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Remove useless #[global_allocator] from rustc and rustdoc.

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This was added in #83152, which has several errors in its comments.

This commit also fix up the comments, which are quite wrong and
misleading.
This commit is contained in:
Nicholas Nethercote 2021-12-23 15:47:32 +11:00
parent e95e084a14
commit bb23bfc2cd
2 changed files with 27 additions and 32 deletions
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41
compiler/rustc/src/main.rs
View File

@ -1,25 +1,32 @@
// Configure jemalloc as the `global_allocator` when configured. This is
// so that we use the sized deallocation apis jemalloc provides
// (namely `sdallocx`).
// A note about jemalloc: rustc uses jemalloc when built for CI and
// 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.
//
// The symbol overrides documented below are also performed so that we can
// ensure that we use a consistent allocator across the rustc <-> llvm boundary
#[cfg(feature = "jemalloc")]
#[global_allocator]
static ALLOC: tikv_jemallocator::Jemalloc = tikv_jemallocator::Jemalloc;
// 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.
//
// 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.
#[cfg(feature = "tikv-jemalloc-sys")]
use tikv_jemalloc_sys as jemalloc_sys;
fn main() {
// Pull in jemalloc when enabled.
//
// Note that we're pulling in a static copy of jemalloc which means that to
// pull it in 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. That means to pull in jemalloc we actually need to
// reference allocation symbols one way or another (as this file is the only
// object code in the rustc executable).
// See the comment at the top of this file for an explanation of this.
#[cfg(feature = "tikv-jemalloc-sys")]
{
use std::os::raw::{c_int, c_void};

18
src/librustdoc/lib.rs
View File

@ -63,14 +63,12 @@ extern crate rustc_trait_selection;
extern crate rustc_typeck;
extern crate test;
// See docs in https://github.com/rust-lang/rust/blob/master/compiler/rustc/src/main.rs
// about jemalloc.
#[cfg(feature = "jemalloc")]
extern crate tikv_jemalloc_sys;
#[cfg(feature = "jemalloc")]
use tikv_jemalloc_sys as jemalloc_sys;
#[cfg(feature = "jemalloc")]
extern crate tikv_jemallocator;
#[cfg(feature = "jemalloc")]
use tikv_jemallocator as jemallocator;
use std::default::Default;
use std::env;
@ -125,15 +123,9 @@ mod visit;
mod visit_ast;
mod visit_lib;
// See docs in https://github.com/rust-lang/rust/blob/master/compiler/rustc/src/main.rs
// about jemallocator
#[cfg(feature = "jemalloc")]
#[global_allocator]
static ALLOC: jemallocator::Jemalloc = jemallocator::Jemalloc;
pub fn main() {
// See docs in https://github.com/rust-lang/rust/blob/master/compiler/rustc/src/main.rs
// about jemalloc-sys
// about jemalloc.
#[cfg(feature = "jemalloc")]
{
use std::os::raw::{c_int, c_void};
@ -152,10 +144,6 @@ pub fn main() {
#[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")]
{
extern "C" {