[llvm] r227300 - [LPM] Rip all of ManagedStatic and ThreadLocal out of the pretty stack

[Chandler Carruth]

On Wed, Jan 28, 2015 at 3:05 PM, Chandler Carruth <chandlerc at gmail.com>
wrote:

> On Wed, Jan 28, 2015 at 2:14 PM, Owen Anderson <resistor at mac.com> wrote:
>
>> Hi Chandler,
>>
>> On Jan 28, 2015, at 1:52 AM, Chandler Carruth <chandlerc at gmail.com>
>> wrote:
>>
>> -static ManagedStatic<sys::ThreadLocal<const PrettyStackTraceEntry> >
>> PrettyStackTraceHead;
>> +// We need a thread local pointer to manage the stack of our stack trace
>> +// objects, but we *really* cannot tolerate destructors running and do
>> not want
>> +// to pay any overhead of synchronizing. As a consequence, we use a raw
>> +// thread-local variable. Some day, we should be able to use a limited
>> subset
>> +// of C++11's thread_local, but compilers aren't up for it today.
>> +// FIXME: This should be moved to a Compiler.h abstraction.
>> +#ifdef _MSC_VER // MSVC supports this with a __declspec.
>> +static __declspec(thread) const PrettyStackTraceEntry
>> +    *PrettyStackTraceHead = nullptr;
>> +#else // Clang, GCC, and all compatible compilers tend to use __thread.
>> +static __thread const PrettyStackTraceEntry *PrettyStackTraceHead =
>> nullptr;
>> +#endif
>>
>>
>> Darwin doesn’t support __thread or thread_local on armv7 or arm64.  This
>> is breaking all Darwin embedded builds.  :-(
>>
>
> So, this is part of C++11 and threading... I don't know how to support a
> system which wants a threaded LLVM but doesn't support TLS. I'm happy to
> make these compile out when threading is disabled of course. Do you have
> any other suggestions?


FWIW, now that I think I have the PPC issue sorted, I've got a patch ready
to go which moves this to Compiler.h and makes it only be TLS when threads
are enabled at build time.
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