Re: Compiling for single-threaded use (implicit threading support difference in 4.9.1 vs. 4.8.1)

[Johan Alfredsson <sduvan dot gcc at gmail dot com>]

On Wed, Oct 15, 2014 at 6:44 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> On 15 October 2014 10:14, Jonathan Wakely wrote:
>> On 14 October 2014 18:29, Johan Alfredsson wrote:
>>> Hi,
>>>
>>> I've noticed that g++ 4.9.1 behaves differently than 4.8.1 with
>>> regards to (implicit) threading support. The 4.8.1 and 4.9.1 compilers
>>> used were configured with identical options (*) to the configure
>>> script (except --prefix) using --enable-threads=posix.
>>>
>>> For the following test-case
>>>
>>> #include <string>
>>> #include <iostream>
>>>
>>> int main() {
>>>     std::string test("test");
>>>     std::cout << test << std::endl;
>>> }
>>>
>>> invoking g++ -O3 test.cc -o test, the 'test' binary is compiled with
>>> multi-threading support using 4.9.1 but not using 4.8.1, e.g. for the
>>> libstdc++ pool allocator a mutex is locked when allocating memory for
>>> the string in the test program above while no such locking is present
>>> in the 'test' binary compiled with 4.8.1. (There is also a difference
>>> in that there is a weak symbol __pthread_key_create in the binary
>>> compiled with 4.9.1 but no such thing for the  4.8.1 case.)
>>
>> Using a mutex in a single-threaded program would be a bug.

Indeed. I don't use mutexes but things like the pool allocator does
even if I don't want/need that (see below).

>>> As my application is single-threaded, I don't want to pay the
>>> performance penalty of mutexes etc. Hence, my question is if it is
>>> possible to explicitly request gcc to compile my application in
>>> single-threaded mode.
>>
>> It should happen automatically, there's no way to request it because
>> there should be no need.
>>
>> I'll try to reproduce what you're seeing.
>
> I can't reproduce the problem with GCC 4.9.1 or trunk. I'm using a
> Fedora 20 x86_64 system, so it's possible there's something different
> on your distro.

Sorry, my mistake. It turned out that librt was implicitly linked in
in the 4.9.1 case. However, the only things I use from librt are high
precision timers, so a switch to ensure no performance hit in my own
code would be great.

Regards,

/Johan