diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index 3f7dcb36cf698..5570d356149dc 100644
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -256,6 +256,13 @@ extern "rust-intrinsic" {
     /// NB: This is very different from the `unreachable!()` macro!
     pub fn unreachable() -> !;
 
+    /// Inform the optimizer that a condition is always true.
+    /// If the condition is false, the behavior is undefined.
+    ///
+    /// No code is generated for this intrisic.
+    #[cfg(not(stage0))]
+    pub fn assume(b: bool);
+
     /// Execute a breakpoint trap, for inspection by a debugger.
     pub fn breakpoint();
 
diff --git a/src/librustc/middle/trans/context.rs b/src/librustc/middle/trans/context.rs
index ee5ba61a295cf..d7ab02f373cf1 100644
--- a/src/librustc/middle/trans/context.rs
+++ b/src/librustc/middle/trans/context.rs
@@ -855,6 +855,7 @@ fn declare_intrinsic(ccx: &CrateContext, key: & &'static str) -> Option<ValueRef
     ifn!("llvm.lifetime.end" fn(t_i64, i8p) -> void);
 
     ifn!("llvm.expect.i1" fn(i1, i1) -> i1);
+    ifn!("llvm.assume" fn(i1) -> void);
 
     // Some intrinsics were introduced in later versions of LLVM, but they have
     // fallbacks in libc or libm and such. Currently, all of these intrinsics
diff --git a/src/librustc/middle/trans/intrinsic.rs b/src/librustc/middle/trans/intrinsic.rs
index f463f258ad550..7951b3be0e573 100644
--- a/src/librustc/middle/trans/intrinsic.rs
+++ b/src/librustc/middle/trans/intrinsic.rs
@@ -81,6 +81,7 @@ pub fn get_simple_intrinsic(ccx: &CrateContext, item: &ast::ForeignItem) -> Opti
         "bswap16" => "llvm.bswap.i16",
         "bswap32" => "llvm.bswap.i32",
         "bswap64" => "llvm.bswap.i64",
+        "assume" => "llvm.assume",
         _ => return None
     };
     Some(ccx.get_intrinsic(&name))
diff --git a/src/librustc/middle/typeck/check/mod.rs b/src/librustc/middle/typeck/check/mod.rs
index d88e01f6e495f..296a6b40e54fc 100644
--- a/src/librustc/middle/typeck/check/mod.rs
+++ b/src/librustc/middle/typeck/check/mod.rs
@@ -5754,6 +5754,8 @@ pub fn check_intrinsic_type(ccx: &CrateCtxt, it: &ast::ForeignItem) {
 
             "return_address" => (0, vec![], ty::mk_imm_ptr(tcx, ty::mk_u8())),
 
+            "assume" => (0, vec![ty::mk_bool()], ty::mk_nil()),
+
             ref other => {
                 span_err!(tcx.sess, it.span, E0093,
                     "unrecognized intrinsic function: `{}`", *other);
diff --git a/src/test/run-pass/intrinsic-assume.rs b/src/test/run-pass/intrinsic-assume.rs
new file mode 100644
index 0000000000000..abf9b94c59d8a
--- /dev/null
+++ b/src/test/run-pass/intrinsic-assume.rs
@@ -0,0 +1,25 @@
+// Copyright 2014 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.
+
+use std::intrinsics::assume;
+
+unsafe fn f(x: i32) -> i32 {
+    assume(x == 34);
+    match x {
+        34 => 42,
+        _  => 30
+    }
+}
+
+fn main() {
+    let x = unsafe { f(34) };
+    assert_eq!(x, 42);
+}
+