I'm experimenting with gcov using mingw gcc 4.4.0. I've been getting some interesting but strange results. A common pattern is something like this...
5162: 66: std::string::iterator i = l_Temp.begin ();
5162: 67: std::string::iterator j = l_Temp.end () - 1;
-: 68: char ch;
-: 69:
20564: 70: while (i < j)
-: 71: {
10240: 72: ch = *i; *i = *j; *j = ch; i++; j--;
-: 73: }
-: 74:
#####: 75: return l_Temp;
-: 76:}
How can that return not be exected at all, given that the loop just before clearly is both executing and exiting? I think I'm a victim of the return value optimisation here, given that this temporary variable is of type std::string.
The trouble is, I'm already specifying -O0 in the compiler options. These are the exact compiler flags I'm using...
-Wno-invalid-offsetof -g -O0 -fprofile-arcs -ftest-coverage
My best guess is that not all optimisations are disabled by -O0 after all. I can start hunting down specific optimisation flags one by one as I notice problems, but this seems a strange thing to need to do.
So - what flags should I be specifying in order to get sane coverage results from gcov?
EDIT
So far, I think I need the following additional flags...
- -fno-default-inline
- -fno-inline
I'm not sure these are both needed, though I think they each disable a different specific type of inline.
I haven't found any way to disable return value optimisations, though. This isn't a big problem, but it's a bit of an annoyance. When aiming for 100% coverage, some files that really do achieve 100% will be reported as less because of this issue. A grep can find the ##### markers and show if they're for return statements, but you still need to do some visual inspection to check that the problem is purely an RVO.