Update all uses of variadic `.Cases` to use the initializer list
overload instead. I plan to mark variadic `.Cases` as deprecated in a
followup PR.

For more context, see https://github.com/llvm/llvm-project/pull/163117.

Add DTLTO linker option `--thinlto-remote-compiler-prepend-arg` to
enable support for the multi-call LLVM driver that requires an
additional option to specify the subcommand, e.g. "llvm clang ...".

Fixes https://github.com/llvm/llvm-project/issues/159125.

Change LLD DTLTO option definitions to match actual option name.

Add `--print-gc-sections=<file>` to redirect garbage collection section
listing to a file, avoiding contamination of stdout with other linker
output. mold has recently added the option.
GNU ld feature request:
https://sourceware.org/bugzilla/show_bug.cgi?id=33331

This ensures subsequent calls to elf::postScanRelocations with a new Ctx
will correctly use an instance with the right internalFile (with the old
one presumably deleted, even). It also avoids having to create a new
instance in elf::getErrorPlace, and will allow more uses of such a dummy
symbol in future commits.

Reviewers: MaskRay

Reviewed By: MaskRay

Pull Request: https://github.com/llvm/llvm-project/pull/150796

https://discourse.llvm.org/t/rfc-profile-guided-static-data-partitioning/83744
proposes to partition a static data section (like `.data.rel.ro`) into
two sections, one grouping the cold ones and the other grouping the
rest.

lld requires all relro sections to be contiguous. To place
`.data.rel.ro.unlikely` in the middle of all relro sections, this change
proposes to add `.data.rel.ro.unlikely` explicitly as RELRO section.

---------

Co-authored-by: Sam Elliott <quic_aelliott@quicinc.com>

Merge the attributes of object files being linked together. The
`.hexagon.attributes` section can be used by loaders and analysis tools.
This is similar to the .riscv.attributes, introduced in
8a900f2438b4a167b98404565ad4da2645cc9330 /
https://reviews.llvm.org/D138550.

This patch introduces support for Integrated Distributed ThinLTO (DTLTO)
in ELF LLD.

DTLTO enables the distribution of ThinLTO backend compilations via
external distribution systems, such as Incredibuild, during the
traditional link step: https://llvm.org/docs/DTLTO.html.

It is expected that users will invoke DTLTO through the compiler driver
(e.g., Clang) rather than calling LLD directly. A Clang-side interface
for DTLTO will be added in a follow-up patch.

Note: Bitcode members of archives (thin or non-thin) are not currently
supported. This will be addressed in a future change. As a consequence
of this lack of support, this patch is not sufficient to allow for
self-hosting an LLVM build with DTLTO. Theoretically,
--start-lib/--end-lib could be used instead of archives in a self-host
build. However, it's unclear how --start-lib/--end-lib can be easily
used with the LLVM build system.

Testing:
- ELF LLD `lit` test coverage has been added, using a mock distributor
  to avoid requiring Clang.
- Cross-project `lit` tests cover integration with Clang.

For the design discussion of the DTLTO feature, see: #126654.

Fixed assertion failure when reading .eh_frame sections, and added
.eh_frame sections to tests.

This reverts commit 1e95349dbe329938d2962a78baa0ec421e9cd7d1.

Original commit message follows:

When code calls a function which then immediately tail calls another
function there is no need to go via the intermediate function. By
branching directly to the target function we reduce the program's working
set for a slight increase in runtime performance.

Normally it is relatively uncommon to have functions that just tail call
another function, but with LLVM control flow integrity we have jump tables
that replace the function itself as the canonical address. As a result,
when a function address is taken and called directly, for example after
a compiler optimization resolves the indirect call, or if code built
without control flow integrity calls the function, the call will go via
the jump table.

The impact of this optimization was measured using a large internal
Google benchmark. The results were as follows:

CFI enabled:  +0.1% ± 0.05% queries per second
CFI disabled: +0.01% queries per second [not statistically significant]

The optimization is enabled by default at -O2 but may also be enabled
or disabled individually with --{,no-}branch-to-branch.

This optimization is implemented for AArch64 and X86_64 only.

lld's runtime performance (real execution time) after adding this
optimization was measured using firefox-x64 from lld-speed-test [1]
with ldflags "-O2 -S" on an Apple M2 Ultra. The results are as follows:

```
    N           Min           Max        Median           Avg        Stddev
x 512     1.2264546     1.3481076     1.2970261     1.2965788   0.018620888
+ 512     1.2561196     1.3839965     1.3214632     1.3209327   0.019443971
Difference at 95.0% confidence
        0.0243538 +/- 0.00233202
        1.87831% +/- 0.179859%
        (Student's t, pooled s = 0.0190369)
```

[1] https://discourse.llvm.org/t/improving-the-reproducibility-of-linker-benchmarking/86057

Reviewers: zmodem, MaskRay

Reviewed By: MaskRay

Pull Request: https://github.com/llvm/llvm-project/pull/145579

This caused assertion failures in applyBranchToBranchOpt():

  llvm/include/llvm/Support/Casting.h:578:
  decltype(auto) llvm::cast(From*)
  [with To = lld::elf::InputSection; From = lld::elf::InputSectionBase]:
  Assertion `isa<To>(Val) && "cast<Ty>() argument of incompatible type!"' failed.

See comment on the PR (https://github.com/llvm/llvm-project/pull/138366)

This reverts commit 491b82a5ec1add78d2c93370580a2f1897b6a364.

This also reverts the follow-up "[lld] Use llvm::partition_point (NFC) (#145209)"

This reverts commit 2ac293f5ac4cf65c0c038bf75a88f1d6715e467d.