Add likely/unlikely intrinsics

Author: Aatch

src/libcore/intrinsics.rs

@@ -602,4 +602,9 @@ extern "rust-intrinsic" {
     /// Returns the value of the discriminant for the variant in 'v',
     /// cast to a `u64`; if `T` has no discriminant, returns 0.
     pub fn discriminant_value<T>(v: &T) -> u64;
+
+    #[cfg(not(stage0))]
+    pub fn likely(v: bool) -> bool;
+    #[cfg(not(stage0))]
+    pub fn unlikely(v: bool) -> bool;
 }

src/librustc_trans/trans/intrinsic.rs

@@ -763,6 +763,17 @@ pub fn trans_intrinsic_call<'a, 'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
             }
         }
 
+        (_, "likely") => {
+            let llfn = ccx.get_intrinsic(&("llvm.expect.i1"));
+            let expected = C_bool(ccx, true);
+            Call(bcx, llfn, &[llargs[0], expected], None, call_debug_location)
+        }
+        (_, "unlikely") => {
+            let llfn = ccx.get_intrinsic(&("llvm.expect.i1"));
+            let expected = C_bool(ccx, false);
+            Call(bcx, llfn, &[llargs[0], expected], None, call_debug_location)
+        }
+
         // This requires that atomic intrinsics follow a specific naming pattern:
         // "atomic_<operation>[_<ordering>]", and no ordering means SeqCst
         (_, name) if name.starts_with("atomic_") => {

src/librustc_typeck/check/mod.rs

@@ -5239,6 +5239,7 @@ pub fn check_intrinsic_type(ccx: &CrateCtxt, it: &ast::ForeignItem) {
                                     tcx.mk_region(ty::ReLateBound(ty::DebruijnIndex::new(1),
                                                                   ty::BrAnon(0))),
                                     param(ccx, 0))], tcx.types.u64),
+            "likely" | "unlikely" => (0, vec![tcx.types.bool], tcx.types.bool),
 
             ref other => {
                 span_err!(tcx.sess, it.span, E0093,