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, 1 insertions, 1 deletions

diff --git a/pad.c b/pad.c
index 3586b64f09..8e7c47ca98 100644
--- a/pad.c
+++ b/pad.c
@@ -1638,7 +1638,7 @@ Perl_pad_swipe(pTHX_ PADOFFSET po, bool refadjust)
 		"Pad 0x%"UVxf"[0x%"UVxf"] swipe:   %ld\n",
 		PTR2UV(PL_comppad), PTR2UV(PL_curpad), (long)po));
 
-    if (PL_curpad[po])
+    if (PL_curpad[po] && !SvPADMY(PL_curpad[po]))
 	SvPADTMP_off(PL_curpad[po]);
     if (refadjust)
 	SvREFCNT_dec(PL_curpad[po]);