[BOLT] Use CDSort and CDSplit
CDSort and CDSplit are the most recent versions of function ordering and function splitting algorithms with some improvements over the previous baseline (ext-tsp and two-way splitting).
LLVM now includes the minor version in the soname, and also changed
the names of shared object files. libLLVM-18.so is now a symlink to
libLLVM.so.18.1. We need to make some changes to support this:
First, we need to run the installed llvm-config binary, rather
than the one from the build directory. This is because the symlink
does not exist in the build directory, but llvm-config requires it.
This looks like an LLVM bug to me, but it's probably a good idea to
use the installed version anyway.
Second, when installing LLVM into the libdir, we need to install
the target of the symlink, ans this is what will get loaded at
runtime.
However, the rust-dev component in particular also needs to
distribute the symlink itself, as download-ci-llvm will end up
invoking llvm-config, which requires the symlink to exist. The
symlink is not shipped in other components.
CDSort and CDSplit are the most recent versions of function ordering and function splitting algorithms with some improvements over the previous baseline (ext-tsp and two-way splitting).
bootstrap major change detection implementation
The use of `changelog-seen` and `bootstrap/CHANGELOG.md` has not been functional in any way for many years. We often do major/breaking changes but never update the changelog file or the `changelog-seen`. This is an alternative method for tracking major or breaking changes and informing developers when such changes occur.
Example output when bootstrap detects a major change:
Build the first LLVM without LTO in try builds
Currently, we perform three LLVM builds in the Linux x64 dist builder, which is used for `try` builds:
1) "Normal" LLVM - takes ~5s to compile thanks to `sccache`, but ~8 minutes to link because of ThinLTO
2) PGO instrumented LLVM - same timings as 1)
3) PGO optimized LLVM - takes about 20 minutes to build
When I tried to disable LTO for build 1), it suddenly takes only about a minute to build, because the linking step is much faster. The first LLVM doesn't really need LTO all that much. Without it, it will be a bit slower to build `rustc` in two subsequent steps, but it seems that the ~7 minutes saved on linking it do win that back.
Btw, we can't use the host LLVM for build 1), because this LLVM then builds `rustc` in PGO instrumented mode, and we need the same compiler when later PGO optimizing `rustc`. And we want to use our in-house LLVM for that I think.
