op.c: Stop copying constants under ithreads

This fixes bugs #21979, #89188, #109746, #114838 and #115388 and
mostly fixes #109744 and #105906 (other issues still remain in those
two tickets).

Because the PADTMP flag was doing double duty, indicating that a
pad slot was in use in addition to indicating a target, constants
could not be shared between pad slots, as freeing one const op (and
turning of its PADTMP flag in pad_free) would mark some other pad
slot as free.

I believe this may have been fixed already by change 3b1c21fabed,
which made const ops use pad_swipe (which removes the SV from the
pad) instead of pad_free (which marks it as available for reuse).  But
the copying still happens.

In any case, as of the previous commit, whether a pad slot for a con-
stant is in use is now stored in the pad name.  Slots in use for const
ops now have &PL_sv_no names.

So there is no longer any reason to copy the constants.

The difference can be observed thus:

Before:

$ ./perl -lIlib -MDevel::Peek -e 'sub foo(){42} Dump foo; Dump foo'
SV = IV(0x7f94ea02ef10) at 0x7f94ea02ef20
  REFCNT = 2
  FLAGS = (PADTMP,IOK,READONLY,pIOK)
  IV = 42
SV = IV(0x7f94ea02eeb0) at 0x7f94ea02eec0
  REFCNT = 1
  FLAGS = (PADTMP,IOK,READONLY,pIOK)
  IV = 42

After:

$ ./perl -lIlib -MDevel::Peek -e 'sub foo(){42} Dump foo; Dump foo'
SV = IV(0x7f894882ef10) at 0x7f894882ef20
  REFCNT = 3
  FLAGS = (IOK,READONLY,pIOK)
  IV = 42
SV = IV(0x7f894882ef10) at 0x7f894882ef20
  REFCNT = 3
  FLAGS = (IOK,READONLY,pIOK)
  IV = 42

Notice the different addresses.

There are still special cases for copying &PL_sv_undef, which I need
to tackle.

Since most constants created by ‘use constant’ have the PADMY flag on
(since they reside in lexical variables inside constant.pm) and PADMY
and PADTMP are exclusive, I have stop turning on PADTMP for constants.
It is no longer necessary now, since before its purpose was to mark
pad entries as being in use.  That means many to-do tests pass.

1 files changed, 0 insertions, 2 deletions

diff --git a/dist/constant/t/constant.t b/dist/constant/t/constant.t
index 6b2ac27dcc..d39c05a6d3 100644
--- a/dist/constant/t/constant.t
+++ b/dist/constant/t/constant.t
@@ -363,7 +363,6 @@ eval q{
 	local $TODO;
 	if ($Config::Config{useithreads}) {
 	    skip "fails under threads", 1 if $] < 5.019001;
-	    $TODO = ' ';
 	}
 	like $@, qr/^Modification of a read-only value attempted at /,
 	    '... and immutable through refgen, too';
@@ -387,7 +386,6 @@ SKIP: {
     local $TODO;
     if ($Config::Config{useithreads}) {
 	skip "fails under threads", 1 if $] < 5.019001;
-	$TODO = ' ';
     }
     like $@, qr/^Modification of a read-only value attempted at /,
 	'... and immutable through refgen, too';