llvm dev - Jan 2020 - Is there some sort of "@llvm.thread_ctors."?

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.

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.

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.
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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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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<mailto: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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