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Upgrade to LLVM 13 #87570

Merged
merged 12 commits into from
Aug 21, 2021
Merged

Upgrade to LLVM 13 #87570

merged 12 commits into from
Aug 21, 2021

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nikic
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@nikic nikic commented Jul 28, 2021

Work in progress update to LLVM 13. Main changes:

  • InlineAsm diagnostics reported using SrcMgr diagnostic kind are now handled. Previously these used a separate diag handler.
  • Codegen tests are updated for additional attributes.
  • Some data layouts have changed.
  • Switch #[used] attribute from llvm.used to llvm.compiler.used to avoid SHF_GNU_RETAIN flag introduced in https://reviews.llvm.org/D97448, which appears to trigger a bug in older versions of gold.
  • Set LLVM_INCLUDE_TESTS=OFF to avoid Python 3.6 requirement.

Upstream issues:

The compile-time impact is mixed, but quite positive as LLVM upgrades go.

The LLVM 13 final release is scheduled for Sep 21st. The current nightly is scheduled for stable release on Oct 21st.

r? @ghost

@nikic
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nikic commented Jul 28, 2021

@bors try @rust-timer queue

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Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 28, 2021
@bors
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bors commented Jul 28, 2021

⌛ Trying commit ce064a239e234c6b926107fd9382b9cda7589e83 with merge 450db03509584046c77604194f41a448ac97e1c5...

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bors commented Jul 28, 2021

☀️ Try build successful - checks-actions
Build commit: 450db03509584046c77604194f41a448ac97e1c5 (450db03509584046c77604194f41a448ac97e1c5)

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Queued 450db03509584046c77604194f41a448ac97e1c5 with parent a28109a, future comparison URL.

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Finished benchmarking try commit (450db03509584046c77604194f41a448ac97e1c5): comparison url.

Summary: This change led to significant mixed results 🤷 in compiler performance.

  • Very large regression in instruction counts (up to 31.7% on incr-unchanged builds of helloworld-opt)
  • Very large improvement in instruction counts (up to -10.5% on incr-unchanged builds of helloworld-check)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf +perf-regression

@rustbot rustbot added perf-regression Performance regression. S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jul 29, 2021
@nikic
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nikic commented Jul 29, 2021

It looks like the large regressions at the start are all slowdowns in run_linker, so this might be related to the missing llvmbc section flag.

@hkratz
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hkratz commented Jul 29, 2021

It looks like the large regressions at the start are all slowdowns in run_linker, so this might be related to the missing llvmbc section flag.

Seems like it. The resulting binaries do get bloated with bitcode.

The resulting stripped binary of compiling the helloworld perf test...

with Rust nightly 2021-07-28:

rustc -C opt-level=3 src/main.rs;strip main;ls -l main
-rwxr-xr-x 1 hans hans 272656 Jul 29 09:54 main

with LLVM 13:

~/dev/rust/build/x86_64-unknown-linux-gnu/stage2/bin/rustc -C opt-level=3 src/main.rs;strip main;ls -l main
-rwxr-xr-x 1 hans hans 6384232 Jul 29 10:07 main

Just backing out the hack and reverting the LLVM commit to allow the SHF_EXCLUDE flag on .llvmbc again works and makes it better:

~/dev/rust/build/x86_64-unknown-linux-gnu/stage2/bin/rustc -C opt-level=3 src/main.rs;strip main;ls -l main
-rwxr-xr-x 1 hans hans 309520 Jul 29 09:56 main

@nikic
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nikic commented Jul 29, 2021

Weird issue encountered via the pgo-branch-weights test:

fn main() {
    println!("{:?}", std::env::args());
}

Running rustc test.rs && ./test works fine. Running rustc -Clink-args=-fuse-ld=gold test.rs && ./test gives empty arguments.

@nikic
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nikic commented Jul 30, 2021

Just backing out the hack and reverting the LLVM commit to allow the SHF_EXCLUDE flag on .llvmbc again works and makes it better:

~/dev/rust/build/x86_64-unknown-linux-gnu/stage2/bin/rustc -C opt-level=3 src/main.rs;strip main;ls -l main
-rwxr-xr-x 1 hans hans 309520 Jul 29 09:56 main

Unfortunately, it seems like this doesn't actually work: Due to https://reviews.llvm.org/D100944 this no longer modifies the section flags on .llvmbc and will instead emit first an empty .llvmbc section with EXCLUDE, and then the actual .llvmbc section with ALLOC. LLVM is then no longer able to find the bitcode.

