Rollup merge of #23322 - dotdash:jemalloc_attrs, r=brson

 When this attribute is applied to a function, its return value gets the
noalias attribute, which is how you tell LLVM that the function returns
a \"new\" pointer that doesn't alias anything accessible to the caller,
i.e. it acts like a memory allocator.

Plain malloc doesn't need this attribute because LLVM already knows
about malloc and adds the attribute itself.

Author: Manishearth

src/liballoc/heap.rs

@@ -198,6 +198,7 @@ mod imp {
     extern {}
 
     extern {
+        #[allocator]
         fn je_mallocx(size: size_t, flags: c_int) -> *mut c_void;
         fn je_rallocx(ptr: *mut c_void, size: size_t, flags: c_int) -> *mut c_void;
         fn je_xallocx(ptr: *mut c_void, size: size_t, extra: size_t, flags: c_int) -> size_t;

src/liballoc/lib.rs

@@ -69,6 +69,7 @@
 
 #![feature(no_std)]
 #![no_std]
+#![feature(allocator)]
 #![feature(lang_items, unsafe_destructor)]
 #![feature(box_syntax)]
 #![feature(optin_builtin_traits)]

src/librustc_trans/trans/base.rs

@@ -33,7 +33,7 @@ use super::ModuleTranslation;
 use back::link::{mangle_exported_name};
 use back::{link, abi};
 use lint;
-use llvm::{BasicBlockRef, Linkage, ValueRef, Vector, get_param};
+use llvm::{AttrHelper, BasicBlockRef, Linkage, ValueRef, Vector, get_param};
 use llvm;
 use metadata::{csearch, encoder, loader};
 use middle::astencode;
@@ -456,6 +456,9 @@ pub fn set_llvm_fn_attrs(ccx: &CrateContext, attrs: &[ast::Attribute], llfn: Val
                                                llvm::FunctionIndex as c_uint,
                                                llvm::ColdAttribute as uint64_t)
             },
+            "allocator" => {
+                llvm::NoAliasAttribute.apply_llfn(llvm::ReturnIndex as c_uint, llfn);
+            }
             _ => used = false,
         }
         if used {
@@ -903,8 +906,10 @@ pub fn trans_external_path<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
                     ccx.sess().bug("unexpected intrinsic in trans_external_path")
                 }
                 _ => {
-                    foreign::register_foreign_item_fn(ccx, fn_ty.abi, t,
-                                                      &name[..])
+                    let llfn = foreign::register_foreign_item_fn(ccx, fn_ty.abi, t, &name[..]);
+                    let attrs = csearch::get_item_attrs(&ccx.sess().cstore, did);
+                    set_llvm_fn_attrs(ccx, &attrs, llfn);
+                    llfn
                 }
             }
         }
@@ -2841,7 +2846,9 @@ pub fn get_item_val(ccx: &CrateContext, id: ast::NodeId) -> ValueRef {
                     let abi = ccx.tcx().map.get_foreign_abi(id);
                     let ty = ty::node_id_to_type(ccx.tcx(), ni.id);
                     let name = foreign::link_name(&*ni);
-                    foreign::register_foreign_item_fn(ccx, abi, ty, &name)
+                    let llfn = foreign::register_foreign_item_fn(ccx, abi, ty, &name);
+                    set_llvm_fn_attrs(ccx, &ni.attrs, llfn);
+                    llfn
                 }
                 ast::ForeignItemStatic(..) => {
                     foreign::register_static(ccx, &*ni)

src/librustc_trans/trans/foreign.rs

@@ -470,8 +470,8 @@ pub fn trans_foreign_mod(ccx: &CrateContext, foreign_mod: &ast::ForeignMod) {
                                                      "foreign fn's sty isn't a bare_fn_ty?")
                     }
 
-                    register_foreign_item_fn(ccx, abi, ty,
-                                             &lname);
+                    let llfn = register_foreign_item_fn(ccx, abi, ty, &lname);
+                    base::set_llvm_fn_attrs(ccx, &foreign_item.attrs, llfn);
                     // Unlike for other items, we shouldn't call
                     // `base::update_linkage` here.  Foreign items have
                     // special linkage requirements, which are handled

src/libsyntax/feature_gate.rs

@@ -83,6 +83,7 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
     ("box_syntax", "1.0.0", Active),
     ("on_unimplemented", "1.0.0", Active),
     ("simd_ffi", "1.0.0", Active),
+    ("allocator", "1.0.0", Active),
 
     ("if_let", "1.0.0", Accepted),
     ("while_let", "1.0.0", Accepted),
@@ -230,6 +231,8 @@ pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType)] = &[
     ("rustc_on_unimplemented", Gated("on_unimplemented",
                                      "the `#[rustc_on_unimplemented]` attribute \
                                       is an experimental feature")),
+    ("allocator", Gated("allocator",
+                        "the `#[allocator]` attribute is an experimental feature")),
     ("rustc_variance", Gated("rustc_attrs",
                              "the `#[rustc_variance]` attribute \
                               is an experimental feature")),