Simplify jemalloc setup

Using the new `override_allocator_on_supported_platforms` feature in
`tikv-jemalloc-sys v0.6.1` we can avoid the manual statics.

compiler/rustc/Cargo.toml

@@ -21,9 +21,9 @@ rustc_public_bridge = { path = "../rustc_public_bridge" }
 # tidy-alphabetical-end
 
 [dependencies.tikv-jemalloc-sys]
-version = "0.6.0"
+version = "0.6.1"
 optional = true
-features = ['unprefixed_malloc_on_supported_platforms']
+features = ['override_allocator_on_supported_platforms']
 
 [features]
 # tidy-alphabetical-start

compiler/rustc/src/main.rs

@@ -7,26 +7,25 @@
 // distribution. The obvious way to do this is with the `#[global_allocator]`
 // mechanism. However, for complicated reasons (see
 // https://github.com/rust-lang/rust/pull/81782#issuecomment-784438001 for some
-// details) that mechanism doesn't work here. Also, we must use a consistent
-// allocator across the rustc <-> llvm boundary, and `#[global_allocator]`
-// wouldn't provide that.
+// details) that mechanism doesn't work here. Also, we'd like to use a
+// consistent allocator across the rustc <-> llvm boundary, and
+// `#[global_allocator]` wouldn't provide that.
 //
-// Instead, we use a lower-level mechanism. rustc is linked with jemalloc in a
-// way such that jemalloc's implementation of `malloc`, `free`, etc., override
-// the libc allocator's implementation. This means that Rust's `System`
-// allocator, which calls `libc::malloc()` et al., is actually calling into
-// jemalloc.
+// Instead, we use a lower-level mechanism, namely the
+// `"override_allocator_on_supported_platforms"` Cargo feature of jemalloc-sys.
+//
+// This makes jemalloc-sys override the libc/system allocator's implementation
+// of `malloc`, `free`, etc.. This means that Rust's `System` allocator, which
+// calls `libc::malloc()` et al., is actually calling into jemalloc.
 //
 // A consequence of not using `GlobalAlloc` (and the `tikv-jemallocator` crate
 // provides an impl of that trait, which is called `Jemalloc`) is that we
 // cannot use the sized deallocation APIs (`sdallocx`) that jemalloc provides.
 // It's unclear how much performance is lost because of this.
 //
-// As for the symbol overrides in `main` below: we're pulling in a static copy
-// of jemalloc. We need to actually reference its symbols for it to get linked.
-// The two crates we link to here, `std` and `rustc_driver`, are both dynamic
-// libraries. So we must reference jemalloc symbols one way or another, because
-// this file is the only object code in the rustc executable.
+// NOTE: Even though Cargo passes `--extern` with `tikv_jemalloc_sys`, we still need to `use` the
+// crate for the compiler to see the `#[used]`, see https://github.com/rust-lang/rust/issues/64402.
+// This is similarly required if we used a crate with `#[global_allocator]`.
 //
 // NOTE: if you are reading this comment because you want to set a custom `global_allocator` for
 // benchmarking, consider using the benchmarks in the `rustc-perf` collector suite instead:
@@ -36,43 +35,9 @@
 // to compare their performance, see
 // https://github.com/rust-lang/rust/commit/b90cfc887c31c3e7a9e6d462e2464db1fe506175#diff-43914724af6e464c1da2171e4a9b6c7e607d5bc1203fa95c0ab85be4122605ef
 // for an example of how to do so.
+#[cfg(feature = "jemalloc")]
+use tikv_jemalloc_sys as _;
 
 fn main() {
-    // See the comment at the top of this file for an explanation of this.
-    #[cfg(feature = "jemalloc")]
-    {
-        use std::os::raw::{c_int, c_void};
-
-        use tikv_jemalloc_sys as jemalloc_sys;
-
-        #[used]
-        static _F1: unsafe extern "C" fn(usize, usize) -> *mut c_void = jemalloc_sys::calloc;
-        #[used]
-        static _F2: unsafe extern "C" fn(*mut *mut c_void, usize, usize) -> c_int =
-            jemalloc_sys::posix_memalign;
-        #[used]
-        static _F3: unsafe extern "C" fn(usize, usize) -> *mut c_void = jemalloc_sys::aligned_alloc;
-        #[used]
-        static _F4: unsafe extern "C" fn(usize) -> *mut c_void = jemalloc_sys::malloc;
-        #[used]
-        static _F5: unsafe extern "C" fn(*mut c_void, usize) -> *mut c_void = jemalloc_sys::realloc;
-        #[used]
-        static _F6: unsafe extern "C" fn(*mut c_void) = jemalloc_sys::free;
-
-        // On OSX, jemalloc doesn't directly override malloc/free, but instead
-        // registers itself with the allocator's zone APIs in a ctor. However,
-        // the linker doesn't seem to consider ctors as "used" when statically
-        // linking, so we need to explicitly depend on the function.
-        #[cfg(target_os = "macos")]
-        {
-            unsafe extern "C" {
-                fn _rjem_je_zone_register();
-            }
-
-            #[used]
-            static _F7: unsafe extern "C" fn() = _rjem_je_zone_register;
-        }
-    }
-
     rustc_driver::main()
 }