diff --git a/src/Cargo.lock b/src/Cargo.lock
index 93bbf0f227b..ec45f45ccb6 100644
--- a/src/Cargo.lock
+++ b/src/Cargo.lock
@@ -61,6 +61,9 @@ dependencies = [
 [[package]]
 name = "build_helper"
 version = "0.1.0"
+dependencies = [
+ "filetime 0.1.10 (registry+https://github.com/rust-lang/crates.io-index)",
+]
 
 [[package]]
 name = "cargotest"
diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index 7c35151a6d2..776b91028a1 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -21,10 +21,10 @@ use std::fs::{self, File};
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
-use build_helper::output;
+use build_helper::{output, mtime};
 use filetime::FileTime;
 
-use util::{exe, libdir, mtime, is_dylib, copy};
+use util::{exe, libdir, is_dylib, copy};
 use {Build, Compiler, Mode};
 
 /// Build the standard library.
diff --git a/src/bootstrap/doc.rs b/src/bootstrap/doc.rs
index d1c9918a733..3dc9b837555 100644
--- a/src/bootstrap/doc.rs
+++ b/src/bootstrap/doc.rs
@@ -22,7 +22,8 @@ use std::io::prelude::*;
 use std::process::Command;
 
 use {Build, Compiler, Mode};
-use util::{up_to_date, cp_r};
+use util::cp_r;
+use build_helper::up_to_date;
 
 /// Invoke `rustbook` as compiled in `stage` for `target` for the doc book
 /// `name` into the `out` path.
diff --git a/src/bootstrap/lib.rs b/src/bootstrap/lib.rs
index 52944f36996..df1218752d1 100644
--- a/src/bootstrap/lib.rs
+++ b/src/bootstrap/lib.rs
@@ -66,6 +66,7 @@
 
 #![deny(warnings)]
 
+#[macro_use]
 extern crate build_helper;
 extern crate cmake;
 extern crate filetime;
@@ -83,24 +84,9 @@ use std::fs::{self, File};
 use std::path::{Component, PathBuf, Path};
 use std::process::Command;
 
-use build_helper::{run_silent, output};
+use build_helper::{run_silent, output, mtime};
 
-use util::{exe, mtime, libdir, add_lib_path};
-
-/// A helper macro to `unwrap` a result except also print out details like:
-///
-/// * The file/line of the panic
-/// * The expression that failed
-/// * The error itself
-///
-/// This is currently used judiciously throughout the build system rather than
-/// using a `Result` with `try!`, but this may change one day...
-macro_rules! t {
-    ($e:expr) => (match $e {
-        Ok(e) => e,
-        Err(e) => panic!("{} failed with {}", stringify!($e), e),
-    })
-}
+use util::{exe, libdir, add_lib_path};
 
 mod cc;
 mod channel;
diff --git a/src/bootstrap/native.rs b/src/bootstrap/native.rs
index 4b6fef8edc1..21fc61cc814 100644
--- a/src/bootstrap/native.rs
+++ b/src/bootstrap/native.rs
@@ -28,7 +28,8 @@ use cmake;
 use gcc;
 
 use Build;
-use util::{self, up_to_date};
+use util;
+use build_helper::up_to_date;
 
 /// Compile LLVM for `target`.
 pub fn llvm(build: &Build, target: &str) {
diff --git a/src/bootstrap/util.rs b/src/bootstrap/util.rs
index 2ab3776ada0..520514f5fc9 100644
--- a/src/bootstrap/util.rs
+++ b/src/bootstrap/util.rs
@@ -20,8 +20,6 @@ use std::path::{Path, PathBuf};
 use std::process::Command;
 use std::time::Instant;
 
-use filetime::FileTime;
-
 /// Returns the `name` as the filename of a static library for `target`.
 pub fn staticlib(name: &str, target: &str) -> String {
     if target.contains("windows") {
@@ -31,13 +29,6 @@ pub fn staticlib(name: &str, target: &str) -> String {
     }
 }
 
-/// Returns the last-modified time for `path`, or zero if it doesn't exist.
-pub fn mtime(path: &Path) -> FileTime {
-    fs::metadata(path).map(|f| {
-        FileTime::from_last_modification_time(&f)
-    }).unwrap_or(FileTime::zero())
-}
-
 /// Copies a file from `src` to `dst`, attempting to use hard links and then
 /// falling back to an actually filesystem copy if necessary.
 pub fn copy(src: &Path, dst: &Path) {
@@ -132,34 +123,6 @@ pub fn add_lib_path(path: Vec<PathBuf>, cmd: &mut Command) {
     cmd.env(dylib_path_var(), t!(env::join_paths(list)));
 }
 
