Integrate:

[ 34214]
In S_mro_get_linear_isa_dfs(), hv_exists_ent() followed by an optional
hv_store_ent() can be replaced with an lvalue fetch.

[ 34215]
In S_mro_get_linear_isa_dfs(), save copying by making a shared hash
key scalar from the key of the hash entry we've just creating.
(Currently the hash is disposed of afterwards, but soon it won't, so
having both point to the same string buffer will also save memory.)
p4raw-link: @34215 on //depot/perl: f46ee2483d022839b174c58babe5076a34c0b367
p4raw-link: @34214 on //depot/perl: 8e45cc2bb9db96fb730868796fbfe1d0f7ece32d

p4raw-id: //depot/maint-5.10/perl@35071
p4raw-integrated: from //depot/perl@34215 'edit in' mro.c (@34214..)
	'merge in' sv.c (@34213..)

2 files changed, 28 insertions, 3 deletions

diff --git a/mro.c b/mro.c
index 83fdd719cf..c960fe4d15 100644
--- a/mro.c
+++ b/mro.c
@@ -186,9 +186,32 @@ S_mro_get_linear_isa_dfs(pTHX_ HV *stash, I32 level)
 	    }
 	    while(subrv_items--) {
 		SV *const subsv = *subrv_p++;
-		if(!hv_exists_ent(stored, subsv, 0)) {
-		    (void)hv_store_ent(stored, subsv, &PL_sv_undef, 0);
-		    av_push(retval, newSVsv(subsv));
+		/* LVALUE fetch will create a new undefined SV if necessary
+		 */
+		HE *const he = hv_fetch_ent(stored, subsv, 1, 0);
+		assert(he);
+		if(HeVAL(he) != &PL_sv_undef) {
+		    /* It was newly created.  Steal it for our new SV, and
+		       replace it in the hash with the "real" thing.  */
+		    SV *const val = HeVAL(he);
+		    HEK *const key = HeKEY_hek(he);
+
+		    HeVAL(he) = &PL_sv_undef;
+		    /* Save copying by making a shared hash key scalar. We
+		       inline this here rather than calling Perl_newSVpvn_share
+		       because we already have the scalar, and we already have
+		       the hash key.  */
+		    assert(SvTYPE(val) == SVt_NULL);
+		    sv_upgrade(val, SVt_PV);
+		    SvPV_set(val, HEK_KEY(share_hek_hek(key)));
+		    SvCUR_set(val, HEK_LEN(key));
+		    SvREADONLY_on(val);
+		    SvFAKE_on(val);
+		    SvPOK_on(val);
+		    if (HEK_UTF8(key))
+			SvUTF8_on(val);
+
+		    av_push(retval, val);
 		}
             }
         }
diff --git a/sv.c b/sv.c
index 3fc9838329..b8e6f921e7 100644
--- a/sv.c
+++ b/sv.c
@@ -7272,6 +7272,8 @@ Perl_newSVpvn_share(pTHX_ const char *src, I32 len, U32 hash)
     if (!hash)
 	PERL_HASH(hash, src, len);
     new_SV(sv);
+    /* The logic for this is inlined in S_mro_get_linear_isa_dfs(), so if it
+       changes here, update it there too.  */
     sv_upgrade(sv, SVt_PV);
     SvPV_set(sv, sharepvn(src, is_utf8?-len:len, hash));
     SvCUR_set(sv, len);