diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index 66a1c97246200..fcb78c479fa27 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -444,10 +444,11 @@ impl<'a> Builder<'a> {
 
             fn run(self, builder: &Builder) -> Interned<PathBuf> {
                 let compiler = self.compiler;
-                let lib = if compiler.stage >= 1 && builder.build.config.libdir.is_some() {
-                    builder.build.config.libdir.clone().unwrap()
+                let config = &builder.build.config;
+                let lib = if compiler.stage >= 1 && config.libdir_relative().is_some() {
+                    builder.build.config.libdir_relative().unwrap()
                 } else {
-                    PathBuf::from("lib")
+                    Path::new("lib")
                 };
                 let sysroot = builder.sysroot(self.compiler).join(lib)
                     .join("rustlib").join(self.target).join("lib");
diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index 2dcc0e0e7cd9f..c85b04ddc0245 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -516,8 +516,7 @@ fn rustc_cargo_env(build: &Build, cargo: &mut Command) {
          .env("CFG_VERSION", build.rust_version())
          .env("CFG_PREFIX", build.config.prefix.clone().unwrap_or_default());
 
-    let libdir_relative =
-        build.config.libdir.clone().unwrap_or(PathBuf::from("lib"));
+    let libdir_relative = build.config.libdir_relative().unwrap_or(Path::new("lib"));
     cargo.env("CFG_LIBDIR_RELATIVE", libdir_relative);
 
     // If we're not building a compiler with debugging information then remove
diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index 812ca6d64fb6a..3cf8f36df25ee 100644
--- a/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@ -17,7 +17,7 @@ use std::collections::{HashMap, HashSet};
 use std::env;
 use std::fs::File;
 use std::io::prelude::*;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 use std::process;
 use std::cmp;
 
@@ -564,6 +564,17 @@ impl Config {
         config
     }
 
+    /// Try to find the relative path of `libdir`.
+    pub fn libdir_relative(&self) -> Option<&Path> {
+        let libdir = self.libdir.as_ref()?;
+        if libdir.is_relative() {
+            Some(libdir)
+        } else {
+            // Try to make it relative to the prefix.
+            libdir.strip_prefix(self.prefix.as_ref()?).ok()
+        }
+    }
+
     pub fn verbose(&self) -> bool {
         self.verbose > 0
     }
diff --git a/src/librustc_driver/driver.rs b/src/librustc_driver/driver.rs
index f344624666a6c..b8a1fe9910540 100644
--- a/src/librustc_driver/driver.rs
+++ b/src/librustc_driver/driver.rs
@@ -660,6 +660,15 @@ pub fn phase_2_configure_and_expand_inner<'a, F>(sess: &'a Session,
         disambiguator,
     );
 
+    if sess.opts.incremental.is_some() {
+        time(time_passes, "garbage collect incremental cache directory", || {
+            if let Err(e) = rustc_incremental::garbage_collect_session_directories(sess) {
+                warn!("Error while trying to garbage collect incremental \
+                       compilation cache directory: {}", e);
+            }
+        });
+    }
+
     // If necessary, compute the dependency graph (in the background).
     let future_dep_graph = if sess.opts.build_dep_graph() {
         Some(rustc_incremental::load_dep_graph(sess, time_passes))
diff --git a/src/librustc_incremental/lib.rs b/src/librustc_incremental/lib.rs
index b53ee1daada42..65fbd9d0bf8f1 100644
--- a/src/librustc_incremental/lib.rs
+++ b/src/librustc_incremental/lib.rs
@@ -46,3 +46,4 @@ pub use persist::in_incr_comp_dir;
 pub use persist::prepare_session_directory;
 pub use persist::finalize_session_directory;
 pub use persist::delete_workproduct_files;
+pub use persist::garbage_collect_session_directories;
diff --git a/src/librustc_incremental/persist/fs.rs b/src/librustc_incremental/persist/fs.rs
index f4171f951f407..795825f180c9d 100644
--- a/src/librustc_incremental/persist/fs.rs
+++ b/src/librustc_incremental/persist/fs.rs
@@ -603,7 +603,7 @@ fn timestamp_to_string(timestamp: SystemTime) -> String {
 }
 
 fn string_to_timestamp(s: &str) -> Result<SystemTime, ()> {
-    let micros_since_unix_epoch = u64::from_str_radix(s, 36);
+    let micros_since_unix_epoch = u64::from_str_radix(s, INT_ENCODE_BASE as u32);
 
     if micros_since_unix_epoch.is_err() {
         return Err(())
@@ -733,6 +733,20 @@ pub fn garbage_collect_session_directories(sess: &Session) -> io::Result<()> {
                                 })
                                 .collect();
 
+    // Delete all session directories that don't have a lock file.
+    for directory_name in session_directories {
+        if !lock_file_to_session_dir.values().any(|dir| *dir == directory_name) {
+            let path = crate_directory.join(directory_name);
+            if let Err(err) = safe_remove_dir_all(&path) {
+                sess.warn(&format!("Failed to garbage collect invalid incremental \
+                                    compilation session directory `{}`: {}",
+                                    path.display(),
+                                    err));
+            }
+        }
+    }
+
+    // Now garbage collect the valid session directories.
     let mut deletion_candidates = vec![];
     let mut definitely_delete = vec![];
 
diff --git a/src/librustc_incremental/persist/mod.rs b/src/librustc_incremental/persist/mod.rs
index 82a43d85bc608..2f864aaefba89 100644
--- a/src/librustc_incremental/persist/mod.rs
+++ b/src/librustc_incremental/persist/mod.rs
@@ -20,9 +20,10 @@ mod save;
 mod work_product;
 mod file_format;
 
-pub use self::fs::prepare_session_directory;
 pub use self::fs::finalize_session_directory;
+pub use self::fs::garbage_collect_session_directories;
 pub use self::fs::in_incr_comp_dir;
+pub use self::fs::prepare_session_directory;
 pub use self::load::dep_graph_tcx_init;
 pub use self::load::load_dep_graph;
 pub use self::load::load_query_result_cache;