diff --git a/src/librustc/middle/trans/base.rs b/src/librustc/middle/trans/base.rs
index 5070d678c9fab..f28b0c1b001d4 100644
--- a/src/librustc/middle/trans/base.rs
+++ b/src/librustc/middle/trans/base.rs
@@ -2753,6 +2753,9 @@ pub fn declare_intrinsics(llmod: ModuleRef) -> HashMap<&'static str, ValueRef> {
     ifn!("llvm.bswap.i32",[Type::i32()], Type::i32());
     ifn!("llvm.bswap.i64",[Type::i64()], Type::i64());
 
+    ifn!("llvm.expect.i32",[Type::i32(), Type::i32()], Type::i32());
+    ifn!("llvm.expect.i64",[Type::i64(), Type::i64()], Type::i64());
+
     return intrinsics;
 }
 
diff --git a/src/librustc/middle/trans/foreign.rs b/src/librustc/middle/trans/foreign.rs
index 20c8e69129d74..4ac315b32bb4c 100644
--- a/src/librustc/middle/trans/foreign.rs
+++ b/src/librustc/middle/trans/foreign.rs
@@ -939,6 +939,8 @@ pub fn trans_intrinsic(ccx: @mut CrateContext,
         "bswap16" => simple_llvm_intrinsic(bcx, "llvm.bswap.i16", 1),
         "bswap32" => simple_llvm_intrinsic(bcx, "llvm.bswap.i32", 1),
         "bswap64" => simple_llvm_intrinsic(bcx, "llvm.bswap.i64", 1),
+        "expect32" => simple_llvm_intrinsic(bcx, "llvm.expect.i32", 2),
+        "expect64" => simple_llvm_intrinsic(bcx, "llvm.expect.i64", 2),
         _ => {
             // Could we make this an enum rather than a string? does it get
             // checked earlier?
diff --git a/src/librustc/middle/trans/type_use.rs b/src/librustc/middle/trans/type_use.rs
index 42d5527ee43bc..0296e7675d9c2 100644
--- a/src/librustc/middle/trans/type_use.rs
+++ b/src/librustc/middle/trans/type_use.rs
@@ -167,6 +167,8 @@ pub fn type_uses_for(ccx: @mut CrateContext, fn_id: def_id, n_tps: uint)
 
                     "bswap16" | "bswap32" | "bswap64" => 0,
 
+                    "expect32" | "expect64" => 0,
+
                     // would be cool to make these an enum instead of
                     // strings!
                     _ => fail!("unknown intrinsic in type_use")
diff --git a/src/librustc/middle/typeck/check/mod.rs b/src/librustc/middle/typeck/check/mod.rs
index 7f486f77447cb..f5fe5caed4df3 100644
--- a/src/librustc/middle/typeck/check/mod.rs
+++ b/src/librustc/middle/typeck/check/mod.rs
@@ -3634,6 +3634,8 @@ pub fn check_intrinsic_type(ccx: @mut CrateCtxt, it: @ast::foreign_item) {
             "bswap16"  => (0, ~[ ty::mk_i16() ], ty::mk_i16()),
             "bswap32"  => (0, ~[ ty::mk_i32() ], ty::mk_i32()),
             "bswap64"  => (0, ~[ ty::mk_i64() ], ty::mk_i64()),
+            "expect32" => (0, ~[ ty::mk_u32(), ty::mk_u32() ], ty::mk_u32()),
+            "expect64" => (0, ~[ ty::mk_u64(), ty::mk_u64() ], ty::mk_u64()),
             ref other => {
                 tcx.sess.span_err(it.span,
                                   fmt!("unrecognized intrinsic function: `%s`",
diff --git a/src/libstd/macros.rs b/src/libstd/macros.rs
index 04058887970d0..64114f1d4c007 100644
--- a/src/libstd/macros.rs
+++ b/src/libstd/macros.rs
@@ -35,7 +35,7 @@ macro_rules! rtdebug (
 
 macro_rules! rtassert (
     ( $arg:expr ) => ( {
-        if !$arg {
+        if ::std::unstable::intrinsics::unlikely(!$arg) {
             rtabort!("assertion failed: %s", stringify!($arg));
         }
     } )
diff --git a/src/libstd/unstable/intrinsics.rs b/src/libstd/unstable/intrinsics.rs
index 1270a80c35499..2f8f5efa42c22 100644
--- a/src/libstd/unstable/intrinsics.rs
+++ b/src/libstd/unstable/intrinsics.rs
@@ -419,6 +419,11 @@ extern "rust-intrinsic" {
     pub fn bswap16(x: i16) -> i16;
     pub fn bswap32(x: i32) -> i32;
     pub fn bswap64(x: i64) -> i64;
+
+    #[cfg(not(stage0))]
+    pub fn expect32(value: u32, expected: u32) -> u32;
+    #[cfg(not(stage0))]
+    pub fn expect64(value: u64, expected: u64) -> u64;
 }
 