@nikic
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nikic commented Jul 30, 2021

The good news is that reverting (just) https://reviews.llvm.org/D100944 does fix the llvmbc issue.

Running rustc test.rs && ./test works fine. Running rustc -Clink-args=-fuse-ld=gold test.rs && ./test gives empty arguments.

It looks like something is going wrong with .init_array:

objdump -t test_ld | grep init_array
0000000000048250 l    d  .init_array	0000000000000000              .init_array
0000000000048258 l     O .init_array	0000000000000000              __frame_dummy_init_array_entry
0000000000048250 l     O .init_array	0000000000000008              _ZN3std3sys4unix4args3imp15ARGV_INIT_ARRAY17hb13aa0f306b867eeE
0000000000048260 l       .init_array	0000000000000000              __init_array_end
0000000000048250 l       .init_array	0000000000000000              __init_array_start

objdump -t test_gold | grep init_array
000000000004a218 l     O .init_array	0000000000000000              __frame_dummy_init_array_entry
000000000004c728 l     O .init_array	0000000000000008              _ZN3std3sys4unix4args3imp15ARGV_INIT_ARRAY17hb13aa0f306b867eeE
000000000004a218 l       .init_array	0000000000000000              .hidden __init_array_start
000000000004a220 l       .init_array	0000000000000000              .hidden __init_array_end

Notably, with ld we have ARGV_INIT_ARRAY sitting between __init_array_start and __init_array_end. With gold it's located after __init_array_end.

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nikic commented Jul 30, 2021

@bors try @rust-timer queue

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Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 30, 2021
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bors commented Jul 30, 2021

⌛ Trying commit 5f2b5c7078195e844f3b07fcad2a13172882c68f with merge 0983094463497eec22d550dad25576a894687002...

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bors commented Jul 30, 2021

☀️ Try build successful - checks-actions
Build commit: 0983094463497eec22d550dad25576a894687002 (0983094463497eec22d550dad25576a894687002)

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Queued 0983094463497eec22d550dad25576a894687002 with parent ef9549b, future comparison URL.

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Finished benchmarking try commit (0983094463497eec22d550dad25576a894687002): comparison url.

Summary: This change led to significant mixed results 🤷 in compiler performance.

  • Very large improvement in instruction counts (up to -10.4% on incr-unchanged builds of helloworld-check)
  • Large regression in instruction counts (up to 9.6% on full builds of match-stress-enum-check)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf +perf-regression

@rustbot rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 31, 2021
@nikic
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nikic commented Aug 1, 2021

Still rather stumped on the gold issue and don't really know how to debug it.

The best lead I have right now is that there are now some adjacent anon globals in the .data.rel.ro section, which were not present previously. Extracting the bitcode from the stdlib rlib, the relevant difference seems to be that a number of panic message constants are now hidden unnamed_addr constant, while they were previously private unnamed_addr constant. This means that these symbols now have external linkage, which they shouldn't have.

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tmiasko commented Aug 7, 2021

Running rustc test.rs && ./test works fine. Running rustc -Clink-args=-fuse-ld=gold test.rs && ./test gives empty arguments.

#[used] attribute now results in SHF_GNU_RETAIN / R flag being set https://reviews.llvm.org/D97448, which seems to be causing the issue when linking with gold < binutils 2.36.

@nikic
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nikic commented Aug 7, 2021

@tmiasko Ooh, nice find!

As suggested in the differential, we might want to switch to llvm.compiler.used then, as all our docs on #[used] make it abundantly clear that the attribute is only supposed to ensure that the symbol makes it into the object file, but the linker is still allowed to drop it.

@leonardo-m
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not sure if that is to be expected, or warrants an issue.

In my opinion this kind of slow little change is better tracked with specialized efforts (like the one for the compiler performance).

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Upgrading to LLVM changes the profiling data format incompatibly compared to LLVM 12, breaking my CI/CD code coverage jobs. Thus nightly users doing source-based code coverage measurement LLVM 13's profiling tools. However, LLVM 13 hasn't been released yet, so it looks like people need to use https://github.com/llvm/llvm-project/releases/tag/llvmorg-13.0.0-rc1 for now.

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We ship the necessary tools as part of llvm-tools-preview as far as I know, so I believe that should always work.

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We ship the necessary tools as part of llvm-tools-preview as far as I know, so I believe that should always work.

Thanks for that pointer!

If you are building non-Rust code then you'll also need LLVM-13 compatible tools to use those new tools, e.g. clang-13.

