Auto merge of #36181 - seanmonstar:likely, r=nikomatsakis

core: add likely and unlikely intrinsics

I'm no good at reading assembly, but I have tried a stage1 compiler with this patch, and it does cause different asm output. Additionally, testing this compiler on my httparse crate with some `likely` usage added in to the branches does affect benchmarks. However, I'm sure a codegen test should be included, if anyone knows what it should look like.

There isn't an entry in `librustc_trans/context.rs` in this diff, because it already exists (`llvm.expect.i1` is used for array indices).

----

Even though this does affect httparse benchmarks, it doesn't seem to affect it the same way GCC's `__builtin_expect` affects picohttpparser. I was confused that the deviation on the benchmarks grew hugely when testing this, especially since I'm absolutely certain that the branchs where I added `likely` were always `true`. I chalk that up to GCC and LLVM handle branch prediction differently.

cc #26179

src/test/codegen/likely.rs

@@ -0,0 +1,41 @@
+// Copyright 2016 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.
+
+// compile-flags: -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+#![feature(core_intrinsics)]
+
+use std::intrinsics::{likely,unlikely};
+
+#[no_mangle]
+pub fn check_likely(x: i32, y: i32) -> Option<i32> {
+    unsafe {
+        // CHECK: call i1 @llvm.expect.i1(i1 %{{.*}}, i1 true)
+        if likely(x == y) {
+            None
+        } else {
+            Some(x + y)
+        }
+    }
+}
+
+#[no_mangle]
+pub fn check_unlikely(x: i32, y: i32) -> Option<i32> {
+    unsafe {
+        // CHECK: call i1 @llvm.expect.i1(i1 %{{.*}}, i1 false)
+        if unlikely(x == y) {
+            None
+        } else {
+            Some(x + y)
+        }
+    }
+}
+