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, 4 insertions, 3 deletions

diff --git a/hv.c b/hv.c
index 3da7910d50..b00712b77d 100644
--- a/hv.c
+++ b/hv.c
@@ -995,12 +995,13 @@ Perl_hv_bucket_ratio(pTHX_ HV *hv)
             return magic_scalarpack(hv, mg);
     }
 
-    sv = sv_newmortal();
-    if (HvUSEDKEYS((HV *)hv))
+    if (HvUSEDKEYS((HV *)hv)) {
+        sv = sv_newmortal();
         Perl_sv_setpvf(aTHX_ sv, "%ld/%ld",
                 (long)HvFILL(hv), (long)HvMAX(hv) + 1);
+    }
     else
-        sv_setiv(sv, 0);
+        sv = &PL_sv_zero;
     
     return sv;
 }