auto merge of rust-lang#6742 : Aatch/rust/mut-noalias, r=thestinger

This marks `&mut` function arguments with the `noalias` attribute. Since the borrow checker enforces this property, this is worth doing.

I'm not sure if the place I'm doing it in is ideal, but it generates the correct code.

Closes rust-lang#6350

Author: bors

src/librustc/middle/trans/base.rs

@@ -1694,7 +1694,20 @@ pub fn create_llargs_for_fn_args(cx: fn_ctxt,
     vec::from_fn(args.len(), |i| {
         unsafe {
             let arg_n = first_real_arg + i;
-            llvm::LLVMGetParam(cx.llfn, arg_n as c_uint)
+            let arg = &args[i];
+            let llarg = llvm::LLVMGetParam(cx.llfn, arg_n as c_uint);
+
+            // Mark `&mut T` as no-alias, as the borrowck pass ensures it's true
+            match arg.ty.node {
+                ast::ty_rptr(_, mt) => {
+                    if mt.mutbl == ast::m_mutbl  {
+                        llvm::LLVMAddAttribute(llarg, lib::llvm::NoAliasAttribute as c_uint);
+                    }
+                }
+                _ => {}
+            }
+
+            llarg
         }
     })
 }