commit 183315165e6b757c62ee03cf8b29ea8fb924f234
author Fangrui Song <i@maskray.me> Thu Feb 25 15:46:37 2021 -0800
committer Copybara-Service <copybara-worker@google.com> Fri Feb 26 22:04:10 2021 -0800
tree 62a3f2663aa388afb991dedffc9de606242cc0ac
parent 5cbd0695f56f7a778dd860913a2339f152a725ae

[ELF] Add -z start-stop-gc to let __start_/__stop_ not retain C identifier name sections

For one metadata section usage, each text section references a metadata section.
The metadata sections have a C identifier name to allow the runtime to collect them via `__start_/__stop_` symbols.

Since `__start_`/`__stop_` references are always present from live sections, the
C identifier name sections appear like GC roots, which means they cannot be
discarded by `ld --gc-sections`.

To make such sections GCable, either SHF_LINK_ORDER or a section group is needed.

SHF_LINK_ORDER is not suitable for the references can be inlined into other functions
(See D97430:
Function A (in the section .text.A) references its `__sancov_guard` section.
Function B inlines A (so now .text.B references `__sancov_guard` - this is invalid with the semantics of SHF_LINK_ORDER).

In the linking stage,
if `.text.A` gets discarded, and `__sancov_guard` is retained via the reference from `.text.B`,
the output will be invalid because `__sancov_guard` references the discarded `.text.A`.
LLD errors "sh_link points to discarded section".
)

A section group have size overhead, and is cumbersome when there is just one metadata section.

Add `-z start-stop-gc` to drop the "__start_/__stop_ references retain
non-SHF_LINK_ORDER non-SHF_GROUP C identifier name sections" rule.
We reserve the rights to switch the default in the future.

Reviewed By: phosek, jrtc27

Differential Revision: https://reviews.llvm.org/D96914

GitOrigin-RevId: 4bbcd63eea4950c38e29f9e29b1d11b7d7894469