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Daniel_V_2
Beginner
72 Views

Bug: C++11 non-POD thread_local causes reinitialization of non-thread_local static vars?

Using a thread_local non-POD type seems to cause all static but non-thread_local variables (which should only be initialized once) to be reconstructed on every access to the thread_local variable.

Observe:

// icpc version 15.0.0 (gcc version 4.9.0 compatibility)
// icpc -std=c++11 tls.cpp && ./a.out
// -- Expected output --
// MakePointer()
// MyInt constructor <pointer>
// -- Actual output --
// MakePointer()
// MyInt constructor <pointer>
// MakePointer() x 10 (= numReps)

#include <cstdio>

// non-POD type
struct MyInt
{
	MyInt()
	{
		printf("MyInt constructor %p\n", this);
	}
	int v = 1;
};

// should be called exactly once
inline void *MakePointer()
{
	printf("MakePointer()\n");
	return nullptr;
}

// normal static initialization, should happen ONCE
static void *pointer = MakePointer();

// thread_local non-POD variable, should be constructed as many times as there are threads
static thread_local MyInt v1;

const int numReps = 10;

int main(int argc, char const *argv[])
{
	for (int i = 0; i < numReps; ++i)
		++v1.v; // each access seems to reinitialize all static variables?!?
	return 0;
}

We have a static variables 'pointer' which should be constructed exactly once (twice if you count zero-initialization of statics and globals) and a non-POD thread_local variables 'v1' which - as this example only used a single thread - be constructed once as well.

Unfortunately - at least using icpc version 15.0.0 (gcc version 4.9.0 compatibility) - every access to the thread_local variable 'v1' seems to cause reinitialization of 'pointer'. This is both a performance problem (depending on the cost of initialization of all statics in your translation unit) and a correctness problem (in my actual use case a static variable shared between threads got re-initialized by later threads while it was being used by earlier ones).

When I change the thread_local 'v1' to a POD-type (e.g. int), this phenomenon no longer seems to occur.

0 Kudos
1 Reply
72 Views

Hi Daniel,

Could you please upgrade your compiler? It works with latest 15.0 U2 release (and 15.0 U1 release). See below:

$ source /opt/intel/composer_xe_2015.0.090/bin/compilervars.sh intel64
$ icc temp.cpp -std=c++11 && ./a.out
MakePointer()
MyInt constructor 0x7f3c2fcd9718
MakePointer()
MakePointer()
MakePointer()
MakePointer()
MakePointer()
MakePointer()
MakePointer()
MakePointer()
MakePointer()
MakePointer()
$ source /opt/intel/composer_xe_2015.1.133/bin/compilervars.sh intel64
$ icc temp.cpp -std=c++11 && ./a.out
MakePointer()
MyInt constructor 0x7fae6756f718
$ source /opt/intel/composer_xe_2015.2.164/bin/compilervars.sh intel64
$ icc temp.cpp -std=c++11 && ./a.out
MakePointer()
MyInt constructor 0x7f1bb853a718
$

Thanks,

Shenghong