@Mark-Simulacrum Mark-Simulacrum added the perf-regression-triaged The performance regression has been triaged. label Sep 12, 2021
prusnak added a commit to prusnak/nixpkgs that referenced this pull request Oct 22, 2021
+ use llvm_13 as this is the default now:
  rust-lang/rust#87570
@prusnak prusnak mentioned this pull request Oct 22, 2021
12 tasks
wip-sync pushed a commit to NetBSD/pkgsrc-wip that referenced this pull request Oct 22, 2021
Pkgsrc changes:
 * Remove one now-longer-applicable patch, adjust a few others
 * Bump bootstrap requirements to 1.55.0.

Upstream changes:

Version 1.56.0 (2021-10-21)
========================

Language
--------

- [The 2021 Edition is now stable.][rust#88100]
  See [the edition guide][rust-2021-edition-guide] for more details.
- [The pattern in `binding @ pattern` can now also introduce new bindings.]
  [rust#85305]
- [Union field access is permitted in `const fn`.][rust#85769]

[rust-2021-edition-guide]: https://doc.rust-lang.org/nightly/edition-guide/rust-2021/index.html

Compiler
--------

- [Upgrade to LLVM 13.][rust#87570]
- [Support memory, address, and thread sanitizers on
  aarch64-unknown-freebsd.][rust#88023]
- [Allow specifying a deployment target version for all iOS targets][rust#87699]
- [Warnings can be forced on with `--force-warn`.][rust#87472]
  This feature is primarily intended for usage by `cargo fix`,
  rather than end users.
- [Promote `aarch64-apple-ios-sim` to Tier 2\*.][rust#87760]
- [Add `powerpc-unknown-freebsd` at Tier 3\*.][rust#87370]
- [Add `riscv32imc-esp-espidf` at Tier 3\*.][rust#87666]

\* Refer to Rust's [platform support page][platform-support-doc] for more
information on Rust's tiered platform support.

Libraries
---------

- [Allow writing of incomplete UTF-8 sequences via stdout/stderr on Windows.]
  [rust#83342]
  The Windows console still requires valid Unicode, but this change allows
  splitting a UTF-8 character across multiple write calls. This allows, for
  instance, programs that just read and write data buffers (e.g. copying a file
  to stdout) without regard for Unicode or character boundaries.
- [Prefer `AtomicU{64,128}` over Mutex for Instant backsliding protection.]
  [rust#83093]
  For this use case, atomics scale much better under contention.
- [Implement `Extend<(A, B)>` for `(Extend<A>, Extend<B>)`][rust#85835]
- [impl Default, Copy, Clone for std::io::Sink and std::io::Empty][rust#86744]
- [`impl From<[(K, V); N]>` for all collections.][rust#84111]
- [Remove `P: Unpin` bound on impl Future for Pin.][rust#81363]
- [Treat invalid environment variable names as non-existent.][rust#86183]
  Previously, the environment functions would panic if given a
  variable name with an internal null character or equal sign (`=`).
  Now, these functions will just treat such names as non-existent
  variables, since the OS cannot represent the existence of a
  variable with such a name.

Stabilised APIs
---------------

- [`std::os::unix::fs::chroot`]
- [`UnsafeCell::raw_get`]
- [`BufWriter::into_parts`]
- [`core::panic::{UnwindSafe, RefUnwindSafe, AssertUnwindSafe}`]
  These APIs were previously stable in `std`, but are now also available
  in `core`.
- [`Vec::shrink_to`]
- [`String::shrink_to`]
- [`OsString::shrink_to`]
- [`PathBuf::shrink_to`]
- [`BinaryHeap::shrink_to`]
- [`VecDeque::shrink_to`]
- [`HashMap::shrink_to`]
- [`HashSet::shrink_to`]

These APIs are now usable in const contexts:

- [`std::mem::transmute`]
- [`[T]::first`][`slice::first`]
- [`[T]::split_first`][`slice::split_first`]
- [`[T]::last`][`slice::last`]
- [`[T]::split_last`][`slice::split_last`]

Cargo
-----

- [Cargo supports specifying a minimum supported Rust version in Cargo.toml.]
  [`rust-version`] This has no effect at present on dependency
  version selection.  We encourage crates to specify their minimum
  supported Rust version, and we encourage CI systems that support
  Rust code to include a crate's specified minimum version in the
  text matrix for that crate by default.

