Allow overriding the TLS model

[Amanieu]

This PR adds the ability to override the default "global-dynamic" TLS model with a more specific one through a target json option or a command-line option. This allows for better code generation in certain situations.

This is similar to the -ftls-model= option in GCC and Clang.

[rust-highfive]

r? @pnkfelix

(rust_highfive has picked a reviewer for you, use r? to override)

[Amanieu]

At the moment get_tls_model is called each time a #[thread_local] static is encountered. I would like to only call this once at the start, that way the TLS model string is validated even if there are no #[thread_local] statics. However I am not sure where I should do this and where I should put the cached value.

[Amanieu]

Ok, I'm now caching the parsed TLS model in the crate context. This PR is ready for merging.

[carols10cents]

r? @michaelwoerister

[michaelwoerister]

r? @alexcrichton

I think @alexcrichton is more qualified to review this.

[alexcrichton]

I don't really know what a TLS model is but it seems fine to expose for now. Can this be placed behind feature gates, however, to require nightly and go through the normal stabilization process?

[Amanieu]

I don't think we have a mechanism for putting a target json attribute under a feature gate.

[Amanieu]

Also, how do I mark a -C option as unstable?

[alexcrichton]

That's ok to leave the target json as-is, those are "pretty unstable" as-is anyway. For the options you can move it to -Z to make it unstable.

[Amanieu]

Done

[alexcrichton]

@bors: r+

[bors]

📌 Commit fdf7ba2 has been approved by alexcrichton

[bors]

⌛ Testing commit fdf7ba2 with merge 7ade24f...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 7ade24f to master...