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

Fri Jun 20 11:14:10 PDT 2014

#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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