add sret + noalias to the out pointer parameter

This brings Rust in line with how `clang` handles return pointers.

Example:

    pub fn bar() -> [uint, .. 8] {
        let a = [0, .. 8];
        a
    }

Before:

    ; Function Attrs: nounwind uwtable
    define void @_ZN3bar17ha4635c6f704bfa334v0.0E([8 x i64]* nocapture, { i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone) #1 {
    "function top level":
      %a = alloca [8 x i64], align 8
      %2 = bitcast [8 x i64]* %a to i8*
      call void @llvm.memset.p0i8.i64(i8* %2, i8 0, i64 64, i32 8, i1 false)
      %3 = bitcast [8 x i64]* %0 to i8*
      call void @llvm.memcpy.p0i8.p0i8.i64(i8* %3, i8* %2, i64 64, i32 8, i1 false)
      ret void
    }

After:

    ; Function Attrs: nounwind uwtable
    define void @_ZN3bar17ha4635c6f704bfa334v0.0E([8 x i64]* noalias nocapture sret, { i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone) #1 {
    "function top level":
      %2 = bitcast [8 x i64]* %0 to i8*
      call void @llvm.memset.p0i8.i64(i8* %2, i8 0, i64 64, i32 8, i1 false)
      ret void
    }

Closes rust-lang#9072
Closes rust-lang#7298
Closes rust-lang#9154

src/librustc/middle/trans/base.rs

@@ -248,6 +248,17 @@ pub fn decl_rust_fn(ccx: &mut CrateContext, inputs: &[ty::t], output: ty::t,
         }
     }
 
+    // The out pointer will never alias with any other pointers, as the object only exists at a
+    // language level after the call. It can also be tagged with SRet to indicate that it is
+    // guaranteed to point to a usable block of memory for the type.
+    if uses_outptr {
+        unsafe {
+            let outptr = llvm::LLVMGetParam(llfn, 0);
+            llvm::LLVMAddAttribute(outptr, lib::llvm::StructRetAttribute as c_uint);
+            llvm::LLVMAddAttribute(outptr, lib::llvm::NoAliasAttribute as c_uint);
+        }
+    }
+
     llfn
 }
 

src/librustc/middle/trans/callee.rs

@@ -20,7 +20,7 @@ use std::vec;
 
 use back::abi;
 use driver::session;
-use lib::llvm::ValueRef;
+use lib::llvm::{ValueRef, NoAliasAttribute, StructRetAttribute};
 use lib::llvm::llvm;
 use metadata::csearch;
 use middle::trans::base;
@@ -707,7 +707,21 @@ pub fn trans_call_inner(in_cx: @mut Block,
             }
 
             // Invoke the actual rust fn and update bcx/llresult.
-            let (llret, b) = base::invoke(bcx, llfn, llargs, []);
+            let mut attrs = ~[];
+            if type_of::return_uses_outptr(in_cx.tcx(), ret_ty) {
+                attrs.push((1, StructRetAttribute));
+            }
+
+            match ty::get(ret_ty).sty {
+                // `~` pointer return values never alias because ownership is transferred
+                ty::ty_uniq(*) |
+                ty::ty_evec(_, ty::vstore_uniq) => {
+                    attrs.push((0, NoAliasAttribute));
+                }
+                _ => ()
+            }
+
+            let (llret, b) = base::invoke(bcx, llfn, llargs, attrs);
             bcx = b;
             llresult = llret;
 

src/librustc/middle/trans/foreign.rs

@@ -497,7 +497,7 @@ pub fn trans_rust_fn_with_foreign_abi(ccx: @mut CrateContext,
             // Rust expects to use an outpointer. If the foreign fn
             // also uses an outpointer, we can reuse it, but the types
             // may vary, so cast first to the Rust type. If the
-            // foriegn fn does NOT use an outpointer, we will have to
+            // foreign fn does NOT use an outpointer, we will have to
             // alloca some scratch space on the stack.
             match foreign_outptr {
                 Some(llforeign_outptr) => {