Its name implies that it's the top allocated element; in fact it's top+1.
PERLVAR(I, tmps_stack, SV **) /* mortals we've made */
PERLVARI(I, tmps_ix, SSize_t, -1)
PERLVARI(I, tmps_floor, SSize_t, -1)
PERLVAR(I, tmps_stack, SV **) /* mortals we've made */
PERLVARI(I, tmps_ix, SSize_t, -1)
PERLVARI(I, tmps_floor, SSize_t, -1)
-PERLVAR(I, tmps_max, SSize_t)
+PERLVAR(I, tmps_max, SSize_t) /* first unalloced slot in tmps stack */
PERLVARI(I, sub_generation, U32, 1) /* incr to invalidate method cache */
PERLVARI(I, sub_generation, U32, 1) /* incr to invalidate method cache */