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

[Nicolai Hähnle via llvm-dev]

Hi Yevgeny,

This would be a very welcome feature for Mesa as well, thank you for 
doing this!


> When several threads compile different modules the compiler options  > (instances of cl::opt) cannot be set individually for each thread. 
That > is because the options are visible to all threads. In other words 
all > options are global.> > It would be convenient if the options were 
specific to LLVMContext and > they were accessed through an instance of 
LLVMContext. This kind of > change would need changes in all source 
files where options are used.> > This patch proposes a solution that 
needs minimal changes in LLVM source > base.> > It is proposed to have a 
thread local set of re-defined option values > mapped by pointers to 
options.
That seems very sensible and pragmatic to me.


> Specifically, every time a program gets/sets a value for an option it is 
> checked if the current thread local context is set for the current 
> thread and the option has its local copy in this context. If so the 
> local copy of the option is accessed, otherwise the global option is 
> accessed. For all programs that existed so far the context is not set 
> and they work with the global options. For new multi-threaded compilers 
> (where every thread compiles its own module) every thread can be linked 
> to its own context (see ContextValues) where any option can have its 
> thread specific value that do not affect the other threads' option 
> values. See the thread_routine() in the test ContextSpecificValues2.

Just to make sure I'm understanding this correctly: When a thread has a 
non-null option context, then options set in the global context will 
have no effect at all for that thread. Right? (That's what I would 
expect and what makes sense to me.)

Cheers,
Nicolai



> 
> This feature allows a configuration flexibility for multi-threaded 
> compilers that can compile every compilation unit in its own thread with 
> different command line options.
> 
> ===
> 
> Thanks.
> 
> -Yevgeny Rouban
> 
> 
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