-/// Returns whether `dst` is up to date given that the file or files in `src`
-/// are used to generate it.
-///
-/// Uses last-modified time checks to verify this.
-pub fn up_to_date(src: &Path, dst: &Path) -> bool {
-    let threshold = mtime(dst);
-    let meta = match fs::metadata(src) {
-        Ok(meta) => meta,
-        Err(e) => panic!("source {:?} failed to get metadata: {}", src, e),
-    };
-    if meta.is_dir() {
-        dir_up_to_date(src, &threshold)
-    } else {
-        FileTime::from_last_modification_time(&meta) <= threshold
-    }
-}
-
-fn dir_up_to_date(src: &Path, threshold: &FileTime) -> bool {
-    t!(fs::read_dir(src)).map(|e| t!(e)).all(|e| {
-        let meta = t!(e.metadata());
-        if meta.is_dir() {
-            dir_up_to_date(&e.path(), threshold)
-        } else {
-            FileTime::from_last_modification_time(&meta) < *threshold
-        }
-    })
-}
-
 /// Returns the environment variable which the dynamic library lookup path
 /// resides in for this platform.
 pub fn dylib_path_var() -> &'static str {
diff --git a/src/build_helper/Cargo.toml b/src/build_helper/Cargo.toml
index 01d704f816b..f8ade0616a5 100644
--- a/src/build_helper/Cargo.toml
+++ b/src/build_helper/Cargo.toml
@@ -6,3 +6,6 @@ authors = ["The Rust Project Developers"]
 [lib]
 name = "build_helper"
 path = "lib.rs"
+
+[dependencies]
+filetime = "0.1"
diff --git a/src/build_helper/lib.rs b/src/build_helper/lib.rs
index 7784584baf4..3dfd2938082 100644
--- a/src/build_helper/lib.rs
+++ b/src/build_helper/lib.rs
@@ -10,9 +10,30 @@
 
 #![deny(warnings)]
 
+extern crate filetime;
+
+use std::fs;
 use std::process::{Command, Stdio};
 use std::path::{Path, PathBuf};
 
+use filetime::FileTime;
+
+/// A helper macro to `unwrap` a result except also print out details like:
+///
+/// * The file/line of the panic
+/// * The expression that failed
+/// * The error itself
+///
+/// This is currently used judiciously throughout the build system rather than
+/// using a `Result` with `try!`, but this may change one day...
+#[macro_export]
+macro_rules! t {
+    ($e:expr) => (match $e {
+        Ok(e) => e,
+        Err(e) => panic!("{} failed with {}", stringify!($e), e),
+    })
+}
+
 pub fn run(cmd: &mut Command) {
     println!("running: {:?}", cmd);
     run_silent(cmd);
@@ -103,6 +124,41 @@ pub fn rerun_if_changed_anything_in_dir(dir: &Path) {
     }
 }
 
+/// Returns the last-modified time for `path`, or zero if it doesn't exist.
+pub fn mtime(path: &Path) -> FileTime {
+    fs::metadata(path).map(|f| {
+        FileTime::from_last_modification_time(&f)
+    }).unwrap_or(FileTime::zero())
+}
+
+/// Returns whether `dst` is up to date given that the file or files in `src`
+/// are used to generate it.
+///
+/// Uses last-modified time checks to verify this.
+pub fn up_to_date(src: &Path, dst: &Path) -> bool {
+    let threshold = mtime(dst);
+    let meta = match fs::metadata(src) {
+        Ok(meta) => meta,
+        Err(e) => panic!("source {:?} failed to get metadata: {}", src, e),
+    };
+    if meta.is_dir() {
+        dir_up_to_date(src, &threshold)
+    } else {
+        FileTime::from_last_modification_time(&meta) <= threshold
+    }
+}
+
+fn dir_up_to_date(src: &Path, threshold: &FileTime) -> bool {
+    t!(fs::read_dir(src)).map(|e| t!(e)).all(|e| {
+        let meta = t!(e.metadata());
+        if meta.is_dir() {
+            dir_up_to_date(&e.path(), threshold)
+        } else {
+            FileTime::from_last_modification_time(&meta) < *threshold
+        }
+    })
+}
+
 fn fail(s: &str) -> ! {
     println!("\n\n{}\n\n", s);
     std::process::exit(1);
diff --git a/src/liballoc_jemalloc/build.rs b/src/liballoc_jemalloc/build.rs
index c982f98e63d..7e616c0ff27 100644
--- a/src/liballoc_jemalloc/build.rs
+++ b/src/liballoc_jemalloc/build.rs
@@ -10,13 +10,15 @@
 
