<div dir="ltr">I should mention that Windows already has a section with callbacks to run on thread startup. I believe it is .CRT$XL[A-Z], and you can see it in use here:<div><a href="https://reviews.llvm.org/D71786">https://reviews.llvm.org/D71786</a> <br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Jan 13, 2020 at 2:29 PM Shoaib Meenai via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div lang="EN-US">
<div class="gmail-m_-3095266050741265124WordSection1">
<p class="MsoNormal">The __start_SECTIONNAME trick is ELF-specifc. Note that the section name has to be a valid C identifier for this to work: it can only contain letters, numbers (except at the start), and underscores. In particular, the section name can’t
have a period (.) in it. LLD ELF’s logic for this is at <a href="https://github.com/llvm/llvm-project/blob/a506f7f9105eec4baac296d21c922457d6f4b52a/lld/ELF/Writer.cpp#L1973" target="_blank">
https://github.com/llvm/llvm-project/blob/a506f7f9105eec4baac296d21c922457d6f4b52a/lld/ELF/Writer.cpp#L1973</a><u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">In COFF, this is usually implemented using grouped sections: if you have sections in your input files named .foo$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 <a href="https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#grouped-sections-object-only" target="_blank">
https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#grouped-sections-object-only</a><u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">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.<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<div style="border-right:none;border-bottom:none;border-left:none;border-top:1pt solid rgb(181,196,223);padding:3pt 0in 0in">
<p class="MsoNormal"><b><span style="font-size:12pt;color:black">From: </span></b><span style="font-size:12pt;color:black">llvm-dev <<a href="mailto:llvm-dev-bounces@lists.llvm.org" target="_blank">llvm-dev-bounces@lists.llvm.org</a>> on behalf of James Y Knight via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>><br>
<b>Reply-To: </b>James Y Knight <<a href="mailto:jyknight@google.com" target="_blank">jyknight@google.com</a>><br>
<b>Date: </b>Thursday, January 9, 2020 at 11:30 AM<br>
<b>To: </b>Yafei Liu <<a href="mailto:yfliu@mobvoi.com" target="_blank">yfliu@mobvoi.com</a>><br>
<b>Cc: </b>llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>><br>
<b>Subject: </b>Re: [llvm-dev] Is there some sort of "@llvm.thread_ctors."?<u></u><u></u></span></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<div>
<p class="MsoNormal">On Thu, Jan 9, 2020 at 6:47 AM Yafei Liu via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<u></u><u></u></p>
</div>
<blockquote style="border-top:none;border-right:none;border-bottom:none;border-left:1pt solid rgb(204,204,204);padding:0in 0in 0in 6pt;margin-left:4.8pt;margin-right:0in">
<div>
<p class="MsoNormal">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.<u></u><u></u></p>
</div>
</blockquote>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">The last is the important part. But that's just an abstraction across the various global constructor mechanisms provided by the underlying platforms.<u></u><u></u></p>
</div>
<blockquote style="border-top:none;border-right:none;border-bottom:none;border-left:1pt solid rgb(204,204,204);padding:0in 0in 0in 6pt;margin-left:4.8pt;margin-right:0in">
<div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">We also know that in C++, the constructor of a static thread_local member will
<b>not</b> 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.<u></u><u></u></p>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">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.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
<blockquote style="border-top:none;border-right:none;border-bottom:none;border-left:1pt solid rgb(204,204,204);padding:0in 0in 0in 6pt;margin-left:4.8pt;margin-right:0in">
<div>
<div>
<p class="MsoNormal">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?<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">I read the LLVM Language Reference Manual, but it doesn't talk much in Thread Local Storage Models. <u></u><u></u></p>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal">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.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">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).<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
<div>
<p class="MsoNormal">If you need to also support shared libraries, then finding all constructors across all libraries gets somewhat more complex...<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
</div>
</div>
</div>
</div>
_______________________________________________<br>
LLVM Developers mailing list<br>
<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a><br>
<a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev" rel="noreferrer" target="_blank">https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a><br>
</blockquote></div>