Only upgrade when necessary

diff --git a/doio.c b/doio.c
index e6e9016..022b499 100644 (file)
--- a/doio.c
+++ b/doio.c
@@ -2275,7 +2275,7 @@ Perl_do_shmio(pTHX_ I32 optype, SV **mark, SV **sp)
        /* suppress warning when reading into undef var (tchrist 3/Mar/00) */
        if (! SvOK(mstr))
            sv_setpvs(mstr, "");
-       sv_upgrade(mstr, SVt_PV);
+       SvUPGRADE(mstr, SVt_PV);
        SvPOK_only(mstr);
        mbuf = SvGROW(mstr, (STRLEN)msize+1);