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

[Jonathan Wakely]

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.

> 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.