Skip no-common-vars optimisation for aliases

The ‘no common vars’ optimisation allows perl to copy the values
straight from the rhs to the lhs in a list assignment.

In ($a,$b) = ($c,$d), that means $c gets assigned to $a,
then $d to $b.

If the same variable occurs on both sides of the expression
(($a,$b)=($b,$a)), then it is necessary to make temporary copies of
the variables on the rhs, before assigning them to the left.

If some variables have been aliased to others, then the common vars
detection can be fooled:

    *x = *y;
    $x = 3;
    ($x, $z) = (1, $y);

That assigns 1 to $x, and then goes to assign $y to $z, but $y is
the same as $x, which has just been clobbered.  So 1 gets assigned
instead of 3.

This commit solves this by recording in each typeglob whether the sca-
lar is an alias of a scalar from elsewhere.

If such a glob is encountered, then the entire expression is ‘tainted’
such that list assignments will assume there might be common vars.

1 files changed, 6 insertions, 0 deletions

diff --git a/gv.h b/gv.h
index 412adc0646..2b29b6d8d2 100644
--- a/gv.h
+++ b/gv.h
@@ -198,6 +198,12 @@ Return the CV from the GV.
 #define GvIMPORTED_CV_on(gv)	(GvFLAGS(gv) |= GVf_IMPORTED_CV)
 #define GvIMPORTED_CV_off(gv)	(GvFLAGS(gv) &= ~GVf_IMPORTED_CV)
 
+#define GPf_ALIASED_SV	1
+
+#define GvALIASED_SV(gv)	(GvGPFLAGS(gv) & GPf_ALIASED_SV)
+#define GvALIASED_SV_on(gv)	(GvGPFLAGS(gv) |= GPf_ALIASED_SV)
+#define GvALIASED_SV_off(gv)	(GvGPFLAGS(gv) &= ~GPf_ALIASED_SV)
+
 #ifndef PERL_CORE
 #  define GvIN_PAD(gv)		0
 #  define GvIN_PAD_on(gv)	NOOP