Add Coverage Analysis (à la DMD)

[JohanEngelen]

This PR adds coverage analysis to LDC2 (cmdline option -cov[=n%]), equal to DMD's coverage analysis (http://dlang.org/code_coverage.html).

Two data structures are added to the module object file for coverage analysis: _d_cover_data and _d_cover_valid. _d_cover_data is an array of the execution count for each line of code (the execution count is atomically incremented at the start of each statement). _d_cover_valid is a bit-array that for each line says whether there is a statement on that line for which the execution count must be printed.
Each module compiled with -cov is registered for coverage output by calling _d_cover_register2() in druntime/rt/cover.d, passing _d_cover_data and _d_cover_valid. Then, druntime will take care of outputting the coverage listing. _d_cover_register2() is called by a shared static constructor.

There is a bug in druntime's interface of _d_cover_register2(). The _d_cover_valid bit array must be passed as a "size_t[] valid", however, the length field of the slice must be the number of bits in the array (number of lines). I will submit a PR to druntime to fix this, but do not know if it will be accepted. The code in this PR is very conservative and creates an oversized array for _d_cover_valid, such that the slice length field is equal to (at least) number of lines.

Tested using MSVC, x64.

[redstar]

Looks nice. I try to do a review this evening.

[dnadlinger]

Nice work! Did you try how your implementation fares against the test cases from the DMD test-suite? (There is a couple of them which are currently commented out. Just search for -cov on a REQUIRED_ARGS line.)

[dnadlinger]

Just post that druntime PR – I suppose the easiest solution would be to turn the slice into a pointer and a separate value for the length in bits?

[JohanEngelen]

@klickverbot I was thinking about another solution for the druntime bug, preserving the function's signature. But I think yours is simpler and better.

[redstar]

In general, the coding looks good. Could you please check the Travis build? There are compile errors with the different LLVM versions we support.
A PR requires a green Travis build. (At least, a "virtual" green build. There is an out-of-memory problem in the std.algorithm unit test which sometimes kills the build. You can ignore this failure.)

[redstar]

Please remove the LDC_GEN_COVERAGE. The file fits on a screen so this does not add information.

[redstar]

Looks good known. Could you please rebase your commits into one? Then I merge this PR.

[redstar]

BTW: Could you please check ldmd2? I think the -cov switch is parsed but I don't know if it passed to ldc2. The help text is currently disabled. You can provide another PR for this.

[JohanEngelen]

@redstar ldmd2 should now correctly handle -cov. I've squashed all commits into a single one.
Thanks!

[JohanEngelen]

@klickverbot See https://issues.dlang.org/show_bug.cgi?id=14417 .

[JohanEngelen]

@redstar "all" Travis builds are green now.
I've also put in a PR to enable the coverage analysis tests: ldc-developers/dmd-testsuite#7

[redstar]

@JohanEngelen Great work! Thank you!