use the new PL_sv_zero in obvious places

In places that do things like mPUSHi(0) or newSViv(0), replace them
with PUSHs(&PL_sv_zero) and &PL_sv_zero, etc.

This avoids the cost of creating and/or mortalising an SV, and/or setting
its value to 0.

This commit causes a subtle change to tainting in various places as a
side-effect. For example, grep in scalar context retunrs 0 if it has no
args.  Formerly the zero value could in theory get tainted:

    @a = ();
    $x = ( ($^X . ""), grep { 1 } @a);

It used to be the case that $x would be tainted; now its not.
In practice this doesn't matter - the zero value was only getting tainted
as a side-effect of tainting's "if anything in the statement uses a
tainted value, taint everything" mechanism, which gives (documented) false
positives. This commit merely removes some such false positives, and
makes the behaviour similar to functions which return &PL_sv_undef/no/yes,
which are also immune to side-effect tainting.

1 files changed, 2 insertions, 2 deletions

diff --git a/gv.c b/gv.c
index 5da09dfe77..9038764719 100644
--- a/gv.c
+++ b/gv.c
@@ -3180,7 +3180,7 @@ Perl_amagic_call(pTHX_ SV *left, SV *right, int method, int flags)
 	 case abs_amg:
 	   if ((cvp[off1=lt_amg] || cvp[off1=ncmp_amg])
 	       && ((cv = cvp[off=neg_amg]) || (cv = cvp[off=subtr_amg]))) {
-	     SV* const nullsv=sv_2mortal(newSViv(0));
+	     SV* const nullsv=&PL_sv_zero;
 	     if (off1==lt_amg) {
 	       SV* const lessp = amagic_call(left,nullsv,
 				       lt_amg,AMGf_noright);
@@ -3204,7 +3204,7 @@ Perl_amagic_call(pTHX_ SV *left, SV *right, int method, int flags)
 	 case neg_amg:
 	   if ((cv = cvp[off=subtr_amg])) {
 	     right = left;
-	     left = sv_2mortal(newSViv(0));
+	     left = &PL_sv_zero;
 	     lr = 1;
 	   }
 	   break;