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, 5 insertions, 5 deletions

diff --git a/pp_sys.c b/pp_sys.c
index 100762c1b7..e6ed385d65 100644
--- a/pp_sys.c
+++ b/pp_sys.c
@@ -655,7 +655,7 @@ PP(pp_open)
     if (ok)
 	PUSHi( (I32)PL_forkprocess );
     else if (PL_forkprocess == 0)		/* we are a new child */
-	PUSHi(0);
+	PUSHs(&PL_sv_zero);
     else
 	RETPUSHUNDEF;
     RETURN;
@@ -3660,7 +3660,7 @@ PP(pp_chdir)
                                 "chdir() on unopened filehandle %" SVf, sv);
                 }
                 SETERRNO(EBADF,RMS_IFI);
-                PUSHi(0);
+                PUSHs(&PL_sv_zero);
                 TAINT_PROPER("chdir");
                 RETURN;
             }
@@ -3683,7 +3683,7 @@ PP(pp_chdir)
             tmps = SvPV_nolen_const(*svp);
         }
         else {
-            PUSHi(0);
+            PUSHs(&PL_sv_zero);
             SETERRNO(EINVAL, LIB_INVARG);
             TAINT_PROPER("chdir");
             RETURN;
@@ -3728,7 +3728,7 @@ PP(pp_chdir)
  nuts:
     report_evil_fh(gv);
     SETERRNO(EBADF,RMS_IFI);
-    PUSHi(0);
+    PUSHs(&PL_sv_zero);
     RETURN;
 #endif
 }
@@ -4871,7 +4871,7 @@ PP(pp_sleep)
           Perl_ck_warner_d(aTHX_ packWARN(WARN_MISC),
                            "sleep() with negative argument");
           SETERRNO(EINVAL, LIB_INVARG);
-          XPUSHi(0);
+          XPUSHs(&PL_sv_zero);
           RETURN;
         } else {
           PerlProc_sleep((unsigned int)duration);