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There were two main places where global variables were used in the Slang implementation:
1. The "standard library" code was generated as a string at run-time, and stored in a global variable so that it could be amortized across compiles.
2. The representation of types uses some globals (well, class `static` members) to store common types (e.g., `void`) and to deal with memory lifetime for things like canonicalized types.
In each case the "simple" fix is to move the relevant state into the `Session` type that controlled their lifetime already (the `Session` destructor was already cleaning up these globals to avoid leaks).
For the standard library stuff this really was easy, but for the types it required threading through the `Session` a bit carefully.
One more case that I found: there was a function-`static` variable used to generate a unique ID for files output when dumping of intermediates is enabled (this is almost strictly a debugging option).
Rather than make this counter per-session (which would lead to different sessions on different threads clobbering the same few files), I went ahead and used an atomic in this case.
Note that the remaining case I had been worried about was any function-`static` counter that might be used in generating unique names.
It turns out that right now the parser doesn't use such a counter (even in cases where it probably should), and the lowering pass already uses a counter local to the pass (again, whether or not this is a good idea).
This change should be a major step toward allowing an application to use Slang in multiple threads, so long as each thread uses a distinct `SlangSession`. The case of using a single session across multiple threads is harder to support, and will require more careful implementation work.
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