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Auto merge of #54592 - GabrielMajeri:no-plt, r=nagisa

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Support for disabling PLT for better function call performance

This PR gives `rustc` the ability to skip the PLT when generating function calls into shared libraries. This can improve performance by reducing branch indirection.

AFAIK, the only advantage of using the PLT is to allow for ELF lazy binding. However, since Rust already [enables full relro for security](https://github.com/rust-lang/rust/pull/43170), lazy binding was disabled anyway.

This is a little known feature which is supported by [GCC](https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html) and [Clang](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-fplt) as `-fno-plt` (some Linux distros [enable it by default](https://git.archlinux.org/svntogit/packages.git/tree/trunk/makepkg.conf?h=packages/pacman#n40) for all builds).

Implementation inspired by [this patch](https://reviews.llvm.org/D39079#change-YvkpNDlMs_LT) which adds `-fno-plt` support to Clang.

## Performance

I didn't run a lot of benchmarks, but these are the results on my machine for a `clap` [benchmark](https://github.com/clap-rs/clap/blob/master/benches/05_ripgrep.rs):

```
 name              control ns/iter  no-plt ns/iter  diff ns/iter  diff %  speedup
 build_app_long    11,097           10,733                  -364  -3.28%   x 1.03
 build_app_short   11,089           10,742                  -347  -3.13%   x 1.03
 build_help_long   186,835          182,713               -4,122  -2.21%   x 1.02
 build_help_short  80,949           78,455                -2,494  -3.08%   x 1.03
 parse_clean       12,385           12,044                  -341  -2.75%   x 1.03
 parse_complex     19,438           19,017                  -421  -2.17%   x 1.02
 parse_lots        431,493          421,421              -10,072  -2.33%   x 1.02
```

A small performance improvement across the board, with no downsides. It's likely binaries which make a lot of function calls into dynamic libraries could see even more improvements. [This comment](https://patchwork.ozlabs.org/patch/468993/#1028255) suggests that, in some cases, `-fno-plt` could improve PIC/PIE code performance by 10%.

## Security benefits

**Bonus**: some of the speculative execution attacks rely on the PLT, by disabling it we reduce a big attack surface and reduce the need for [`retpoline`](https://reviews.llvm.org/D41723).

## Remaining PLT calls

The compiled binaries still have plenty of PLT calls, coming from C/C++ libraries. Building dependencies with `CFLAGS=-fno-plt CXXFLAGS=-fno-plt` removes them.
This commit is contained in:
bors 2018-10-11 19:38:15 +00:00
commit 77af314083
12 changed files with 89 additions and 7 deletions
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10
src/test/codegen/naked-functions.rs
View file

@ -15,7 +15,7 @@
#![crate_type = "lib"]
#![feature(naked_functions)]
// CHECK: Function Attrs: naked uwtable
// CHECK: Function Attrs: naked
// CHECK-NEXT: define void @naked_empty()
#[no_mangle]
#[naked]
@ -24,7 +24,7 @@ pub fn naked_empty() {
// CHECK-NEXT: ret void
}
// CHECK: Function Attrs: naked uwtable
// CHECK: Function Attrs: naked
#[no_mangle]
#[naked]
// CHECK-NEXT: define void @naked_with_args(i{{[0-9]+}})
@ -35,7 +35,7 @@ pub fn naked_with_args(a: isize) {
// CHECK: ret void
}
// CHECK: Function Attrs: naked uwtable
// CHECK: Function Attrs: naked
// CHECK-NEXT: define i{{[0-9]+}} @naked_with_return()
#[no_mangle]
#[naked]
@ -45,7 +45,7 @@ pub fn naked_with_return() -> isize {
0
}
// CHECK: Function Attrs: naked uwtable
// CHECK: Function Attrs: naked
// CHECK-NEXT: define i{{[0-9]+}} @naked_with_args_and_return(i{{[0-9]+}})
#[no_mangle]
#[naked]
@ -57,7 +57,7 @@ pub fn naked_with_args_and_return(a: isize) -> isize {
a
}
// CHECK: Function Attrs: naked uwtable
// CHECK: Function Attrs: naked
// CHECK-NEXT: define void @naked_recursive()
#[no_mangle]
#[naked]

27
src/test/codegen/no-plt.rs Normal file
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@ -0,0 +1,27 @@
// Copyright 2018 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.
// compile-flags: -C relocation-model=pic -Z plt=no
#![crate_type = "lib"]
// We need a function which is normally called through the PLT.
extern "C" {
// CHECK: Function Attrs: nounwind nonlazybind
fn getenv(name: *const u8) -> *mut u8;
}
// Ensure the function gets referenced.
pub unsafe fn call_through_plt() -> *mut u8 {
getenv(b"\0".as_ptr())
}
// Ensure intrinsics also skip the PLT
// CHECK: !"RtLibUseGOT"