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

Yevgeny Rouban via llvm-dev llvm-dev at lists.llvm.org
Fri Oct 19 03:58:24 PDT 2018

Hello LLVM Developers.

We at Azul Systems are working on a multi-threaded LLVM based compiler. It
can run several compilations each of which compiles its own module in its
own thread.

One of the limitation we face is that all threads use the same options
(instances of cl::opt). In other words, the options are global and cannot
be changed for one thread and left unchanged for the others.

One solution I propose in the patch

https://reviews.llvm.org/D53424 Enable thread specific cl::opt values for
multi-threaded support

As the change affects many source files (though slightly) I decided to
share it with wider audience. Any less intrusive solution is welcome.

Here is the patch description for your convenience:


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

It is proposed to have a thread local set of re-defined option values
mapped by pointers to options.

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.

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.



-Yevgeny Rouban
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