Compatibility notes
-------------------

- [Update to new argument parsing rules on Windows.][rust#87580]
  This adjusts Rust's standard library to match the behavior of the standard
  libraries for C/C++. The rules have changed slightly over time, and this PR
  brings us to the latest set of rules (changed in 2008).
- [Disallow the aapcs calling convention on aarch64][rust#88399]
  This was already not supported by LLVM; this change surfaces this lack of
  support with a better error message.
- [Make `SEMICOLON_IN_EXPRESSIONS_FROM_MACROS` warn by default][rust#87385]
- [Warn when an escaped newline skips multiple lines.][rust#87671]
- [Calls to `libc::getpid` / `std::process::id` from `Command::pre_exec`
  may return different values on glibc <= 2.24.][rust#81825] Rust
  now invokes the `clone3` system call directly, when available,
  to use new functionality available via that system call. Older
  versions of glibc cache the result of `getpid`, and only update
  that cache when calling glibc's clone/fork functions, so a direct
  system call bypasses that cache update. glibc 2.25 and newer no
  longer cache `getpid` for exactly this reason.

Internal changes
----------------
These changes provide no direct user facing benefits, but represent
significant improvements to the internals and overall performance
of rustc and related tools.

- [LLVM is compiled with PGO in published x86_64-unknown-linux-gnu
  artifacts.][rust#88069] This improves the performance of most Rust builds.

- [Unify representation of macros in internal data structures.][rust#88019]
  This change fixes a host of bugs with the handling of macros by the compiler,
  as well as rustdoc.

[`std::os::unix::fs::chroot`]: https://doc.rust-lang.org/stable/std/os/unix/fs/fn.chroot.html
[`Iterator::intersperse`]: https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.intersperse
[`Iterator::intersperse_with`]: https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.intersperse
[`UnsafeCell::raw_get`]: https://doc.rust-lang.org/stable/std/cell/struct.UnsafeCell.html#method.raw_get
[`BufWriter::into_parts`]: https://doc.rust-lang.org/stable/std/io/struct.BufWriter.html#method.into_parts
[`core::panic::{UnwindSafe, RefUnwindSafe, AssertUnwindSafe}`]: rust-lang/rust#84662
[`Vec::shrink_to`]: https://doc.rust-lang.org/stable/std/vec/struct.Vec.html#method.shrink_to
[`String::shrink_to`]: https://doc.rust-lang.org/stable/std/string/struct.String.html#method.shrink_to
[`OsString::shrink_to`]: https://doc.rust-lang.org/stable/std/ffi/struct.OsString.html#method.shrink_to
[`PathBuf::shrink_to`]: https://doc.rust-lang.org/stable/std/path/struct.PathBuf.html#method.shrink_to
[`BinaryHeap::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/struct.BinaryHeap.html#method.shrink_to
[`VecDeque::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/struct.VecDeque.html#method.shrink_to
[`HashMap::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/hash_map/struct.HashMap.html#method.shrink_to
[`HashSet::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/hash_set/struct.HashSet.html#method.shrink_to
[`std::mem::transmute`]: https://doc.rust-lang.org/stable/std/mem/fn.transmute.html
[`slice::first`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.first
[`slice::split_first`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_first
[`slice::last`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.last
[`slice::split_last`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_last
[`rust-version`]: https://doc.rust-lang.org/nightly/cargo/reference/manifest.html#the-rust-version-field
[rust#87671]: rust-lang/rust#87671
[rust#86183]: rust-lang/rust#86183
[rust#87385]: rust-lang/rust#87385
[rust#88100]: rust-lang/rust#88100
[rust#86860]: rust-lang/rust#86860
[rust#84039]: rust-lang/rust#84039
[rust#86492]: rust-lang/rust#86492
[rust#88363]: rust-lang/rust#88363
[rust#85305]: rust-lang/rust#85305
[rust#87832]: rust-lang/rust#87832
[rust#88069]: rust-lang/rust#88069
[rust#87472]: rust-lang/rust#87472
[rust#87699]: rust-lang/rust#87699
[rust#87570]: rust-lang/rust#87570
[rust#88023]: rust-lang/rust#88023
[rust#87760]: rust-lang/rust#87760
[rust#87370]: rust-lang/rust#87370
[rust#87580]: rust-lang/rust#87580
[rust#83342]: rust-lang/rust#83342
[rust#83093]: rust-lang/rust#83093
[rust#88177]: rust-lang/rust#88177
[rust#88548]: rust-lang/rust#88548
[rust#88551]: rust-lang/rust#88551
[rust#88299]: rust-lang/rust#88299
[rust#88220]: rust-lang/rust#88220
[rust#85835]: rust-lang/rust#85835
[rust#86879]: rust-lang/rust#86879
[rust#86744]: rust-lang/rust#86744
[rust#84662]: rust-lang/rust#84662
[rust#86593]: rust-lang/rust#86593
[rust#81050]: rust-lang/rust#81050
[rust#81363]: rust-lang/rust#81363
[rust#84111]: rust-lang/rust#84111
[rust#85769]: rust-lang/rust#85769 (comment)
[rust#88490]: rust-lang/rust#88490
[rust#88269]: rust-lang/rust#88269
[rust#84176]: rust-lang/rust#84176
[rust#88399]: rust-lang/rust#88399
[rust#88227]: rust-lang/rust#88227
[rust#88200]: rust-lang/rust#88200
[rust#82776]: rust-lang/rust#82776
[rust#88077]: rust-lang/rust#88077
[rust#87728]: rust-lang/rust#87728
[rust#87050]: rust-lang/rust#87050
[rust#87619]: rust-lang/rust#87619
[rust#81825]: rust-lang/rust#81825 (comment)
[rust#88019]: rust-lang/rust#88019
[rust#87666]: rust-lang/rust#87666
netbsd-srcmastr pushed a commit to NetBSD/pkgsrc that referenced this pull request Nov 20, 2021
Pkgsrc changes:
 * Bump bootstrap kit version to 1.55.0.
 * Adjust patches as needed, some no longer apply (so removed)
 * Update checksum adjustments.
 * Avoid rust-llvm on SunOS
 * Optionally build docs
 * Remove reference to closed/old PR#54621

