diff --git a/src/librustc/session/mod.rs b/src/librustc/session/mod.rs
index d3005ff2ded2..dfde3ded46d1 100644
--- a/src/librustc/session/mod.rs
+++ b/src/librustc/session/mod.rs
@@ -30,13 +30,16 @@ use syntax::{ast, codemap};
 use syntax::feature_gate::AttributeType;
 
 use rustc_back::target::Target;
+use llvm;
 
 use std::path::{Path, PathBuf};
 use std::cell::{Cell, RefCell};
 use std::collections::{HashMap, HashSet};
 use std::env;
+use std::ffi::CString;
 use std::rc::Rc;
 use std::fmt;
+use libc::c_int;
 
 pub mod config;
 pub mod filesearch;
@@ -491,9 +494,63 @@ pub fn build_session_(sopts: config::Options,
         imported_macro_spans: RefCell::new(HashMap::new()),
     };
 
+    init_llvm(&sess);
+
     sess
 }
 
+fn init_llvm(sess: &Session) {
+    unsafe {
+        // Before we touch LLVM, make sure that multithreading is enabled.
+        use std::sync::Once;
+        static INIT: Once = Once::new();
+        static mut POISONED: bool = false;
+        INIT.call_once(|| {
+            if llvm::LLVMStartMultithreaded() != 1 {
+                // use an extra bool to make sure that all future usage of LLVM
+                // cannot proceed despite the Once not running more than once.
+                POISONED = true;
+            }
+
+            configure_llvm(sess);
+        });
+
+        if POISONED {
+            bug!("couldn't enable multi-threaded LLVM");
+        }
+    }
+}
+
+unsafe fn configure_llvm(sess: &Session) {
+    let mut llvm_c_strs = Vec::new();
+    let mut llvm_args = Vec::new();
+
+    {
+        let mut add = |arg: &str| {
+            let s = CString::new(arg).unwrap();
+            llvm_args.push(s.as_ptr());
+            llvm_c_strs.push(s);
+        };
+        add("rustc"); // fake program name
+        if sess.time_llvm_passes() { add("-time-passes"); }
+        if sess.print_llvm_passes() { add("-debug-pass=Structure"); }
+
+        // FIXME #21627 disable faulty FastISel on AArch64 (even for -O0)
+        if sess.target.target.arch == "aarch64" { add("-fast-isel=0"); }
+
+        for arg in &sess.opts.cg.llvm_args {
+            add(&(*arg));
+        }
+    }
+
+    llvm::LLVMInitializePasses();
+
+    llvm::initialize_available_targets();
+
+    llvm::LLVMRustSetLLVMOptions(llvm_args.len() as c_int,
+                                 llvm_args.as_ptr());
+}
+
 pub fn early_error(output: config::ErrorOutputType, msg: &str) -> ! {
     let mut emitter: Box<Emitter> = match output {
         config::ErrorOutputType::HumanReadable(color_config) => {
diff --git a/src/librustc_trans/back/write.rs b/src/librustc_trans/back/write.rs
index e6a51eb7c87e..8a915f044053 100644
--- a/src/librustc_trans/back/write.rs
+++ b/src/librustc_trans/back/write.rs
@@ -31,7 +31,7 @@ use std::str;
 use std::sync::{Arc, Mutex};
 use std::sync::mpsc::channel;
 use std::thread;
-use libc::{c_uint, c_int, c_void};
+use libc::{c_uint, c_void};
 
 pub fn llvm_err(handler: &errors::Handler, msg: String) -> ! {
     match llvm::last_error() {
@@ -984,36 +984,6 @@ pub fn run_assembler(sess: &Session, outputs: &OutputFilenames) {
     }
 }
 
-pub unsafe fn configure_llvm(sess: &Session) {
-    let mut llvm_c_strs = Vec::new();
-    let mut llvm_args = Vec::new();
-
-    {
-        let mut add = |arg: &str| {
-            let s = CString::new(arg).unwrap();
-            llvm_args.push(s.as_ptr());
-            llvm_c_strs.push(s);
-        };
-        add("rustc"); // fake program name
-        if sess.time_llvm_passes() { add("-time-passes"); }
-        if sess.print_llvm_passes() { add("-debug-pass=Structure"); }
-
-        // FIXME #21627 disable faulty FastISel on AArch64 (even for -O0)
-        if sess.target.target.arch == "aarch64" { add("-fast-isel=0"); }
-
-        for arg in &sess.opts.cg.llvm_args {
-            add(&(*arg));
-        }
-    }
-
-    llvm::LLVMInitializePasses();
-
-    llvm::initialize_available_targets();
-
-    llvm::LLVMRustSetLLVMOptions(llvm_args.len() as c_int,
-                                 llvm_args.as_ptr());
-}
-
 pub unsafe fn with_llvm_pmb(llmod: ModuleRef,
                             config: &ModuleConfig,
                             f: &mut FnMut(llvm::PassManagerBuilderRef)) {
diff --git a/src/librustc_trans/base.rs b/src/librustc_trans/base.rs
index 17230eff6e63..6304a0eacd3b 100644
--- a/src/librustc_trans/base.rs
+++ b/src/librustc_trans/base.rs
@@ -2709,26 +2709,6 @@ pub fn trans_crate<'tcx>(tcx: &TyCtxt<'tcx>,
         tcx.sess.opts.debug_assertions
     };
 
-    // Before we touch LLVM, make sure that multithreading is enabled.
-    unsafe {
-        use std::sync::Once;
-        static INIT: Once = Once::new();
-        static mut POISONED: bool = false;
-        INIT.call_once(|| {
-            if llvm::LLVMStartMultithreaded() != 1 {
-                // use an extra bool to make sure that all future usage of LLVM
-                // cannot proceed despite the Once not running more than once.
-                POISONED = true;
-            }
-
-            ::back::write::configure_llvm(&tcx.sess);
-        });
-
-        if POISONED {
-            bug!("couldn't enable multi-threaded LLVM");
-        }
-    }
-
     let link_meta = link::build_link_meta(&tcx, name);
 
     let codegen_units = tcx.sess.opts.cg.codegen_units;