https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109237
--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
For delete_trivially_dead_insns we have
for (insn = get_last_insn (); insn; insn = prev)
{
int live_insn = 0;
prev = PREV_INSN (insn);
if (!INSN_P (insn))
continue;
live_insn = insn_live_p (insn, counts);
/* If this is a dead insn, delete it and show registers in it aren't
being used. */
if (! live_insn && dbg_cnt (delete_trivial_dead))
{
if (DEBUG_INSN_P (insn))
...
but insn_live_p has "interesting" behavior for debug_insns:
else if (DEBUG_INSN_P (insn))
{
rtx_insn *next;
if (DEBUG_MARKER_INSN_P (insn))
return true;
for (next = NEXT_INSN (insn); next; next = NEXT_INSN (next))
if (NOTE_P (next))
continue;
else if (!DEBUG_INSN_P (next))
return true;
/* If we find an inspection point, such as a debug begin stmt,
we want to keep the earlier debug insn. */
else if (DEBUG_MARKER_INSN_P (next))
return true;
else if (INSN_VAR_LOCATION_DECL (insn) == INSN_VAR_LOCATION_DECL
(next))
return false;
return true;
that's ending up doing quadratic work that would have been better done
by keeping track of some state during the backwards walk of the insn
stream.
