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

[Fāng-ruì Sòng via cfe-dev]

On Wed, Feb 24, 2021 at 10:40 AM David Blaikie <dblaikie at gmail.com> wrote:
>
> (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.

I chatted with Saleem Abdulrasool, who is in favor of (1), too.

I am going to send these patches:

(a) Add CodeGenModule::addUsedOrCompilerUsedGlobal (which uses
llvm.compiler.used for ELF and llvm.used for the others). Migrate some
addUsedGlobal call sites to use addUsedOrCompilerUsedGlobal.
(b) Add __attribute__((retain))
(c) Change llvm.used to use SHF_GNU_RETAIN if integrated assembler or
binutils>=2.36

Currently llvm.used/llvm.compiler.used should have no difference on
ELF, so (a) & (b) do not affect users who don't use 'retain'.

(c) will change the binary format representation of llvm.used, so
there is some risk if the consumer is not prepared for multiple
sections of the same name (which means they already break with
-fno-unique-section-names, but the option is rare).
On very large C/C++ projects, llvm.used has usually 0 or 1 element.
ObjC can have multiple llvm.used but that should work. So if there is
risk, the risk for other frontends.
I don't see a way to avoid that, but they can switch to llvm.compiler.used.

Non-ELF users should not observe anything different.

> 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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