Warnings on macOS Mojave

Question

Hi Miles,

When I install ITensor on macOS Mojave 10.14.2, there are some warnings like:

Compiling itensor/iqtensor.cc with optimizations
iqtensor.cc:428:26: warning: moving a temporary object prevents copy elision [-Wpessimizing-move]
newind.nextIndex(std::move(dag(I)));
^
iqtensor.cc:428:26: note: remove std::move call here
newind.nextIndex(std::move(dag(I)));
^~~~~~~~~~ ~
In file included from iqtensor.cc:11:
In file included from /Users/jin/itensor/itensor/iqtensor.h:7:
In file included from /Users/jin/itensor/itensor/itensor.h:7:
In file included from /Users/jin/itensor/itensor/itensorinterface.h:582:
./itensorinterface_impl.h:737:67: warning: local variable 'B' will be copied despite being returned by name [-Wreturn-std-move]
operator(ITensorT const& A, ITensorT&& B) { B = A; return B; }
^
/Users/jin/itensor/itensor/iqtensor_impl.h:125:23: note: in instantiation of function template specialization 'itensor::operator' requested here
return setElt(iv) * toITensor(T);
^
./itensorinterfaceimpl.h:737:67: note: call 'std::move' explicitly to avoid copying
operator(ITensorT const& A, ITensorT&& B) { B *= A; return B; }
^
std::move(B)
2 warnings generated.

Any suggestions for how to fix it? Thanks.

Jin

Comments

I removed "std::move" in "std::move(dag(I))" and added "std::move" into "return B" as suggested. The warnings disappear. But I don't know if this will mess up something or not.

---

Thanks for reporting this! When I compile on macOS Mojave 10.14.2 with the clang++ compiler, I also see the first warning you report above ("iqtensor.cc:428:26: warning: moving a temporary object prevents copy elision"), but I don't see the second warning. It may just be a difference in the compiler (different compilers will warn about different things).

We will look into making changes so these warnings don't appear, but for now you can likely just ignore them. If you want to check to make sure your installation is working properly, from your base ITensor directory, enter the unittest directory with `cd unittest` and type `make` to compile and run all of the tests. If all of the tests compile and pass, your installation of ITensor is working correctly! Another good test is to enter the sample directory from your base ITensor directory with `cd sample` and type `make`, and then run the sample DMRG runs with commands like `./dmrg`. This is a good test that the DMRG code in ITensor is working correctly.

Cheers,
Matt

---

Hi Matt,

I also compile with the clang++ compiler (version 7.0.1). All tests passed and the sample dmrg is working correctly. But they both generate the socond warning when I compile.

Jin

Answer 1

(Just marking this as answered, via Matt Fishman's comments above.)

Jin, I second Matt's comment that we appreciate you reporting this, as these are very likely mis-used or unnecessary std::move commands that we will remove to get rid of these warnings as well. Thanks!