Upstream changes:

Version 1.56.1 (2021-11-01)
===========================

- New lints to detect the presence of bidirectional-override Unicode
  codepoints in the compiled source code ([CVE-2021-42574])

[CVE-2021-42574]: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-42574

Version 1.56.0 (2021-10-21)
========================

Language
--------

- [The 2021 Edition is now stable.][rust#88100]
  See [the edition guide][rust-2021-edition-guide] for more details.
- [The pattern in `binding @ pattern` can now also introduce new bindings.]
  [rust#85305]
- [Union field access is permitted in `const fn`.][rust#85769]

[rust-2021-edition-guide]:
  https://doc.rust-lang.org/nightly/edition-guide/rust-2021/index.html

Compiler
--------

- [Upgrade to LLVM 13.][rust#87570]
- [Support memory, address, and thread sanitizers on aarch64-unknown-freebsd.]
  [rust#88023]
- [Allow specifying a deployment target version for all iOS targets][rust#87699]
- [Warnings can be forced on with `--force-warn`.][rust#87472]
  This feature is primarily intended for usage by `cargo fix`, rather than
  end users.
- [Promote `aarch64-apple-ios-sim` to Tier 2\*.][rust#87760]
- [Add `powerpc-unknown-freebsd` at Tier 3\*.][rust#87370]
- [Add `riscv32imc-esp-espidf` at Tier 3\*.][rust#87666]

\* Refer to Rust's [platform support page][platform-support-doc] for more
information on Rust's tiered platform support.

Libraries
---------

- [Allow writing of incomplete UTF-8 sequences via stdout/stderr on Windows.]
  [rust#83342]
  The Windows console still requires valid Unicode, but this change allows
  splitting a UTF-8 character across multiple write calls. This allows, for
  instance, programs that just read and write data buffers (e.g. copying a file
  to stdout) without regard for Unicode or character boundaries.
- [Prefer `AtomicU{64,128}` over Mutex for Instant backsliding protection.]
  [rust#83093]
  For this use case, atomics scale much better under contention.
- [Implement `Extend<(A, B)>` for `(Extend<A>, Extend<B>)`][rust#85835]
- [impl Default, Copy, Clone for std::io::Sink and std::io::Empty][rust#86744]
- [`impl From<[(K, V); N]>` for all collections.][rust#84111]
- [Remove `P: Unpin` bound on impl Future for Pin.][rust#81363]
- [Treat invalid environment variable names as non-existent.][rust#86183]
  Previously, the environment functions would panic if given a
  variable name with an internal null character or equal sign (`=`).
  Now, these functions will just treat such names as non-existent
  variables, since the OS cannot represent the existence of a
  variable with such a name.

