Auto merge of #116352 - Kobzol:rustc-driver-bolt, r=Mark-Simulacrum

Optimize `librustc_driver.so` with BOLT

This PR optimizes `librustc_driver.so` on 64-bit Linux CI with BOLT.

### Code
One thing that's not clear yet to me how to resolve is how to best pass a linker flag that we need for BOLT (the second commit). It is currently passed unconditionally, which is not a good idea. We somehow have to:
1) Only pass it when we actually plan to use BOLT. How to best do that? `config.toml` entry? Environment variable? CLI flag for bootstrap? BOLT optimization is done by `opt-dist`, therefore bootstrap doesn't know about it by default.
2) Only pass it to `librustc_driver.so` (see performance below).

Some discussion of this flag already happened on [Zulip](https://rust-lang.zulipchat.com/#narrow/stream/326414-t-infra.2Fbootstrap/topic/Adding.20a.20one-off.20linker.20flag).

### Performance
Latest perf. results can be found [here](https://github.com/rust-lang/rust/pull/102487#issuecomment-1743469053). Note that instruction counts are not very interesting here, there are only regressions on hello world programs. Probably caused by a larger C++ libstd (?).

Summary:
- :heavy_check_mark: `-1.8%` mean improvement in cycle counts across many primary benchmarks.
- :heavy_check_mark: `-1.8%` mean Max-RSS improvement.
- :heavy_multiplication_x: 34 MiB (+48%) artifact size regression of `librustc_driver.so`.
  - This is caused by building `librustc_driver.so` with relocations (which are required for BOLT). Hopefully, it will be [fixed](https://discourse.llvm.org/t/bolt-rfc-a-new-mode-to-rewrite-entire-binary/68674) in the future with BOLT improvements, but now trying to reduce this size increase is [tricky](https://github.com/rust-lang/rust/pull/114649).
  - Note that the size of this file was recently reduced in https://github.com/rust-lang/rust/pull/115554 by pretty much the same amount (33 MiB). So the size after this PR is basically the same as it was for the last ~year.
- :heavy_multiplication_x: 1.4 MiB (+53%) artifact size regression of `rustc`.
  - This is annoying and pretty much unnecessary. It is caused by the way relocations are currently applied in this PR, because they are applied both to `librustc_driver.so` (where they are needed) and for `rustc` (where they aren't needed), since both are built with a single cargo invocation. We might need e.g. some tricks in the bootstrap `rustc` shim to only apply the relocation flag for the shared library and not for `rustc`.

### CI time
CI (try build) got slower by ~5 minutes, which is fine, IMO. It can be further reduced by running LLVM and `librustc_driver` BOLT profile gathering at the same time (now they are gathered separately for LLVM and `librustc_driver`).

r? `@Mark-Simulacrum`

Also CC `@onur-ozkan,` primarily for the bootstrap linker flag issue.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/bootstrap/flags.rs b/src/bootstrap/flags.rs
index e0291e407b3..5a6a5f37fc6 100644
--- a/src/bootstrap/flags.rs
+++ b/src/bootstrap/flags.rs
@@ -152,6 +152,9 @@ pub struct Flags {
     /// generate PGO profile with llvm built for rustc
     #[arg(global(true), long)]
     pub llvm_profile_generate: bool,
+    /// Enable BOLT link flags
+    #[arg(global(true), long)]
+    pub enable_bolt_settings: bool,
     /// Additional reproducible artifacts that should be added to the reproducible artifacts archive.
     #[arg(global(true), long)]
     pub reproducible_artifact: Vec<String>,