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

Fri Jun 20 11:14:31 PDT 2014

Sorry, I mean only disable this for THREADS-WIN32, not threads-posix.

On Fri, Jun 20, 2014 at 11:14 AM, Zachary Turner <zturner at google.com> wrote:

> #2 is better if we can detect threads-win32 vs threads-posix on MinGW, and
> only disable this for threads-posix.  We can check for
> _GLIBCXX_HAS_GTHREADS, but that seems somewhat hackish, so I wonder if
> there's a better way.
>
> To handle the switching, I guess we'll have to go back to the original
> option of having llvm::mutex, llvm::recursive_mutex, etc, and then
> conditionally typedefing them.  Kinda sucks, but still better than getting
> rid of it entirely.
>
>
> On Fri, Jun 20, 2014 at 11:09 AM, Reid Kleckner <rnk at google.com> wrote:
>
>> 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
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>>
>>>>
>>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140620/5d461cff/attachment.html>