Refactor some code in `BPSectionOrderer.cpp` in preparation for
https://github.com/llvm/llvm-project/pull/107348.
* Rename `constructNodesForCompression()` -> `getUnsForCompression()`
and return a `SmallVector` directly rather than populating a vector
alias
* Pass `duplicateSectionIdxs` as a pointer to make it possible to skip
finding (nearly) duplicate sections
* Combine `duplicate{Function,Data}SectionIdxs` into one variable
* Compute all `BPFunctionNode` vectors at the end (like
`nodesForStartup`)
There should be no functional change.
Add the "Separate" option `--irpgo-profile-sort <profile` instead of
just the "Joined" option `--irpgo-profile-sort=<profile>`. This is
useful if the path has a `,` for some reason which would break when
trying to use `-Wl,--irpgo-profile-sort=<profile-with-comma>`.
While I'm here, use `static_cast<>` instead of the C style cast
introduced in https://github.com/llvm/llvm-project/pull/100627
Add the lld flags `--irpgo-profile-sort=<profile>` and
`--compression-sort={function,data,both}` to order functions to improve
startup time, and functions or data to improve compressed size,
respectively.
We use Balanced Partitioning to determine the best section order using
traces from IRPGO profiles (see
https://discourse.llvm.org/t/rfc-temporal-profiling-extension-for-irpgo/68068
for details) to improve startup time and using hashes of section
contents to improve compressed size.
In our recent LLVM talk (https://www.youtube.com/watch?v=yd4pbSTjwuA),
we showed that this can reduce page faults during startup by 40% on a
large iOS app and we can reduce compressed size by 0.8-3%.
More details can be found in https://dl.acm.org/doi/10.1145/3660635
---------
Co-authored-by: Vincent Lee <thevinster@users.noreply.github.com>
