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

Fri Jun 20 11:39:13 PDT 2014

I suppose there's also an option #3: "declare that LLVM only supports the
threads-posix flavor".   As long as configure script gives a clear error
message...

It should be noted, though, that MinGW's pthreads mutexes are likely to
perform worse than LLVM's home-grown ones:
http://sourceforge.net/p/mingw-w64/bugs/344/

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

> 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
>>>>>
>>>>>
>>>>
>>>
>>
>
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