[PATCH] D97447: Add GNU attribute 'retain'

[Fangrui Song via Phabricator via cfe-commits]

MaskRay created this revision.
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For ELF targets, GCC 11 will set SHF_GNU_RETAIN on the section of a
`__attribute__((retain))` function/variable to prevent linker garbage
collection. (See AttrDocs.td for the linker support).

This patch adds `retain` functions/variables to the `llvm.used` list, which has
the desired linker GC semantics. Note: `retain` does not imply `used`,
so an unused function/variable can be dropped by Sema.

Before 'retain' was introduced, previous ELF solutions require inline asm or
linker tricks, e.g.  `asm volatile(".reloc 0, R_X86_64_NONE, target");`
(architecture dependent) or define a non-local symbol in the section and use
`ld -u`. There was no elegant source-level solution.

As of this patch, llvm.used and llvm.compiler.used have the same
behaviors for ELF. `__attribute__((retain))` does not set `SHF_GNU_RETAIN` yet.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D97447

Files:
  clang/include/clang/Basic/Attr.td
  clang/include/clang/Basic/AttrDocs.td
  clang/lib/CodeGen/CGDecl.cpp
  clang/lib/CodeGen/CodeGenModule.cpp
  clang/lib/Sema/SemaDecl.cpp
  clang/test/CodeGen/attr-retain.c
  clang/test/CodeGenCXX/attr-retain.cpp
  clang/test/Sema/attr-retain.c

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