Stabilised APIs
---------------

- [`std::os::unix::fs::chroot`]
- [`UnsafeCell::raw_get`]
- [`BufWriter::into_parts`]
- [`core::panic::{UnwindSafe, RefUnwindSafe, AssertUnwindSafe}`]
  These APIs were previously stable in `std`, but are now also available
  in `core`.
- [`Vec::shrink_to`]
- [`String::shrink_to`]
- [`OsString::shrink_to`]
- [`PathBuf::shrink_to`]
- [`BinaryHeap::shrink_to`]
- [`VecDeque::shrink_to`]
- [`HashMap::shrink_to`]
- [`HashSet::shrink_to`]

These APIs are now usable in const contexts:

- [`std::mem::transmute`]
- [`[T]::first`][`slice::first`]
- [`[T]::split_first`][`slice::split_first`]
- [`[T]::last`][`slice::last`]
- [`[T]::split_last`][`slice::split_last`]

Cargo
-----

- [Cargo supports specifying a minimum supported Rust version in Cargo.toml.]
  [`rust-version`]
  This has no effect at present on dependency version selection.
  We encourage crates to specify their minimum supported Rust
  version, and we encourage CI systems that support Rust code to
  include a crate's specified minimum version in the text matrix
  for that crate by default.

Compatibility notes
-------------------

- [Update to new argument parsing rules on Windows.][rust#87580]
  This adjusts Rust's standard library to match the behavior of the standard
  libraries for C/C++. The rules have changed slightly over time, and this PR
  brings us to the latest set of rules (changed in 2008).
- [Disallow the aapcs calling convention on aarch64][rust#88399]
  This was already not supported by LLVM; this change surfaces this lack of
  support with a better error message.
- [Make `SEMICOLON_IN_EXPRESSIONS_FROM_MACROS` warn by default][rust#87385]
- [Warn when an escaped newline skips multiple lines.][rust#87671]
- [Calls to `libc::getpid` / `std::process::id` from `Command::pre_exec`
  may return different values on glibc <= 2.24.][rust#81825]
  Rust now invokes the `clone3` system call directly, when available,
  to use new functionality available via that system call. Older
  versions of glibc cache the result of `getpid`, and only update
  that cache when calling glibc's clone/fork functions, so a direct
  system call bypasses that cache update. glibc 2.25 and newer no
  longer cache `getpid` for exactly this reason.

Internal changes
----------------
These changes provide no direct user facing benefits, but represent
significant improvements to the internals and overall performance
of rustc and related tools.

- [LLVM is compiled with PGO in published x86_64-unknown-linux-gnu artifacts.]
  [rust#88069]
  This improves the performance of most Rust builds.
- [Unify representation of macros in internal data structures.][rust#88019]
  This change fixes a host of bugs with the handling of macros by the compiler,
  as well as rustdoc.

[`std::os::unix::fs::chroot`]: https://doc.rust-lang.org/stable/std/os/unix/fs/fn.chroot.html
[`Iterator::intersperse`]: https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.intersperse
[`Iterator::intersperse_with`]: https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.intersperse
[`UnsafeCell::raw_get`]: https://doc.rust-lang.org/stable/std/cell/struct.UnsafeCell.html#method.raw_get
[`BufWriter::into_parts`]: https://doc.rust-lang.org/stable/std/io/struct.BufWriter.html#method.into_parts
[`core::panic::{UnwindSafe, RefUnwindSafe, AssertUnwindSafe}`]: rust-lang/rust#84662
[`Vec::shrink_to`]: https://doc.rust-lang.org/stable/std/vec/struct.Vec.html#method.shrink_to
[`String::shrink_to`]: https://doc.rust-lang.org/stable/std/string/struct.String.html#method.shrink_to
[`OsString::shrink_to`]: https://doc.rust-lang.org/stable/std/ffi/struct.OsString.html#method.shrink_to
[`PathBuf::shrink_to`]: https://doc.rust-lang.org/stable/std/path/struct.PathBuf.html#method.shrink_to
[`BinaryHeap::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/struct.BinaryHeap.html#method.shrink_to
[`VecDeque::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/struct.VecDeque.html#method.shrink_to
[`HashMap::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/hash_map/struct.HashMap.html#method.shrink_to
[`HashSet::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/hash_set/struct.HashSet.html#method.shrink_to
[`std::mem::transmute`]: https://doc.rust-lang.org/stable/std/mem/fn.transmute.html
[`slice::first`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.first
[`slice::split_first`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_first
[`slice::last`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.last
[`slice::split_last`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_last
[`rust-version`]: https://doc.rust-lang.org/nightly/cargo/reference/manifest.html#the-rust-version-field
[rust#87671]: rust-lang/rust#87671
[rust#86183]: rust-lang/rust#86183
[rust#87385]: rust-lang/rust#87385
[rust#88100]: rust-lang/rust#88100
[rust#86860]: rust-lang/rust#86860
[rust#84039]: rust-lang/rust#84039
[rust#86492]: rust-lang/rust#86492
[rust#88363]: rust-lang/rust#88363
[rust#85305]: rust-lang/rust#85305
[rust#87832]: rust-lang/rust#87832
[rust#88069]: rust-lang/rust#88069
[rust#87472]: rust-lang/rust#87472
[rust#87699]: rust-lang/rust#87699
[rust#87570]: rust-lang/rust#87570
[rust#88023]: rust-lang/rust#88023
[rust#87760]: rust-lang/rust#87760
[rust#87370]: rust-lang/rust#87370
[rust#87580]: rust-lang/rust#87580
[rust#83342]: rust-lang/rust#83342
[rust#83093]: rust-lang/rust#83093
[rust#88177]: rust-lang/rust#88177
[rust#88548]: rust-lang/rust#88548
[rust#88551]: rust-lang/rust#88551
[rust#88299]: rust-lang/rust#88299
[rust#88220]: rust-lang/rust#88220
[rust#85835]: rust-lang/rust#85835
[rust#86879]: rust-lang/rust#86879
[rust#86744]: rust-lang/rust#86744
[rust#84662]: rust-lang/rust#84662
[rust#86593]: rust-lang/rust#86593
[rust#81050]: rust-lang/rust#81050
[rust#81363]: rust-lang/rust#81363
[rust#84111]: rust-lang/rust#84111
[rust#85769]: rust-lang/rust#85769 (comment)
[rust#88490]: rust-lang/rust#88490
[rust#88269]: rust-lang/rust#88269
[rust#84176]: rust-lang/rust#84176
[rust#88399]: rust-lang/rust#88399
[rust#88227]: rust-lang/rust#88227
[rust#88200]: rust-lang/rust#88200
[rust#82776]: rust-lang/rust#82776
[rust#88077]: rust-lang/rust#88077
[rust#87728]: rust-lang/rust#87728
[rust#87050]: rust-lang/rust#87050
[rust#87619]: rust-lang/rust#87619
[rust#81825]: rust-lang/rust#81825 (comment)
[rust#88019]: rust-lang/rust#88019
[rust#87666]: rust-lang/rust#87666

