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

Zachary Turner zturner at google.com
Fri Jun 20 12:34:08 PDT 2014 

Hmm, that's no good either.  Someone pointed out to me that the reason for
implementing this as a semaphore probably has to do with the fact that
std::mutex can be used with a std::condition_variable, and you can't make
that work with a critical section.  Still, there are better ways to
implement it, because on Vista+ Windows provides a native condition
variable object.  Still though, it's unfortunate.

I'm not really sure what the right solution is.  I'll let this thread bake
for a little while and get some more comments, but I may end up reverting
this patch if there's no good solution.


On Fri, Jun 20, 2014 at 11:39 AM, Vadim Chugunov <vadimcn at gmail.com> wrote:

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