On the 32-bit win7 target, we use OS TLS instead of native TLS, due to
issues with how the OS handles alignment. Unfortunately, this caused
issues due to the TLS destructors not running, causing memory leaks
among other problems.
On Windows, to support OS TLS, the TlsAlloc family of function is used
by Rust. This function does not support TLS destructors at all. However,
rust has some code to emulate those destructors, by leveraging the TLS
support functionality found in the MSVC CRT (specifically, in tlssup.c
of the CRT). Specifically, the CRT provides the ability to register
callbacks that are called (among other things) on thread destruction. By
registering our own callback, we can run through a list of registered
destructors functions to execute.
To use this functionality, the user must do two things:
1. They must put the address to their callback in a section between
`.CRT$XLB` and `.CRT$XLY`.
2. They must add a reference to `_tls_used` (or `__tls_used` on x86) to
make sure the TLS support code in tlssup.c isn't garbage collected by
the linker.
Prior to this commit, this second bit wasn't being done properly by the
Rust TLS support code. Instead of adding a reference to _tls_used, it
instead had a reference to its own callback to prevent it from getting
GC'd by the linker. While this is _also_ necessary, not having a
reference on _tls_used made the entire support non-functional.
This commit reworks the code to:
1. Add an unconditional `#[used]` attribute on the CALLBACK, which
should be enough to prevent it from getting GC'd by the linker.
2. Add a reference to `_tls_used`, which should pull the TLS support
code into the Rust programs and not let it be GC'd by the linker.
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