irgen: Generate explicit instantiations

[ChrisDodd]

• explicit instantiations for all Vector and IndexedVector used in any .def file

[fruffy]

This seems to be a better approach than #4680 because it automatically applies to all IR nodes with these characteristics.

I would add the comment/context from #4680 to the PR.

@vlstill can you confirm that this has a similar effect as your changes?

[vlstill]

Sorry I got caught by other work. I have to check that this does not break anything is someone tried to instantiate (Indexed)Vector before including the ir-generated.h which now contains the extern declarations. I'm not sure what C++ says about such cases but it seems to be allowed.

EDIT: no problem there.

[vlstill]

Hmm, I'm seeing only part of the improvement in this branch compared to my branch (after just one run, will confirm once the rest finishes). Could it be that for this to work fully the extern template declarations need to be before the use? Otherwise I think the template would still be (partially) instantiated with the class that uses it (e.g. member size needs to be known, constructor/destructor, things called from the ir-generated.h directly). And only after that the compiler would find the extern declaration.

Is it possible to move the declarations at the beginning, or is the ir-generator too much single-pass?

[vlstill]

OK, so these are the results for release build (similar to #4674 (comment)):

# Original (#4681^ = this PR minus the last commit)
  Time (mean ± σ):     595.677 s ± 10.207 s    [User: 12206.865 s, System: 629.514 s]
  Range (min … max):   582.688 s … 609.877 s    5 runs

# Vector + IndexedVector manual (#4680, more precisely this RP minus the last commit + the commit from #4680)
  Time (mean ± σ):     541.040 s ± 11.445 s    [User: 11295.180 s, System: 602.781 s]
  Range (min … max):   523.936 s … 553.141 s    5 runs

# Vector + IndexedVector in ir-generator (#4681)
  Time (mean ± σ):     558.003 s ±  7.345 s    [User: 11216.258 s, System: 604.529 s]
  Range (min … max):   549.559 s … 568.076 s    5 runs

So this PR stands in-between the two previous benchmarks, although significantly closer to #4680. I agree this is the better approach overall (and downstream tools that have new IR nodes with Vector/IndexedVector will benefit more from it), but it would be good to move the declarations at the beginning to see if that helps (or maybe put them in a separate header and include that one directly to vector.h/indexed_vector.h.

[ChrisDodd]

Is it possible to move the declarations at the beginning, or is the ir-generator too much single-pass?

I'll try moving them earlier, but I think the instantiations need to be after the declarations of the classes that are used as parameters to the template?

[ChrisDodd]

Is it possible to move the declarations at the beginning, or is the ir-generator too much single-pass?

Ok, I've moved the extern template declarations to the top of the ir-generated.h file -- this puts them before all the class definitions (and uses of the templates) -- see if that affects the build speed.

[vlstill]

Now the results are pretty much the same:

# this PR
  Time (mean ± σ):     566.911 s ±  3.272 s    [User: 11686.466 s, System: 588.923 s]
  Range (min … max):   562.361 s … 570.533 s    5 runs

# my approach
  Time (mean ± σ):     564.324 s ±  3.548 s    [User: 12161.493 s, System: 618.834 s]
  Range (min … max):   561.535 s … 570.435 s    5 runs

(I had to re-measure both as the PR base moved).

So the same. Thank you for this.