+static HV *
+S_get_isa_hash(pTHX_ HV *const stash)
+{
+ dVAR;
+ struct mro_meta *const meta = HvMROMETA(stash);
+
+ PERL_ARGS_ASSERT_GET_ISA_HASH;
+
+ if (!meta->isa) {
+ AV *const isa = mro_get_linear_isa(stash);
+ if (!meta->isa) {
+ HV *const isa_hash = newHV();
+ /* Linearisation didn't build it for us, so do it here. */
+ SV *const *svp = AvARRAY(isa);
+ SV *const *const svp_end = svp + AvFILLp(isa) + 1;
+ const HEK *const canon_name = HvNAME_HEK(stash);
+
+ while (svp < svp_end) {
+ (void) hv_store_ent(isa_hash, *svp++, &PL_sv_undef, 0);
+ }
+
+ (void) hv_common(isa_hash, NULL, HEK_KEY(canon_name),
+ HEK_LEN(canon_name), HEK_FLAGS(canon_name),
+ HV_FETCH_ISSTORE, &PL_sv_undef,
+ HEK_HASH(canon_name));
+ (void) hv_store(isa_hash, "UNIVERSAL", 9, &PL_sv_undef, 0);
+
+ SvREADONLY_on(isa_hash);
+
+ meta->isa = isa_hash;
+ }
+ }
+ return meta->isa;
+}
+