cfgbuild: Fix DEBUG_INSN handling in find_bb_boundaries [PR106719]

cfgbuild: Fix DEBUG_INSN handling in find_bb_boundaries [PR106719]

The following testcase FAILs on aarch64-linux.  We have some atomic
instruction followed by 2 DEBUG_INSNs (if -g only of course) followed
by NOTE_INSN_EPILOGUE_BEG followed by some USE insn.
Now, split3 pass replaces the atomic instruction with a code sequence
which ends with a conditional jump and the split3 pass calls
find_many_sub_basic_blocks.
For -g0, find_bb_boundaries sees the flow_transfer_insn (the new conditional
jump), then NOTE_INSN_EPILOGUE_BEG which can live in between basic blocks
and then the USE insn, so splits block after the NOTE_INSN_EPILOGUE_BEG
and puts the NOTE in between the blocks.
For -g, if sees a DEBUG_INSN after the flow_transfer_insn, so sets
debug_insn to it, then walks over another DEBUG_INSN, NOTE_INSN_EPILOGUE_BEG
until it finally sees the USE insn, and triggers the:
          rtx_insn *prev = PREV_INSN (insn);

          /* If the first non-debug inside_basic_block_p insn after a control
             flow transfer is not a label, split the block before the debug
             insn instead of before the non-debug insn, so that the debug
             insns are not lost.  */
          if (debug_insn && code != CODE_LABEL && code != BARRIER)
            prev = PREV_INSN (debug_insn);
code I've added for PR81325.  If there are only DEBUG_INSNs, that is
the right thing to do, but if in between debug_insn and insn there are
notes which can stay in between basic blocks or simnilarly JUMP_TABLE_DATA
or their associated CODE_LABELs, it causes -fcompare-debug differences.

The following patch fixes it by clearing debug_insn if JUMP_TABLE_DATA
or associated CODE_LABEL is seen (I'm afraid there is no good answer
what to do with DEBUG_INSNs before those; the code then removes them:
              /* Clean up the bb field for the insns between the blocks.  */
              for (x = NEXT_INSN (flow_transfer_insn);
                   x != BB_HEAD (fallthru->dest);
                   x = next)
                {
                  next = NEXT_INSN (x);
                  /* Debug insns should not be in between basic blocks,
                     drop them on the floor.  */
                  if (DEBUG_INSN_P (x))
                    delete_insn (x);
                  else if (!BARRIER_P (x))
                    set_block_for_insn (x, NULL);
                }
but if there are NOTEs, the patch just reorders the NOTEs and DEBUG_INSNs,
such that the NOTEs come first (so that they stay in between basic blocks
like with -g0) and DEBUG_INSNs after those (so that bb is split before
them, so they will be in the basic block after NOTE_INSN_BASIC_BLOCK).

2022-12-08  Jakub Jelinek  <jakub@redhat.com>

	PR debug/106719
	* cfgbuild.cc (find_bb_boundaries): If there are NOTEs in between
	debug_insn (seen after flow_transfer_insn) and insn, move NOTEs
	before all the DEBUG_INSNs and split after NOTEs.  If there are
	other insns like jump table data, clear debug_insn.

	* gcc.dg/pr106719.c: New test.