[llvm-dev] [RFC] Enable thread specific cl::opt values for multi-threaded support

[Yevgeny Rouban via llvm-dev]

Hello Chris.

> One of the perennial problems with cl::opt is that ...
My RFC does not change the way the options are initialized and registered.
So the static initialization problem is not made worse by this RFC.

> Personally I very much prefer the idea of having *Context objects be
responsible for options rather than thread-local storage ...
This RFC defines a *Context* for options. In the patch the class is called
ContextValues. Essentially, it is similar to *Context objects*.
With the RFC we will be able to make further changes for specific options
to get them from any specific Context, not only from the thread local one.
The thread local is proposed as the default context to keep unchanged all
places where cl::opts are accessed.

If we have a flag:
static cl::opt<bool> SomeFlag; // static global

Flag access code looks like the following:
if (  SomeFlag ) ... // global flag access

With the RFC (thread specific cl::opt values) this code being unchanged
becomes equivalent to:
if (  SomeFlag.getValue(cl::ContextValues::GetThreadOptionContext()) ) ...
// thread local flag access with fallback to global flag access

Then we can manually change this source to:
if (  SomeFlag.getValue(SomeSpecificContext) ) ... // context specific flag
access with fallback to thread local flag access

In this example SomeSpecificContext can be specific to LLVMContext, Pass,
...

Thanks.
-Yevgeny Rouban
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