[cfe-dev] __attribute__((retain)) && llvm.used/llvm.compiler.used

[Fāng-ruì Sòng via cfe-dev]

Currently __attribute__((used)) lowers to llvm.used.

* On Mach-O, a GlobalObject in llvm.used gets the S_ATTR_NO_DEAD_STRIP
attribute, which prevents linker GC (dead stripping).
* On COFF, a non-local-linkage GlobalObject[1] in llvm.used gets the
/INCLUDE: linker option (similar to ELF `ld -u`), which prevents
linker GC.
  It should be possible to work with local linkage GlobalObject's as
well but that will require a complex COMDAT dance.
* On ELF, a global object llvm.used can be discarded by
ld.bfd/gold/ld.lld --gc-sections.
  (If the section is a C identifier name, __start_/__stop_ relocations
from a live input section can retain the section, even if its defined
symbols are not referenced. [2] .
  I understand that some folks use `__attribute__((used,
section("C_ident")))` and expect the sections to be similar to GC
roots, however,
  non-C-identifier cases are very common, too. They don't get
__start_/__stop_ linker magic and the sections can always be GCed.
  )

In LangRef, the description of llvm.used contains:

> If a symbol appears in the @llvm.used list, then the compiler, assembler, and **linker** are required to treat the symbol as if there is a reference to the symbol that it cannot see (which is why they have to be named). For example, if a variable has internal linkage and no references other than that from the @llvm.used list, it cannot be deleted. This is commonly used to represent references from inline asms and other things the compiler cannot “see”, and corresponds to “attribute((used))” in GNU C.

Note that the "linker" part does not match the reality on ELF targets.
It does match the reality on Mach-O and partially on COFF.

llvm.compiler.used:

> The @llvm.compiler.used directive is the same as the @llvm.used directive, except that it only prevents the compiler from touching the symbol. On targets that support it, this allows an **intelligent linker to optimize references to the symbol without being impeded** as it would be by @llvm.used.

Note that this explicitly mentions linker GC, so this appears to be
the closest thing to __attribute__((used)) on ELF.
However, LangRef also says:

> This is a rare construct that should only be used in rare circumstances, and should not be exposed to source languages.



My goal is to implement __attribute__((retain)) (which will be in GCC
11) on ELF. GCC folks think that 'used' and 'retain are orthogonal.
(see https://reviews.llvm.org/D96838#2578127)

Shall we

1. Lift the source language restriction on llvm.compiler.used and
change __attribute__((used)) to use llvm.compiler.used on ELF.
2. Or add a metadata (like https://reviews.llvm.org/D96837)?


I lean to option 1 to leverage the existing mechanism.
The downside is that clang codegen will have some target inconsistency
(llvm.compiler.used on ELF while llvm.used on others).



[1]: The implementation additionally allows GlobalAlias.
[2]: See https://maskray.me/blog/2021-01-31-metadata-sections-comdat-and-shf-link-order
"C identifier name sections" for details.