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

[Reid Kleckner]

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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