[PATCH] D64537: [WebAssembly] Implement thread-local storage (local-exec model)

Guanzhong Chen via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Tue Jul 16 17:18:03 PDT 2019

quantum added a comment.

In D64537#1588699 <https://reviews.llvm.org/D64537#1588699>, @dschuff wrote:

> I had a reply that got eaten here, so I'm going to keep trolling you on your CL since we don't have a design doc for this.
>  The `offset` field of a data segment initializer can be a `global.get` on an imported global.  (https://webassembly.github.io/spec/core/valid/instructions.html#constant-expressions). Since each thread is separately instantiated with separate JS, we could have a global import like `__tls_base` which has a different value in each thread. Then we wouldn't need to manually call the init code anywhere. Would there be other advantages or disadvantages for that?

I already answered this one:

In D64537#1586614 <https://reviews.llvm.org/D64537#1586614>, @quantum wrote:

> In D64537#1586556 <https://reviews.llvm.org/D64537#1586556>, @dschuff wrote:
> > The `offset` field of a segment can be a constant expression <https://webassembly.github.io/spec/core/valid/instructions.html#constant-expressions> which can be a `global.get` of an imported global. So we could have an imported global `__tls_base` which is different for each thread, and have an active segment with that as its segment offset?
> I didn't know that it could have been a constant expression. I don't think this would have worked very well on the main thread though, since we need to run `malloc` before we can compute `__tls_base`. I think this requires the global to be mutable, if only because we need to be able to initialize it on the main thread.

The problem I found with the import approach is that the main thread cannot use `global.get` of an imported global to get the location of TLS, mainly because it needs to be `malloc`'d. To use `malloc`, the main thread needs to be initialized. This will result in a circular dependency.




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