Dedicated SV copying code in place of Perl_sv_setsv_flags

[richardleach]

Perl_sv_setsv_flags is the heavyweight function for assigning the value(s) of a source SV to a destination SV. It contains many branches for preparing the destination SV prior to assignment. However:

• If the destination SV has just been created, much of that logic isn't needed.
• When cloning a SV, simple assignments (particularly IVs and PVs) dominate.

This set of commits:

• Extracts the "is this CoWable?" test from Perl_sv_setsv_flags into a macro.
• Adds Perl_sv_freshcopy_flags and two static helper functions.
• Modifies Perl_newSVsv_flags and Perl_sv_mortalcopy_flags to use them.
• Standardizes a number of call sites that did their own things but really should use Perl_newSVsv_flags or Perl_sv_mortalcopy_flags.

Using perl's test harness as a guide:

• Bodyless code handles 45% of calls to Perl_newSVsv_flags and 57% of calls to Perl_sv_mortalcopy_flags.
• The SVt_PV/SVp_POK code handles 32% of calls to Perl_newSVsv_flags and 36% of calls to Perl_sv_mortalcopy_flags.
• S_sv_freshcopy_flags code handles 95% of the remainder in Perl_newSVsv_flags and 91% of the remainder in to Perl_sv_mortalcopy_flags.

With these changes compared with a build of blead:

• perl -e 'for (1..100_000) { my $x = [ (1) x 1000 ]; }' runs 10% faster

• perl -e 'for (1..100_000) { my $x = [ ("Perl") x 250 ]; }' runs 45% faster

---

• This set of changes does require a perldelta entry and I'll write one post-merge.

[Leont]

Cloning is rather unfortunate choice of words, given that it has a very specific meaning in our codebase that is quite different from what this PR is about. Renaming the PR may be helpful.

[richardleach]

I've made a lot of changes following earlier comments - thanks for those - and have finally force-pushed.

These changes aren't complete. For example:

• Measured performance seems worse than in the PR version, so I need to look into that
• Might change sflag handling/ SvFLAGS(dsv) setting
• Not settled on struct membet initialisation
• Might still rename the function that is currently Perl_newSVsv_flags and have newSVsv_flags be a macro that checks (ssv) before calling the sv.c function.

[tonycoz]

The last commit could use a better commit message than "bulk88 comments" :)

[richardleach]

I've squashed recent comment-addressing commits. Will have a final read over the source code comments at the weekend. Hopefully then, apart from any clarifications on the open conversations or further review comments, this will be ready to merge.

[richardleach]

Rebased to clear merge conflicts.

[richardleach]

Rebased for perldelta conflict.

Ready for any final reviews.

[tonycoz]

the last commit's message indicates it should be squashed

[richardleach]

Thanks, I've hopefully addressed those comments and squashed all trailing commits.

[richardleach]

Many thanks for all the reviews @tonycoz & @bulk88 - it's a relief to finally get this in before the github-actions bot adds the hasConflicts flag again!