Version 1.55.0 (2021-09-09)
============================

Language
--------
- [You can now write open "from" range patterns (`X..`), which will start
  at `X` and will end at the maximum value of the integer.][83918]
- [You can now explicitly import the prelude of different editions
  through `std::prelude` (e.g. `use std::prelude::rust_2021::*;`).][86294]

Compiler
--------
- [Added tier 3\* support for `powerpc64le-unknown-freebsd`.][83572]

\* Refer to Rust's [platform support page][platform-support-doc] for more
   information on Rust's tiered platform support.

Libraries
---------

- [Updated std's float parsing to use the Eisel-Lemire algorithm.][86761]
  These improvements should in general provide faster string parsing of floats,
  no longer reject certain valid floating point values, and reduce
  the produced code size for non-stripped artifacts.
- [`string::Drain` now implements `AsRef<str>` and `AsRef<[u8]>`.][86858]

Stabilised APIs
---------------

- [`Bound::cloned`]
- [`Drain::as_str`]
- [`IntoInnerError::into_error`]
- [`IntoInnerError::into_parts`]
- [`MaybeUninit::assume_init_mut`]
- [`MaybeUninit::assume_init_ref`]
- [`MaybeUninit::write`]
- [`array::map`]
- [`ops::ControlFlow`]
- [`x86::_bittest`]
- [`x86::_bittestandcomplement`]
- [`x86::_bittestandreset`]
- [`x86::_bittestandset`]
- [`x86_64::_bittest64`]
- [`x86_64::_bittestandcomplement64`]
- [`x86_64::_bittestandreset64`]
- [`x86_64::_bittestandset64`]

The following previously stable functions are now `const`.

- [`str::from_utf8_unchecked`]


Cargo
-----
- [Cargo will now deduplicate compiler diagnostics to the terminal when invoking
  rustc in parallel such as when using `cargo test`.][cargo/9675]
- [The package definition in `cargo metadata` now includes the `"default_run"`
  field from the manifest.][cargo/9550]
- [Added `cargo d` as an alias for `cargo doc`.][cargo/9680]
- [Added `{lib}` as formatting option for `cargo tree` to print the `"lib_name"`
  of packages.][cargo/9663]

