Auto merge of #113592 - Kobzol:pgo-script-bolt, r=Mark-Simulacrum

Move BOLT from `bootstrap` to `opt-dist`

Currently, we use BOLT to optimize LLVM for x64 Linux. The BOLT instrumentation and optimization step is implemented in `bootstrap`, but it was always quite hacky, because BOLT works quite differently than PGO. Rather than building an instrumented artifact, it takes an already built artifact and instruments it in-place. This is not a good fit for the bootstrap caching mechanism, and it meant that we had to run BOLT "on-the-fly" when packaging LLVM artifacts into the created sysroot.

The BOLT code was also really only used by the PGO script (now called `opt-dist`) and nothing else, so it was quite hardcoded for this one single usage. And even if someone wanted to use the `--llvm-bolt-profile-[use/generate]` bootstrap flags outside of the PGO script, they would also need to implement profile gathering, as this is not performed by bootstrap anyway.

I think that it could be more practical to move the BOLT logic to the `opt-dist` tool instead. This simplifies bootstrap, removes unneeded implementation of BOLT caching (we will now do it exactly once - no need to check if it has been performed already when bootstrap copies `libLLVM.so` around multiple times) and removes two BOLT-specific bootstrap flags, and also one special case for building LLVM (instead I pass the linker flags with `--set llvm.ldflags` from `opt-dist` now).

There are also a few disadvantages to this new approach:
- We have to guess/find the path to the built `libLLVM.so` file (but currently this is quite easy, it's just in `stage2/lib`).
- We have to provide the BOLT profile externally to bootstrap, so that it is packaged into the reproducible artifacts archive. Doesn't seem like a big deal to me.
- We are depending on some inner behavior of boostrap in `opt-dist` (namely, that `libLLVM.so` is hardlinked). But we do that for many other things in the `opt-dist` tool anyway, it's tied quite closely to bootstrap.

I would like to go back to my attempts to also use BOLT for `librustc_driver.so`, and I think that it might be a bit simpler if I also do it from the `opt-dist` tool, so this is the first step towards that.

Anyway, let me know what you think about this. It's just a refactoring in a way, no big deal.

r? bootstrap

0 files changed, 0 insertions, 0 deletions