 #[cfg(target_endian = "little")] pub fn to_le16(x: i16) -> i16 { x }
@@ -434,3 +439,44 @@ extern "rust-intrinsic" {
 #[cfg(target_endian = "big")]    pub fn to_be32(x: i32) -> i32 { x }
 #[cfg(target_endian = "little")] pub fn to_be64(x: i64) -> i64 { unsafe { bswap64(x) } }
 #[cfg(target_endian = "big")]    pub fn to_be64(x: i64) -> i64 { x }
+
+#[cfg(not(stage0))]
+#[inline(always)]
+pub fn likely(cond: bool) -> bool {
+    unsafe {
+        expect32(cond as u32, 1) as bool
+    }
+}
+
+#[cfg(not(stage0))]
+#[inline(always)]
+pub fn unlikely(cond: bool) -> bool {
+    unsafe {
+        expect32(cond as u32, 0) as bool
+    }
+}
+
+#[cfg(stage0)]
+#[inline(always)]
+pub fn likely(cond: bool) -> bool {
+    cond
+}
+
+#[cfg(stage0)]
+#[inline(always)]
+pub fn unlikely(cond: bool) -> bool {
+    cond
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_likely_unlikely() {
+        assert_eq!(true, likely(true));
+        assert_eq!(false, likely(false));
+        assert_eq!(true, unlikely(true));
+        assert_eq!(false, unlikely(false));
+    }
+}
diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index a6c5526b5a9b4..7f8836fa5a667 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -799,18 +799,18 @@ pub fn std_macros() -> @str {
 
     macro_rules! assert(
         ($cond:expr) => {
-            if !$cond {
+            if ::std::unstable::intrinsics::unlikely(!$cond) {
                 ::std::sys::FailWithCause::fail_with(
                     \"assertion failed: \" + stringify!($cond), file!(), line!())
             }
         };
         ($cond:expr, $msg:expr) => {
-            if !$cond {
+            if ::std::unstable::intrinsics::unlikely(!$cond) {
                 ::std::sys::FailWithCause::fail_with($msg, file!(), line!())
             }
         };
         ($cond:expr, $( $arg:expr ),+) => {
-            if !$cond {
+            if ::std::unstable::intrinsics::unlikely(!$cond) {
                 ::std::sys::FailWithCause::fail_with(fmt!( $($arg),+ ), file!(), line!())
             }
         }
@@ -822,7 +822,9 @@ pub fn std_macros() -> @str {
                 let given_val = $given;
                 let expected_val = $expected;
                 // check both directions of equality....
-                if !((given_val == expected_val) && (expected_val == given_val)) {
+                if ::std::unstable::intrinsics::unlikely(
+                        !((given_val == expected_val) && (expected_val == given_val)))
+                {
                     fail!(\"assertion failed: `(left == right) && (right == \
                     left)` (left: `%?`, right: `%?`)\", given_val, expected_val);
                 }
@@ -838,10 +840,10 @@ pub fn std_macros() -> @str {
                 let given_val = $given;
                 let expected_val = $expected;
                 // check both directions of equality....
-                if !(
+                if ::std::unstable::intrinsics::unlikely(!(
                     given_val.approx_eq(&expected_val) &&
                     expected_val.approx_eq(&given_val)
-                ) {
+                )) {
                     fail!(\"left: %? does not approximately equal right: %?\",
                           given_val, expected_val);
                 }
@@ -855,10 +857,10 @@ pub fn std_macros() -> @str {
                 let expected_val = $expected;
                 let epsilon_val = $epsilon;
                 // check both directions of equality....
-                if !(
+                if ::std::unstable::intrinsics::unlikely(!(
                     given_val.approx_eq_eps(&expected_val, &epsilon_val) &&
                     expected_val.approx_eq_eps(&given_val, &epsilon_val)
-                ) {
+                )) {
                     fail!(\"left: %? does not approximately equal right: %? with epsilon: %?\",
                           given_val, expected_val, epsilon_val);
                 }