[LLVMdev] [PATCH] Replace the Execution Engine's mutex with std::recursive_mutex

[Reid Kleckner]

OK, sounds like we're screwed.

There's two options:
1. Revert and give up on C++11 threading libraries for now.
2. Do what Eric suggests.  Move all the mutex usage under #ifdef
LLVM_ENABLE_THREADS, and disable LLVM_ENABLE_THREADS by default on MinGW.
 MinGW plus LLVM_ENABLE_THREADS would become unsupported.

Do people have objections to 2?  I don't really like it either.


On Fri, Jun 20, 2014 at 10:33 AM, Yaron Keren <yaron.keren at gmail.com> wrote:

> The whole "mutex" and "shared_mutex" files are #ifdef
> _GLIBCXX_HAS_GTHREADS so if no _GLIBCXX_HAS_GTHREADS there are no mutexes
> and no call_once. thread lives in "thread" which is also #ifdef
> _GLIBCXX_HAS_GTHREADS.
> "condition_variable" and "future" are the same.
>
> I have tested gcc 4.8.2 predefines and _GLIBCXX_HAS_GTHREADS isn't there
> nor it is defined anywhere with the win32 version. I have also compiled a
> small test and indeed it failed with
>
>   a.cpp:4:3: error: 'mutex' is not a member of 'std'.
>
> Just for fun, I tried to compile it with -D_GLIBCXX_HAS_GTHREADS but then
> it failed on bunch of other errors starting with
>
>   error: '__gthread_time_t' was not declared in this scope
>
> so gthreads isn't there.
>
> As to popularity, compare the download graphs for 32 bit:
>
>
> http://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win32/Personal%20Builds/mingw-builds/4.9.0/
>
> and 64 bit:
>
>
> http://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win64/Personal%20Builds/mingw-builds/4.9.0/
>
> in 32 bit the posix version rules, whereas in 64 bit it is a close winner.
> If you go back to 4.8.2 the pattern is similar.
>
> The win32 version does not support anything thread-related so it's not
> C++11 compliant?
>
> Yaron
>
>
>
> 2014-06-20 19:55 GMT+03:00 Reid Kleckner <rnk at google.com>:
>
>> It sounds like this version of libstdc++ doesn't support
>> std::recursive_mutex from C++11.  This is really unfortunate, because we
>> were hoping that moving to C++11 would allow us to use standard, portable
>> threading primitives.
>>
>> Does this version of MinGW have any C++11 threading support?  Is it just
>> recursive_mutex that is missing, or do we have to avoid std::mutex,
>> std::call_once, etc?  lld has been using all of these things for some time
>> now, and in theory we have the same baseline toolchain requirements.
>>
>> If it's just std::recursive_mutex, how long do you think it would take to
>> implement that for mingw's libstdc++?
>>
>> Do you have a sense of which version of mingw is more popular, the
>> pthreads variant or the win32 threads variant?  If the overwhelming
>> majority use the win32 threads variant, I don't think we can break it.
>>
>>
>> On Fri, Jun 20, 2014 at 9:49 AM, Zachary Turner <zturner at google.com>
>> wrote:
>>
>>> I kind of feel like we should drop support for this configuration.  Here
>>> are the reasons why:
>>>
>>> 1) clang, lld, and other LLVM-based tools already make use of
>>> std::recursive_mutex and std::mutex, so if those types don't exist in this
>>> one configuration, we have already (even if inadvertently) made a statement
>>> that we don't support that configuration.
>>>
>>> 2) We chose C++11 as the baseline because all compilers should support
>>> it.  This functionality in particular is pretty egregious to not support,
>>> considering how simple it is.
>>>
>>> 3) Not supporting this configuration does not mean we don't support GCC
>>> / MinGW, it only means we don't support GCC / MinGW / threads-win32.
>>> There is still the threads-posix flavor of this platform which works fine
>>> on Windows.
>>>
>>> #3 is a little unfortunate and backwards, since on Windows we should be
>>> encouraging native Windows implementations of things and discouraging posix
>>> emulation, but in this case the functionality just isn't implemented.
>>>
>>>
>>> On Fri, Jun 20, 2014 at 9:26 AM, Zachary Turner <zturner at google.com>
>>> wrote:
>>>
>>>> +llvmdev.
>>>>
>>>> I find this pretty surprising.  Actually, we already use std::mutex and
>>>> std::recursive_mutex in clang, lld, and other llvm projects, it's just a
>>>> coincidence that it hadn't been introduced into LLVM until my commits.
>>>>
>>>> I'm not sure what the right thing to do here is.  If I understand
>>>> correctly, it seems like in order to encounter this, a) you must be using
>>>> GCC, b) you must be using the MinGW flavor of GCC, and c) you must be using
>>>> the threads-win32 flavor of this toolchain.   Only if all 3 of those are
>>>> true, then std::mutex and std::recursive_mutex don't exist.
>>>>
>>>> Anybody else have thoughts on whether this necessitates reverting the
>>>> mutex changes, or whether this toolchain configuration should be supported?
>>>>
>>>>
>>>> On Fri, Jun 20, 2014 at 12:07 AM, Vadim Chugunov <vadimcn at gmail.com>
>>>> wrote:
>>>>
>>>>> FYI - this commit broke LLVM build using [[
>>>>> http://stackoverflow.com/questions/13212342/whats-the-difference-between-thread-posixs-and-thread-win32-in-gcc-port-of-windo
>>>>> | win32 threads ]] flavor of the mingw toolchain.  I am getting [[
>>>>> http://stackoverflow.com/questions/14191566/c-mutex-in-namespace-std-does-not-name-a-type
>>>>> | error: 'recursive_mutex' in namespace 'std' does not name a type ]].
>>>>> Not sure if this would be considered a problem for LLVM...
>>>>>
>>>>> http://reviews.llvm.org/D4196
>>>>>
>>>>>
>>>>>
>>>>
>>>
>>
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