http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49710
--- Comment #5 from Jan Hubicka <hubicka at gcc dot gnu.org> 2011-12-28 19:37:38 UTC --- OK, loop hiearchy looks as follows: loop_0 (header = 0, latch = 1, niter = ) { bb_2 (preds = {bb_0 }, succs = {bb_3 }) bb_6 (preds = {bb_5 }, succs = {bb_13 }) bb_12 (preds = {bb_4 }, succs = {bb_1 }) loop_4 (header = 13, latch = 14, niter = ) { bb_13 (preds = {bb_6 bb_14 }, succs = {bb_14 }) bb_14 (preds = {bb_13 }, succs = {bb_13 }) } loop_1 (header = 3, latch = 9, niter = ) { bb_3 (preds = {bb_2 bb_9 }, succs = {bb_4 }) bb_9 (preds = {bb_8 }, succs = {bb_3 }) loop_2 (header = 4, latch = 11, niter = ) { bb_4 (preds = {bb_3 bb_11 }, succs = {bb_12 bb_5 }) bb_5 (preds = {bb_4 }, succs = {bb_6 bb_7 }) bb_7 (preds = {bb_5 }, succs = {bb_10 }) bb_11 (preds = {bb_10 }, succs = {bb_4 }) loop_3 (header = 10, latch = 15, niter = ) { bb_8 (preds = {bb_10 }, succs = {bb_9 bb_15 }) bb_15 (preds = {bb_8 }, succs = {bb_10 }) bb_10 (preds = {bb_7 bb_15 }, succs = {bb_8 bb_11 }) } } } } We remove path from 10 to 8, that is closing the loop of loop_3. Basic blocks removed are 8 9 and 15. Finally we fail on BB 3 that is believed to be in loop 1, but header is null at this point because of code in delete_basic_block: 504 /* If we remove the header or the latch of a loop, mark the loop for 405 removal by setting its header and latch to NULL. */ 506 if (loop->latch == bb 507 || loop->header == bb) 508 { 509 loop->header = NULL; 510 loop->latch = NULL; 511 } OK, so it seems that fix_bb_placements is not ready to see loops marked for removal. I guess the catch is that loop peeling renders bb 3 unreachable. I however do not understand how loop peeling can make this happen, perhaps folding of the header condition is done? Honza