Rustdoc
-------
- [Added "Go to item on exact match" search option.][85876]
- [The "Implementors" section on traits no longer shows redundant
  method definitions.][85970]
- [Trait implementations are toggled open by default.][86260] This should
  make the implementations more searchable by tools like `CTRL+F` in
  your browser.
- [Intra-doc links should now correctly resolve associated items (e.g. methods)
  through type aliases.][86334]
- [Traits which are marked with `#[doc(hidden)]` will no longer appear in the
  "Trait Implementations" section.][86513]


Compatibility Notes
-------------------
- [std functions that return an `io::Error` will no longer use the
  `ErrorKind::Other` variant.][85746] This is to better reflect that these
  kinds of errors could be categorised [into newer more specific `ErrorKind`
  variants][79965], and that they do not represent a user error.
- [Using environment variable names with `process::Command` on Windows now
  behaves as expected.][85270] Previously using envionment variables with
  `Command` would cause them to be ASCII-uppercased.
- [Rustdoc will now warn on using rustdoc lints that aren't prefixed
  with `rustdoc::`][86849]

[86849]: rust-lang/rust#86849
[86513]: rust-lang/rust#86513
[86334]: rust-lang/rust#86334
[86260]: rust-lang/rust#86260
[85970]: rust-lang/rust#85970
[85876]: rust-lang/rust#85876
[83572]: rust-lang/rust#83572
[86294]: rust-lang/rust#86294
[86858]: rust-lang/rust#86858
[86761]: rust-lang/rust#86761
[85769]: rust-lang/rust#85769
[85746]: rust-lang/rust#85746
[85305]: rust-lang/rust#85305
[85270]: rust-lang/rust#85270
[84111]: rust-lang/rust#84111
[83918]: rust-lang/rust#83918
[79965]: rust-lang/rust#79965
[87370]: rust-lang/rust#87370
[87298]: rust-lang/rust#87298
[cargo/9663]: rust-lang/cargo#9663
[cargo/9675]: rust-lang/cargo#9675
[cargo/9550]: rust-lang/cargo#9550
[cargo/9680]: rust-lang/cargo#9680
[cargo/9663]: rust-lang/cargo#9663
[`array::map`]: https://doc.rust-lang.org/stable/std/primitive.array.html#method.map
[`Bound::cloned`]: https://doc.rust-lang.org/stable/std/ops/enum.Bound.html#method.cloned
[`Drain::as_str`]: https://doc.rust-lang.org/stable/std/string/struct.Drain.html#method.as_str
[`IntoInnerError::into_error`]: https://doc.rust-lang.org/stable/std/io/struct.IntoInnerError.html#method.into_error
[`IntoInnerError::into_parts`]: https://doc.rust-lang.org/stable/std/io/struct.IntoInnerError.html#method.into_parts
[`MaybeUninit::assume_init_mut`]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.assume_init_mut
[`MaybeUninit::assume_init_ref`]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.assume_init_ref
[`MaybeUninit::write`]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.write
[`Seek::rewind`]: https://doc.rust-lang.org/stable/std/io/trait.Seek.html#method.rewind
[`ops::ControlFlow`]: https://doc.rust-lang.org/stable/std/ops/enum.ControlFlow.html
[`str::from_utf8_unchecked`]: https://doc.rust-lang.org/stable/std/str/fn.from_utf8_unchecked.html
[`x86::_bittest`]: https://doc.rust-lang.org/stable/core/arch/x86/fn._bittest.html
[`x86::_bittestandcomplement`]: https://doc.rust-lang.org/stable/core/arch/x86/fn._bittestandcomplement.html
[`x86::_bittestandreset`]: https://doc.rust-lang.org/stable/core/arch/x86/fn._bittestandreset.html
[`x86::_bittestandset`]: https://doc.rust-lang.org/stable/core/arch/x86/fn._bittestandset.html
[`x86_64::_bittest64`]: https://doc.rust-lang.org/stable/core/arch/x86_64/fn._bittest64.html
[`x86_64::_bittestandcomplement64`]: https://doc.rust-lang.org/stable/core/arch/x86_64/fn._bittestandcomplement64.html
[`x86_64::_bittestandreset64`]: https://doc.rust-lang.org/stable/core/arch/x86_64/fn._bittestandreset64.html
[`x86_64::_bittestandset64`]: https://doc.rust-lang.org/stable/core/arch/x86_64/fn._bittestandset64.html
nvzqz added a commit to nvzqz/embed-plist-rs that referenced this pull request Jan 9, 2022
This adds the `S_ATTR_NO_DEAD_STRIP` section attribute, which forces
"unused" code to remain in the binary. This is paired with `S_REGULAR`
to provide a required section type.

This fixes #1.

This may be caused by rust-lang/rust#87570.
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