Performance of pIter != cont.end() in for loop

Question

I was getting through \"Exceptional C++\" by Herb Sutter lately, and I have serious doubts about a particular recommendation he gives in Item 6 - Temporary Objects.

Answer 1

Contrary to popular belief, I don't see any difference between VC++ and gcc in this respect. I did a quick check with both g++ 4.7.2 and MS C++ 17 (aka VC++ 2012).

In both cases I compared the code generated with the code as in the question (with headers and such added to let it compile), to the following code:

string FindAddr(list emps, string name) 
{
    auto end = emps.end();
    for (list::iterator i = emps.begin(); i != end; i++)
    {
        if( *i == name )
        {
            return i->addr;
        }
    }
    return "";
}

In both cases the result was essentially identical for the two pieces of code. VC++ includes line-number comments in the code, which changed because of the extra line, but that was the only difference. With g++ the output files were identical.

Doing the same with std::vector instead of std::list, gave pretty much the same result -- no significant difference. For some reason, g++ did switch the order of operands for one instruction, from cmp esi, DWORD PTR [eax+4] to cmp DWORD PTR [eax+4], esi, but (again) this is utterly irrelevant.

Bottom line: no, you're not likely to gain anything from manually hoisting the code out of the loop with a modern compiler (at least with optimization enabled -- I was using /O2b2 with VC++ and /O3 with g++; comparing optimization with optimization turned off seems pretty pointless to me).