Re: [perl #75436] Segfault with lexical $_ and reverse() since e1f795dc

On 29 May 2010 20:19, Ævar Arnfjörð Bjarmason <perlbug-followup@perl.org> wrote:
> This segfaults in everything from 5.10 to blead:
>
>    perl -e 'sub eek { my $_ = $_[0]; reverse } eek(1)'

Thanks for tracing this down.

Some background :
reverse() without arguments is special, because it defaults to $_ only
when called in scalar context. That means that, unlike all other
builtins that default to $_, you can't know at compile time whether it
will read $_ or not, and you can't put this info in the optree. So you
have to figure it out later, at runtime.

find_rundefsvoffset() locates a lexical $_ in the pad at runtime;
however we don't know if it's a "my" or an "our", and in this case the
segfault is produced in the part of the statement:
PAD_COMPNAME_FLAGS_isOUR(padoff_du), which doesn't work, because
apparently it's intended for compile-time, not run-time.

The path below fixes the segfault, but with it the behaviour of the
following one-liner is incorrect:

$ bleadperl -le 'sub eek { our $_ = $_[0]; reverse } print 1+ eek(12);print'
1
12

(the first value should be 22 = 1 + 21 = 1 + reverse(12))
(This is incorrect but still better than a segfault as in blead currently...)

I'd appreciate a hint -- how can I test at run-time the SVpad_*  flags
of a variable in the pad ?

--- a/pp.c
+++ b/pp.c
@@ -5498,8 +5498,7 @@ PP(pp_reverse)
            sv_setsv(TARG, (SP > MARK)
                    ? *SP
                    : (padoff_du = find_rundefsvoffset(),
-                       (padoff_du == NOT_IN_PAD
-                        || PAD_COMPNAME_FLAGS_isOUR(padoff_du))
+                       padoff_du == NOT_IN_PAD
                        ? DEFSV : PAD_SVl(padoff_du)));

            if (! SvOK(TARG) && ckWARN(WARN_UNINITIALIZED))

• [perl #75436] Segfault with lexical $_ and reverse() since e1f795dc by Ævar Arnfjörð Bjarmason
• Re: [perl #75436] Segfault with lexical $_ and reverse() since e1f795dc by Rafael Garcia-Suarez
• Re: [perl #75436] Segfault with lexical $_ and reverse() since e1f795dc by Vincent Pit
• Re: [perl #75436] Segfault with lexical $_ and reverse() since e1f795dc by Rafael Garcia-Suarez