 #![deny(warnings)]
 
+#[macro_use]
 extern crate build_helper;
 extern crate gcc;
 
-use std::{env, fs};
-use std::path::PathBuf;
+use std::env;
+use std::fs::{self, File};
+use std::path::{Path, PathBuf};
 use std::process::Command;
-use build_helper::{run, rerun_if_changed_anything_in_dir};
+use build_helper::{run, rerun_if_changed_anything_in_dir, up_to_date};
 
 fn main() {
     println!("cargo:rustc-cfg=cargobuild");
@@ -69,12 +71,13 @@ fn main() {
     } else if !target.contains("windows") && !target.contains("musl") {
         println!("cargo:rustc-link-lib=pthread");
     }
-    if !cfg!(stage0) && target == host {
+    let src_dir = env::current_dir().unwrap().join("../jemalloc");
+    rerun_if_changed_anything_in_dir(&src_dir);
+    let timestamp = build_dir.join("rustbuild.timestamp");
+    if up_to_date(&Path::new("build.rs"), &timestamp) && up_to_date(&src_dir, &timestamp) {
         return
     }
 
-    let src_dir = env::current_dir().unwrap().join("../jemalloc");
-    rerun_if_changed_anything_in_dir(&src_dir);
     let compiler = gcc::Config::new().get_compiler();
     // only msvc returns None for ar so unwrap is okay
     let ar = build_helper::cc2ar(compiler.path(), &target).unwrap();
@@ -184,4 +187,6 @@ fn main() {
             .file("pthread_atfork_dummy.c")
             .compile("libpthread_atfork_dummy.a");
     }
+
+    t!(File::create(&timestamp));
 }
diff --git a/src/libstd/build.rs b/src/libstd/build.rs
index 112e48921cb..a0844821709 100644
--- a/src/libstd/build.rs
+++ b/src/libstd/build.rs
@@ -10,13 +10,15 @@
 
 #![deny(warnings)]
 
-extern crate gcc;
+#[macro_use]
 extern crate build_helper;
+extern crate gcc;
 
-use std::{env, fs};
-use std::path::PathBuf;
+use std::env;
+use std::fs::{self, File};
+use std::path::{Path, PathBuf};
 use std::process::Command;
-use build_helper::{run, rerun_if_changed_anything_in_dir};
+use build_helper::{run, rerun_if_changed_anything_in_dir, up_to_date};
 
 fn main() {
     println!("cargo:rustc-cfg=cargobuild");
@@ -71,12 +73,13 @@ fn build_libbacktrace(host: &str, target: &str) {
 
     println!("cargo:rustc-link-lib=static=backtrace");
     println!("cargo:rustc-link-search=native={}/.libs", build_dir.display());
-    if !cfg!(stage0) && target == host {
+    let src_dir = env::current_dir().unwrap().join("../libbacktrace");
+    rerun_if_changed_anything_in_dir(&src_dir);
+    let timestamp = build_dir.join("rustbuild.timestamp");
+    if up_to_date(&Path::new("build.rs"), &timestamp) && up_to_date(&src_dir, &timestamp) {
         return
     }
 
-    let src_dir = env::current_dir().unwrap().join("../libbacktrace");
-    rerun_if_changed_anything_in_dir(&src_dir);
     let compiler = gcc::Config::new().get_compiler();
     // only msvc returns None for ar so unwrap is okay
     let ar = build_helper::cc2ar(compiler.path(), target).unwrap();
@@ -103,4 +106,6 @@ fn build_libbacktrace(host: &str, target: &str) {
                 .current_dir(&build_dir)
                 .arg(format!("INCDIR={}", src_dir.display()))
                 .arg("-j").arg(env::var("NUM_JOBS").expect("NUM_JOBS was not set")));
+
+    t!(File::create(&timestamp));
 }