sv.c - add SvPVCLEAR_FRESH macro for new PVs with buffer

SvPVCLEAR(tmpstr) is a macro that calls sv_setpv_bufsize(tmpstr,0,0).
If called on a newly minted PV/PVIV/PVNV/PVMG that has a fresh PV
buffer assigned, many of the checks in SvPVCLEAR are unnecessary:
    * SvTHINKFIRST - won't be set
    * upgrade to SVt_PV - won't be necessary
    * SvGROW - we assert that this isn't necessary

It then does some useful, but small actions:
    * sets SvCUR(sv, 0);
    * sets the last byte in the PV buffer to \0
    * sets SvPOK_only_UTF8
    * does SvTAINT

Then does a final irrelevant check:
    * is tmpstr a PVCV - no it is not

This commit introduces SvPVCLEAR_FRESH as a drop-in replacement
for compatible SVs, that only does the relevant small actions.

1 files changed, 18 insertions, 0 deletions

diff --git a/sv.h b/sv.h
index d4d5681541..1cffa66611 100644
--- a/sv.h
+++ b/sv.h
@@ -2292,10 +2292,28 @@ existing size, but instead it is the total size C<sv> should be.
 Ensures that sv is a SVt_PV and that its SvCUR is 0, and that it is
 properly null terminated. Equivalent to sv_setpvs(""), but more efficient.
 
+=for apidoc Am|char *|SvPVCLEAR_FRESH|SV* sv
+
+Like SvPVCLEAR, but optimized for newly-minted SVt_PV/PVIV/PVNV/PVMG
+that already have a PV buffer allocated, but no SvTHINKFIRST.
+
 =cut
 */
 
 #define SvPVCLEAR(sv) sv_setpv_bufsize(sv,0,0)
+#define SvPVCLEAR_FRESH(sv)                             \
+        STMT_START {                                    \
+            assert(SvTYPE(sv) >= SVt_PV);               \
+            assert(SvTYPE(sv) <= SVt_PVMG);             \
+            assert(!SvTHINKFIRST(sv));                  \
+            assert(SvPVX(sv));                          \
+            SvCUR_set(sv, 0  );                         \
+            *(SvEND(sv))= '\0';                         \
+            (void)SvPOK_only_UTF8(sv);                  \
+            SvTAINT(sv);                                \
+            SvPVX(sv);                                  \
+        } STMT_END
+
 #define SvSHARE(sv) PL_sharehook(aTHX_ sv)
 #define SvLOCK(sv) PL_lockhook(aTHX_ sv)
 #define SvUNLOCK(sv) PL_unlockhook(aTHX_ sv)