Record alignment of class in dictionary and use in TStreamerInfo

[pcanal]

This fixes #21667 and thus addresses CMS issue seen at github.com/cms-sw/cmssw/pull/49969

With the current version storage of collection whose content needs to be aligned with more than 4096 is not supported. Extending it to higher number is trivial but require specific handling for each value so we had to stop at an arbitrary value. The error is seen at dictionary compilation time:

..../root/meta/alignment/evolution_C_ACLiC_dict.cxx:483:21: error: static assertion failed due to requirement 'alignof(std::pair<const int, Wrapper>) <= 4096': Class with
      alignment strictly greater than 4096 are currently not supported in CollectionProxy. Please report this case to the developers
  483 |       static_assert(alignof(map<int,Wrapper>::value_type) <= 64,
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[github-actions]

Test Results

    21 files      21 suites   3d 3h 1m 42s ⏱️
 3 835 tests  3 808 ✅  1 💤  26 ❌
72 233 runs  71 901 ✅ 18 💤 314 ❌

For more details on these failures, see this check.

Results for commit dbf6175.

♻️ This comment has been updated with latest results.

[jblomer]

Thanks!

I think it would be nice to have a separate test for AlignOf just for the cling part that exercises the different special cases (forward declaration etc.)

[jblomer]

I think it would be good to resolve the question in this PR.

[jblomer]

assert return value >0.

[pcanal]

This is a good question ... Currently (see updated doc) TClingClassInfo return -1 for error case but 0 for semi-error cases (eg. a namespace). It is unclear whether we want to treat those as an exception (i.e. the caller would get an assert if this is not called on a Class.

[jblomer]

This may be useful for RNTuple, too. Perhaps we can move it as a utility class to Core.

[pcanal]

The custom allocator has been removed because it leads to the in-memory layout to be different from regular vector.

[pcanal]

Actually it is still there but not used. Should I recommend or do you recommend I move it?