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

[David Blaikie via cfe-dev]

(best to include folks from previous conversations in threads - sometimes
we can't all keep up to date with all the threads happening - so I've added
John McCall here, and echristo since he might have some thoughts on this
too)

I'd lean towards (1) too myself - give the LLVM constructs consistent
semantics, and deal with the platform differences in the frontend during
the mapping down to LLVM.

On Wed, Feb 24, 2021 at 1:09 AM Fāng-ruì Sòng via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On 2021-02-24, Fāng-ruì Sòng wrote:
> >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.
>
> It is too late here and I did not think of it clearly;-)
>
> Clarify:
>
> 1. Lift the source language restriction on llvm.compiler.used, let
> llvm.used use SHF_GNU_RETAIN on ELF, and change __attribute__((used)) to
> use llvm.compiler.used on ELF.
>
>
> __attribute__((retain)) has semantics which are not described by
> llvm.used/llvm.compiler.used.  To facilitate linker GC,
> __attribute__((retain))
> causes the section to be placed in a unique section. The separate section
> behavior can be undesired in some cases (e.g. poorly written Linux kernel
> linker
> scripts which expect one section per name).
>
> So in the -fno-function-sections -fno-data-sections case, a retained
> function/variable does not cause the whole .text/.data/.rodata to be
> retained.
>
> The test llvm/test/CodeGen/X86/elf-retain.ll in
> https://reviews.llvm.org/D96837
> demonstrates the behavior.  So I am not particularly clear that we should
> use
> llvm.compiler.used/llvm.used to describe __attribute__((retain)) .
>
> >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.
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