rustc: Allow changing the default allocator

This commit is an implementation of [RFC 1183][rfc] which allows swapping out
the default allocator on nightly Rust. No new stable surface area should be
added as a part of this commit.

[rfc]: https://github.com/rust-lang/rfcs/pull/1183

Two new attributes have been added to the compiler:

* `#![needs_allocator]` - this is used by liballoc (and likely only liballoc) to
  indicate that it requires an allocator crate to be in scope.
* `#![allocator]` - this is a indicator that the crate is an allocator which can
  satisfy the `needs_allocator` attribute above.

The ABI of the allocator crate is defined to be a set of symbols that implement
the standard Rust allocation/deallocation functions. The symbols are not
currently checked for exhaustiveness or typechecked. There are also a number of
restrictions on these crates:

* An allocator crate cannot transitively depend on a crate that is flagged as
  needing an allocator (e.g. allocator crates can't depend on liballoc).
* There can only be one explicitly linked allocator in a final image.
* If no allocator is explicitly requested one will be injected on behalf of the
  compiler. Binaries and Rust dylibs will use jemalloc by default where
  available and staticlibs/other dylibs will use the system allocator by
  default.

Two allocators are provided by the distribution by default, `alloc_system` and
`alloc_jemalloc` which operate as advertised.

Closes #27389

src/liballoc_jemalloc/lib.rs

@@ -0,0 +1,96 @@
+// Copyright 2015 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.
+
+#![cfg_attr(stage0, feature(custom_attribute))]
+#![crate_name = "alloc_jemalloc"]
+#![crate_type = "rlib"]
+#![staged_api]
+#![no_std]
+#![cfg_attr(not(stage0), allocator)]
+#![unstable(feature = "alloc_jemalloc",
+            reason = "this library is unlikely to be stabilized in its current \
+                      form or name")]
+#![feature(allocator)]
+#![feature(libc)]
+#![feature(no_std)]
+#![feature(staged_api)]
+
+extern crate libc;
+
+use libc::{c_int, c_void, size_t};
+
+#[link(name = "jemalloc", kind = "static")]
+extern {
+    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;
+    fn je_sdallocx(ptr: *mut c_void, size: size_t, flags: c_int);
+    fn je_nallocx(size: size_t, flags: c_int) -> size_t;
+}
+
+// -lpthread needs to occur after -ljemalloc, the earlier argument isn't enough
+#[cfg(all(not(windows),
+          not(target_os = "android"),
+          not(target_env = "musl")))]
+#[link(name = "pthread")]
+extern {}
+
+// The minimum alignment guaranteed by the architecture. This value is used to
+// add fast paths for low alignment values. In practice, the alignment is a
+// constant at the call site and the branch will be optimized out.
+#[cfg(all(any(target_arch = "arm",
+              target_arch = "mips",
+              target_arch = "mipsel",
+              target_arch = "powerpc")))]
+const MIN_ALIGN: usize = 8;
+#[cfg(all(any(target_arch = "x86",
+              target_arch = "x86_64",
+              target_arch = "aarch64")))]
+const MIN_ALIGN: usize = 16;
+
+// MALLOCX_ALIGN(a) macro
+fn mallocx_align(a: usize) -> c_int { a.trailing_zeros() as c_int }
+
+fn align_to_flags(align: usize) -> c_int {
+    if align <= MIN_ALIGN { 0 } else { mallocx_align(align) }
+}
+
+#[no_mangle]
+pub extern fn __rust_allocate(size: usize, align: usize) -> *mut u8 {
+    let flags = align_to_flags(align);
+    unsafe { je_mallocx(size as size_t, flags) as *mut u8 }
+}
+
+#[no_mangle]
+pub extern fn __rust_reallocate(ptr: *mut u8, _old_size: usize, size: usize,
+                                align: usize) -> *mut u8 {
+    let flags = align_to_flags(align);
+    unsafe { je_rallocx(ptr as *mut c_void, size as size_t, flags) as *mut u8 }
+}
+
+#[no_mangle]
+pub extern fn __rust_reallocate_inplace(ptr: *mut u8, _old_size: usize,
+                                        size: usize, align: usize) -> usize {
+    let flags = align_to_flags(align);
+    unsafe { je_xallocx(ptr as *mut c_void, size as size_t, 0, flags) as usize }
+}
+
+#[no_mangle]
+pub extern fn __rust_deallocate(ptr: *mut u8, old_size: usize, align: usize) {
+    let flags = align_to_flags(align);
+    unsafe { je_sdallocx(ptr as *mut c_void, old_size as size_t, flags) }
+}
+
+#[no_mangle]
+pub extern fn __rust_usable_size(size: usize, align: usize) -> usize {
+    let flags = align_to_flags(align);
+    unsafe { je_nallocx(size as size_t, flags) as usize }
+}