg.flag("-DLLVM_RUSTLLVM");
+    }
+
     cfg.file("../rustllvm/ExecutionEngineWrapper.cpp")
        .file("../rustllvm/PassWrapper.cpp")
        .file("../rustllvm/RustWrapper.cpp")