Re: gcov/-ftest-coverage instrumentation for uninstantiated C++ function templates

[Andi Hellmund <mail at andihellmund dot com>]

Hey Manuel,

> I would like to be able to change this behaviour so non-instantiated 
> code templates are considered as blocks (I think this is the term used 
> by GCC/GCov). This would help me greatly to uncover unused/untested 
> codes in a header/template-only library.
>
> First of all: Is this feasible with the GCC infrastructure?
I am not sure about that. It could be harder to get this implemented 
since the required information might not be available when you require it.

> For this to work, one would probably need access to the internal 
> representation of the program. It is my understanding that there are 
> at least two distinct representations of a C++ program: (1) The AST 
> that only contains the parse tree, before any template instantiation 
> and (2) the intermediate language representation (GIMPLE?) that will 
> contain a representation of the instantiated templates. On which level 
> is coverage instrumentation added right now?
Yes, that's true. You get the details of both representations by using 
the additional compiler flags

-fdump-translation-unit (prints out an extended version of GENERIC (= AST))
-fdump-tree-gimple-raw (prints out the second IR known as GIMPLE)

The instrumentation of the source code is finally done on the GIMPLE IR. 
You could print out the IR by specifiying -fdump-tree-tree_profile

> Second: Where would I start looking?
>
> So far, I have discovered gcov*.{h,c} in the gcc directory. However, I 
> have not yet found where the .gcno files are written out in the compiler.
There was a recent discussion on the gcc-help mailing list about the 
basic internals of GCOV. Please find the thread here: 
http://gcc.gnu.org/ml/gcc-help/2011-04/msg00072.html
> Third: How hard would it be and how would I proceed?
>
> Naively, my first guess at an approach would be: Locate where the 
> .gcno file is written out. After writing out all real blocks, identify 
> all uninitialized template functions, or member functions that are 
> members of templates, etc. etc. Add a "block" to the .gcno file for 
> each such function.
As listed above, the .gcno file is written in coverage.c, though you 
should look at this one.

I hope that this helps at least a bit to get a step further ...

Best regards,
Andi