[llvm-dev] Is there some sort of "@llvm.thread_ctors."?

[Reid Kleckner via llvm-dev]

I should mention that Windows already has a section with callbacks to run
on thread startup. I believe it is .CRT$XL[A-Z], and you can see it in use
here:
https://reviews.llvm.org/D71786

On Mon, Jan 13, 2020 at 2:29 PM Shoaib Meenai via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> The __start_SECTIONNAME trick is ELF-specifc. Note that the section name
> has to be a valid C identifier for this to work: it can only contain
> letters, numbers (except at the start), and underscores. In particular, the
> section name can’t have a period (.) in it. LLD ELF’s logic for this is at
> https://github.com/llvm/llvm-project/blob/a506f7f9105eec4baac296d21c922457d6f4b52a/lld/ELF/Writer.cpp#L1973
>
>
>
> In COFF, this is usually implemented using grouped sections: if you have
> sections in your input files named .foo$A, .foo$B, etc., the linker places
> them in the output section .foo but orders them according to the input
> section name (e.g. all .foo$B contents will end up after all .foo$A
> contents). You can therefore have your start of list symbol in .foo$A, your
> end of list symbol in .foo$Z, and your list contents in .foo$B (or any
> other letter that’s not A or Z), and iterate over the list that way. More
> details on grouped sections can be found at
> https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#grouped-sections-object-only
>
>
>
> I’m not familiar with how to do this on Mach-O, but I believe there are
> dynamic linker APIs for traversing the contents of a particular section
> that can be used for this purpose.
>
>
>
> *From: *llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of James Y
> Knight via llvm-dev <llvm-dev at lists.llvm.org>
> *Reply-To: *James Y Knight <jyknight at google.com>
> *Date: *Thursday, January 9, 2020 at 11:30 AM
> *To: *Yafei Liu <yfliu at mobvoi.com>
> *Cc: *llvm-dev <llvm-dev at lists.llvm.org>
> *Subject: *Re: [llvm-dev] Is there some sort of "@llvm.thread_ctors."?
>
>
>
>
>
>
>
> On Thu, Jan 9, 2020 at 6:47 AM Yafei Liu via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> We know that in C++, the constructor of a static member will get called
> when the program starts up. I checked the generated IR code and found this
> is implemented by defining a __cxx_global_var_init() function and marked it
> as section ".text.startup" and assign it to @llvm.global_ctors.
>
>
>
> The last is the important part. But that's just an abstraction across the
> various global constructor mechanisms provided by the underlying platforms.
>
>
>
> We also know that in C++, the constructor of a static thread_local member
> will *not* get called when the program starts, but the first time this
> member is used, I also checked the IR code, this is implemented by calling
> it's constructor at the usage, and then call __cxa_thread_atexit to
> register it's destructor.
>
>
>
> This behavior is not guaranteed by C++. It would also be acceptable
> according to the standard to construct the thread-locals eagerly upon
> thread startup, as you want to do. However, yes, clang does always lazy
> initialize them.
>
>
>
> Now I want to add thread_local feature to my own language, and I want my
> thread_local member acts simlar like a static member, that is,  when a new
> thread is created, the constructors of thread_local members will
> automatically get called, instead get called when used?
>
>
>
> I read the LLVM Language Reference Manual, but it doesn't talk much
> in Thread Local Storage Models.
>
>
>
> No platform I know of provides such a mechanism that you (or llvm) could
> hook into. If you want this, you'll need to create your own thread-startup
> routine that calls your own thread-local initializers.
>
>
>
> The most obvious way to do this when there's no shared-libraries in the
> mix would be to emit the initializer-function pointers into a custom named
> section, and then iterate over that section in your thread startup routine.
> The linker automatically creates __start_SECTIONNAME and __stop_SECTIONNAME
> symbols, so you can just iterate on the pointers between them. (At least
> this works in ELF, not sure if it's the same on COFF/MachO).
>
>
>
> If you need to also support shared libraries, then finding all
> constructors across all libraries gets somewhat more complex...
>
>
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