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

[Yaron Keren]

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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