Bolt can be installed starting with LLVM 15 - README potentially needs an update

[lilianmoraru]

The README.md mentions the need to build bolt yourself - this is not required any more, at least on Debian-based distros. It is available in the LLVM repositories( https://apt.llvm.org/ ) starting with LLVM 15(maybe even LLVM 14, have not checked).

Example install on Ubuntu(matching the version used by rustc), starting with rustc 1.65:

# `grep "LLVM version"` if we worry that the LLVM version won't be always the last in the list
export LLVM_MAJOR_VERSION="$(rustc -vV | grep "LLVM version" | cut -d ' ' -f3 | cut -d '.' -f1)"
# `tail -1`  if we worry that `LLVM version` won't be English-only in the future
export LLVM_MAJOR_VERSION="$(rustc -vV | tail -1 | cut -d ' ' -f3 | cut -d '.' -f1)"

export UBUNTU_CODENAME="$(lsb_release -c | cut -d ':' -f2 | xargs)"
echo "deb http://apt.llvm.org/${UBUNTU_CODENAME}/ llvm-toolchain-${UBUNTU_CODENAME}-${LLVM_MAJOR_VERSION} main" | sudo tee -a /etc/apt/sources.list
echo "deb-src http://apt.llvm.org/${UBUNTU_CODENAME}/ llvm-toolchain-${UBUNTU_CODENAME}-${LLVM_MAJOR_VERSION} main" | sudo tee -a /etc/apt/sources.list
wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -

sudo apt install bolt-${LLVM_MAJOR_VERSION}

dpkg -L bolt-15:

/.
/usr
/usr/bin
/usr/lib
/usr/lib/llvm-15
/usr/lib/llvm-15/bin
/usr/lib/llvm-15/bin/llvm-bolt
/usr/lib/llvm-15/bin/llvm-bolt-heatmap
/usr/lib/llvm-15/bin/merge-fdata
/usr/share
/usr/share/doc
/usr/share/doc/bolt-15
/usr/share/doc/bolt-15/NEWS.Debian.gz
/usr/share/doc/bolt-15/changelog.Debian.gz
/usr/share/doc/bolt-15/copyright
/usr/bin/llvm-bolt-15
/usr/bin/llvm-bolt-heatmap-15
/usr/bin/llvm-boltdiff-15
/usr/bin/merge-fdata-15
/usr/bin/perf2bolt-15
/usr/lib/llvm-15/bin/llvm-boltdiff
/usr/lib/llvm-15/bin/perf2bolt

This might also need an update:

cargo-pgo/src/bolt/mod.rs

Line 12 in 2e28cf4

"Build LLVM with BOLT and add its `bin` directory to PATH."

[Kobzol]

Thanks for letting me know! I will check it out, modify the README, and check if LLVM autodetection in cargo-pgo can use the version from Debian packages.

[Kobzol]

Hmm, while the package can be installed, when I actually try to use BOLT, it fails with this error:

BOLT-ERROR: library not found: /usr/lib/libbolt_rt_instr.a

Does it work for you?

[lilianmoraru]

It seems to be provided by libbolt-15-dev(bolt-${LLVM_MAJOR_VERSION}-dev).

dpkg -L libbolt-15-dev:

/.
/usr
/usr/lib
/usr/lib/llvm-15
/usr/lib/llvm-15/lib
/usr/lib/llvm-15/lib/libbolt_rt_hugify.a
/usr/lib/llvm-15/lib/libbolt_rt_instr.a
/usr/share
/usr/share/doc
/usr/share/doc/libbolt-15-dev
/usr/share/doc/libbolt-15-dev/NEWS.Debian.gz
/usr/share/doc/libbolt-15-dev/changelog.Debian.gz
/usr/share/doc/libbolt-15-dev/copyright

This is the hack that I had to do(after trying with linker flags), which I am not particularly happy or proud about:

sudo ln -sf /usr/lib/llvm-15/lib/libbolt_rt_instr.a /usr/lib/libbolt_rt_instr.a

It found it and then llvm-bolt threw SIGSEGV on my test on fd-find 😄

[Kobzol]

I see 😅 Well, I think that I will wait a bit until modifying the readme, as this installation option still seems to be quite unstable. It would be best if BOLT would be built on the LLVM GitHub releases page, I pinged the LLVM maintainers about this, but it hasn't been implemented yet.

[lilianmoraru]

The self-built version of LLVM 15 is also crashing on fd-find but works fine against cargo-pgo(the apt packages as well).

[Kobzol]

Even ignoring the crash, I think that it might not be production ready if you have to link library files manually :D But I can at least add a link to this issue to README: https://github.com/Kobzol/cargo-pgo/blob/main/README.md#bolt-installation.