[PATCH] D97448: ELF: Create unique SHF_GNU_RETAIN sections for llvm.used global objects

[Fangrui Song via Phabricator via llvm-commits]

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If a global object is listed in `@llvm.used`, place it in a unique section with
the `SHF_GNU_RETAIN` flag. The section is a GC root under `ld --gc-sections`
with LLD>=13 or GNU ld>=2.36.

For front ends which do not expect to see multiple sections of the same name,
consider emitting `@llvm.compiler.used` instead of `@llvm.used`.

We don't restrict the feature to ELFOSABI_GNU and ELFOSABI_FREEBSD for now.
See the rationale in D95749 <https://reviews.llvm.org/D95749>.

The integrated assembler has supported SHF_GNU_RETAIN since D95730 <https://reviews.llvm.org/D95730>.
GNU as>=2.36 supports section flag 'R'.
We don't need to worry about GNU ld support because older GNU ld just ignores
the unknown SHF_GNU_RETAIN.

With this change, `__attribute__((retain))` functions/variables emitted
by clang will get the SHF_GNU_RETAIN flag.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D97448

Files:
  llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
  llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
  llvm/lib/Target/RISCV/RISCVTargetObjectFile.cpp
  llvm/test/CodeGen/PowerPC/func-addr-consts.ll
  llvm/test/CodeGen/PowerPC/no-dead-strip.ll
  llvm/test/CodeGen/X86/elf-retain.ll

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