diff --git a/config.toml.example b/config.toml.example
index 5152a6c988582..f12ff76284589 100644
--- a/config.toml.example
+++ b/config.toml.example
@@ -14,6 +14,12 @@
 # =============================================================================
 [llvm]
 
+# Indicates whether LLVM rebuild should be skipped when running bootstrap. If
+# this is `false` then the compiler's LLVM will be rebuilt whenever the built
+# version doesn't have the correct hash. If it is `true` then LLVM will never
+# be rebuilt. The default value is `false`.
+#skip-rebuild = false
+
 # Indicates whether the LLVM build is a Release or Debug build
 #optimize = true
 
diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index 11bfc7a47cc48..0970a50bee47a 100644
--- a/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@ -67,6 +67,7 @@ pub struct Config {
     pub backtrace_on_ice: bool,
 
     // llvm codegen options
+    pub llvm_skip_rebuild: bool,
     pub llvm_assertions: bool,
     pub llvm_optimize: bool,
     pub llvm_thin_lto: bool,
@@ -244,6 +245,7 @@ struct Install {
 #[derive(Deserialize, Default)]
 #[serde(deny_unknown_fields, rename_all = "kebab-case")]
 struct Llvm {
+    skip_rebuild: Option<bool>,
     optimize: Option<bool>,
     thin_lto: Option<bool>,
     release_debuginfo: Option<bool>,
@@ -490,6 +492,7 @@ impl Config {
 
         // Store off these values as options because if they're not provided
         // we'll infer default values for them later
+        let mut llvm_skip_rebuild = None;
         let mut llvm_assertions = None;
         let mut debug = None;
         let mut debug_assertions = None;
@@ -511,6 +514,7 @@ impl Config {
             }
             set(&mut config.ninja, llvm.ninja);
             llvm_assertions = llvm.assertions;
+            llvm_skip_rebuild = llvm.skip_rebuild;
             set(&mut config.llvm_optimize, llvm.optimize);
             set(&mut config.llvm_thin_lto, llvm.thin_lto);
             set(&mut config.llvm_release_debuginfo, llvm.release_debuginfo);
@@ -617,6 +621,8 @@ impl Config {
         set(&mut config.initial_rustc, build.rustc.map(PathBuf::from));
         set(&mut config.initial_cargo, build.cargo.map(PathBuf::from));
 
+        config.llvm_skip_rebuild = llvm_skip_rebuild.unwrap_or(false);
+
         let default = false;
         config.llvm_assertions = llvm_assertions.unwrap_or(default);
 
diff --git a/src/bootstrap/native.rs b/src/bootstrap/native.rs
index afee154fe7143..4426ef5fb7e21 100644
--- a/src/bootstrap/native.rs
+++ b/src/bootstrap/native.rs
@@ -70,6 +70,13 @@ impl Step for Llvm {
         let done_stamp = out_dir.join("llvm-finished-building");
 
         if done_stamp.exists() {
+            if builder.config.llvm_skip_rebuild {
+                builder.info("Warning: \
+                              Using a potentially stale build of LLVM; \
+                              This may not behave well.");
+                return build_llvm_config;
+            }
+
             if let Some(llvm_commit) = llvm_info.sha() {
                 let done_contents = t!(fs::read(&done_stamp));
 
diff --git a/src/bootstrap/test.rs b/src/bootstrap/test.rs
index 58dc8ffc17da2..f2a7d1d00cf28 100644
--- a/src/bootstrap/test.rs
+++ b/src/bootstrap/test.rs
@@ -384,9 +384,6 @@ impl Step for Miri {
             );
             cargo.arg("--bin").arg("cargo-miri").arg("--").arg("miri").arg("setup");
 
-            // Tell `cargo miri` not to worry about the sysroot mismatch (we built with
-            // stage1 but run with stage2).
-            cargo.env("MIRI_SKIP_SYSROOT_CHECK", "1");
             // Tell `cargo miri setup` where to find the sources.
             cargo.env("XARGO_RUST_SRC", builder.src.join("src"));
             // Debug things.
diff --git a/src/liballoc/slice.rs b/src/liballoc/slice.rs
index 2f6d10c027be3..7b83658fca60d 100644
--- a/src/liballoc/slice.rs
+++ b/src/liballoc/slice.rs
@@ -450,7 +450,8 @@ impl<T> [T] {
         // and `rem` is the remaining part of `n`.
 
         // Using `Vec` to access `set_len()`.
-        let mut buf = Vec::with_capacity(self.len().checked_mul(n).expect("capacity overflow"));
+        let capacity = self.len().checked_mul(n).expect("capacity overflow");
+        let mut buf = Vec::with_capacity(capacity);
 
         // `2^expn` repetition is done by doubling `buf` `expn`-times.
         buf.extend(self);
@@ -476,7 +477,7 @@ impl<T> [T] {
 
         // `rem` (`= n - 2^expn`) repetition is done by copying
         // first `rem` repetitions from `buf` itself.
-        let rem_len = self.len() * n - buf.len(); // `self.len() * rem`
+        let rem_len = capacity - buf.len(); // `self.len() * rem`
         if rem_len > 0 {
             // `buf.extend(buf[0 .. rem_len])`:
             unsafe {
@@ -487,8 +488,7 @@ impl<T> [T] {
                     rem_len,
                 );
                 // `buf.len() + rem_len` equals to `buf.capacity()` (`= self.len() * n`).
-                let buf_cap = buf.capacity();
-                buf.set_len(buf_cap);
+                buf.set_len(capacity);
             }
         }
         buf
diff --git a/src/tools/miri b/src/tools/miri
index 048af409232fc..b1e97df8ee3d6 160000
--- a/src/tools/miri
+++ b/src/tools/miri
@@ -1 +1 @@
-Subproject commit 048af409232fc2d7f8fbe5469080dc8bb702c498
+Subproject commit b1e97df8ee3d67b74b3408579f29b017c4c317ea