Add test to catch performance regressions

src/librustc_trans/trans/intrinsic.rs

@@ -362,7 +362,7 @@ pub fn trans_intrinsic_call<'a, 'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
         (_, "init") => {
             let tp_ty = *substs.types.get(FnSpace, 0);
             if !return_type_is_void(ccx, tp_ty) {
-                // Just zero out the stack slot
+                // Just zero out the stack slot. (See comment on base::memzero for explaination)
                 zero_mem(bcx, llresult, tp_ty);
             }
             C_nil(ccx)

src/test/run-pass/init-large-type.rs

@@ -0,0 +1,25 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Makes sure that zero-initializing large types is reasonably fast,
+// Doing it incorrectly causes massive slowdown in LLVM during
+// optimisation.
+
+#![feature(intrinsics)]
+
+extern "rust-intrinsic" {
+    pub fn init<T>() -> T;
+}
+
+const SIZE: usize = 512 * 1024 * 1024;
+
+fn main() {
+    let _memory: [u8; SIZE] = unsafe { init() };
+}