optimize CLEAR_ERRSV

-gv_add_by_type is special only for non-GVs, and add AV to GV, otherwise
it is just assigning to GP slot (in this case through GvSV), which
CLEAR_ERRSV already did in another branch, so inline the gv_add_by_type
-dont compute GvSV multiple times, GvSV contains 2 derefs, after this patch
it will contains just 1 deref ("*svp") without an offset (deref without
offset is smaller in x86 machine code than deref with offset)
-SvREFCNT_dec_NN for efficiency
-move SvPOK_only closer to SvMAGICAL, this ensures SvFLAGS is read and
written only once, not 2 reads and 1 write, this is specifically for
RISC-ish cpus
-the goto clresv_newemptypv is because VC optimizer in -O1 didn't combine
the branches

perl521.dll VC 2003 before .text section size in machine code bytes
0xc8a63, after 0xc88e3

1 files changed, 10 insertions, 7 deletions

diff --git a/perl.h b/perl.h
index 9fe2fb57f2..7d93332f0d 100644
--- a/perl.h
+++ b/perl.h
@@ -1269,19 +1269,22 @@ EXTERN_C char *crypt(const char *, const char *);
 
 #define ERRSV GvSVn(PL_errgv)
 
+/* contains inlined gv_add_by_type */
 #define CLEAR_ERRSV() STMT_START {					\
-    if (!GvSV(PL_errgv)) {						\
-	sv_setpvs(GvSV(gv_add_by_type(PL_errgv, SVt_PV)), "");		\
-    } else if (SvREADONLY(GvSV(PL_errgv))) {				\
-	SvREFCNT_dec(GvSV(PL_errgv));					\
-	GvSV(PL_errgv) = newSVpvs("");					\
+    SV ** const svp = &GvSV(PL_errgv);					\
+    if (!*svp) {							\
+	goto clresv_newemptypv;						\
+    } else if (SvREADONLY(*svp)) {					\
+	SvREFCNT_dec_NN(*svp);						\
+	clresv_newemptypv:						\
+	*svp = newSVpvs("");						\
     } else {								\
-	SV *const errsv = GvSV(PL_errgv);				\
+	SV *const errsv = *svp;						\
 	sv_setpvs(errsv, "");						\
+	SvPOK_only(errsv);						\
 	if (SvMAGICAL(errsv)) {						\
 	    mg_free(errsv);						\
 	}								\
-	SvPOK_only(errsv);						\
     }									\
     } STMT_END