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

Author: bors

src/libcore/intrinsics.rs

@@ -194,6 +194,20 @@ extern "rust-intrinsic" {
     /// own, or if it does not enable any significant optimizations.
     pub fn assume(b: bool);
 
+    #[cfg(not(stage0))]
+    /// Hints to the compiler that branch condition is likely to be true.
+    /// Returns the value passed to it.
+    ///
+    /// Any use other than with `if` statements will probably not have an effect.
+    pub fn likely(b: bool) -> bool;
+
+    #[cfg(not(stage0))]
+    /// Hints to the compiler that branch condition is likely to be false.
+    /// Returns the value passed to it.
+    ///
+    /// Any use other than with `if` statements will probably not have an effect.
+    pub fn unlikely(b: bool) -> bool;
+
     /// Executes a breakpoint trap, for inspection by a debugger.
     pub fn breakpoint();
 

src/librustc_trans/intrinsic.rs

@@ -136,6 +136,14 @@ pub fn trans_intrinsic_call<'a, 'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
         (Some(llfn), _) => {
             Call(bcx, llfn, &llargs, call_debug_location)
         }
+        (_, "likely") => {
+            let expect = ccx.get_intrinsic(&("llvm.expect.i1"));
+            Call(bcx, expect, &[llargs[0], C_bool(ccx, true)], call_debug_location)
+        }
+        (_, "unlikely") => {
+            let expect = ccx.get_intrinsic(&("llvm.expect.i1"));
+            Call(bcx, expect, &[llargs[0], C_bool(ccx, false)], call_debug_location)
+        }
         (_, "try") => {
             bcx = try_intrinsic(bcx, llargs[0], llargs[1], llargs[2], llresult,
                                 call_debug_location);

src/librustc_typeck/check/intrinsic.rs

@@ -285,6 +285,8 @@ pub fn check_intrinsic_type(ccx: &CrateCtxt, it: &hir::ForeignItem) {
                 (1, vec![param(ccx, 0), param(ccx, 0)], param(ccx, 0)),
 
             "assume" => (0, vec![tcx.types.bool], tcx.mk_nil()),
+            "likely" => (0, vec![tcx.types.bool], tcx.types.bool),
+            "unlikely" => (0, vec![tcx.types.bool], tcx.types.bool),
 
             "discriminant_value" => (1, vec![
                     tcx.mk_imm_ref(tcx.mk_region(ty::ReLateBound(ty::DebruijnIndex::new(1),

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)
+        }
+    }
+}
+