Code coverage missing a branch if a function takes reference parameter

Question

I have a following function which takes a reference parameter:

#include <iostream>

class A { static void TestA(const int &y) };

void A::TestA(const int &y) { std::cout << y; }

int main()
{
   A::TestA(2);
   return 0;
}

In my (lcov) code coverage with google unit tests, it is saying missing a branch with TestA() function, and symbols list have a stack_chk_fail symbol added. If I change the function parameter to non-reference then coverage is 100%.

I am using g++ compiler.

Am I missing anything ?

Thanks

Answer 1

The compiler inlines Test into main (because that's what a good compiler does). However, it also has to create code for Test because it has external linkage. Effectively, the code for the function exists twice: Once inlined into main and once in the code for Test that the linker can link with other compilation units.

If your compiler is bad with code attribution (in the debug symbols) for inlined functions (hello MSVC?) then your profiler will give you exactly the result you see: Executing the program does not lead to coverage for Test because no piece of the binary that is executed (i.e. main) has any line attribution into Test.

Changing the parameter type may affect inlining, but it's more likely that it changes how debug symbols are generated.

To verify this, step through the program with a debugger, with a breakpoint in Test. If that breakpoint is not hit when running main, your coverage tool won't see coverage for that line either. Or, if you really want, look into the debug symbols manually to see which lines have attribution. In Visual Studio you can also look at the disassembly while debugging (it will show the associated code lines).

For the above reasons you will generally get more reliable coverage results if you do coverage runs with debug builds (where e.g. inlining won't happen).