Add assume intrinsic

Adds an `assume` intrinsic that gets translated to llvm.assume. It is
used on a boolean expression and allows the optimizer to assume that
the expression is true.

This implements #18051.

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();
 

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

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))

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);

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);
+}
+