Hi all,
Question: Would it be acceptable to introduce a new "counter" variety,
together with options and UI support, that simply records a "visited" state?
Have there been any previous efforts in this space that I've missed?
I'm currently looking at the feasibility of reducing the memory
footprint of coverage testing. I don't have any figures or testcases
yet, but the customer states that coverage testing works, except that
the memory usage can be a problem on their devices, so I'm exploring
what options we have to address this.
Looking at the implementation, we currently use one 32- or 64-bit
counter (depending on the size of "long", I think) for each arc in the
program. Obviously this memory overhead is a small proportion of the
text size, but even so, it might be overkill for simple coverage testing
(as opposed to profiling). However, the code inserted to atomically
update the counter might end up being quite a large proportion of the
text, so that's worth looking at too.
If we add a new "counter" that uses one byte per arc, then that's not
really enough bits for a useful counter, but is more than enough for a
"visited" flag. Required storage would reduce by a factor of 4 or 8, and
since nothing ever transitions from "visited" to "unvisited" there will
be no need for atomic updates, which should reduce the code overhead.
Alternatively, if we use one bit per arc then storage will reduce even
further, but now we need to do read-modify-write again, so maybe the
saving is entirely negated.
As for the file format, we can either support the new counter all the
way through libgcc and gcov, or we can expand it to look like a regular
counter on stream-out. This second approach would be a lot less effort,
and means we retain backwards compatibility with pre-installed gcov
tools (and even PGO will still be able to locate the most unlikely
branches?), but the output will be presented as "1" iteration and might
possibly end up being misleading (at least to the unwary user).
Which approach is likely to be more acceptable to the maintainers?
Thanks in advance
Andrew
