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https://llvm.org/docs/HowToBuildWindowsItaniumPrograms.html
This is a weird middle ground between `*-windows-gnu` and `*-windows-msvc`. It
uses the C++ ABI of the former while using the system libraries of the latter.
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`__xl_a` is just a global variable containing a null function pointer. There's
nothing magical about it or its name at all.
The section names used on `__xl_a` and `__xl_b` (`.CRT$XLA` and `.CRT$XLZ`) are
the real magic here. The compiler emits TLS variables into `.CRT$XL<x>`
sections, where `x` is an uppercase letter between A and Z (exclusive). The
linker then sorts those sections alphabetically (due to the `$`), and the result
is a neat array of TLS initialization callbacks between `__xl_a` and `__xl_z`.
That array is null-terminated, though! Normally, `__xl_z` serves as the null
terminator; however, by pointing `AddressesOfCallBacks` to `__xl_a`, which just
contains a null function pointer, we've effectively made it so that the PE
loader will just immediately stop invoking TLS callbacks. Fix that by pointing
to the first actual TLS callback instead (or `__xl_z` if there are none).
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LLVM does not use it when emitting code for the MinGW ABI.
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If they're typed as `u8`, they can be aligned to anything. We want at least
pointer size alignment.
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Just to be consistent with Linux.
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