Re: Deleting jump tables and CFG

[law at redhat dot com]

In message <20031201171121.GB15555@kam.mff.cuni.cz>, Jan Hubicka writes:
 >>     Unfrortuantely it is not easy to track and delete all references to
 >>     the dead jumptable immediately when it becomes unreachable or
 >>     simplifies to simplejump as there can be code computing the jumptable
 >>     destination.  Thus the strategy is to remove the jumptables after
 >>     liveness analysis via purge_dead_jumptables call.  This is causing
 >>     problems all the time.  It would be nice to move the jumptables out of
 >>     the instruction chain (as rest of datas are dealt with) so the dead
 >>     jumptables won't affect other optimizations.
 >> 
 >> The problem is that code to delete unreachable blocks is deleting the label
 >s
 >> referenced by a jumptable but not the table itself.  So when jump.c runs, i
 >t
 >> blows up since the CODE_LABELs have been GC'ed.
 >
 >Do you have some testcase to look at?
 >This looks like quite nasty problem.  Perhaps we can manage
 >delete_unreachable_blocks to turn affected CODE_LABELs into
 >NOTE_INSN_DEAD_LABEL instead of deleting them, but it is not completely
 >trivial to track down what labels are accessed via jumptables and we
 >don't have any machinery to delete those notes, so we should not create
 >too many of these...
Err, shouldn't the reference to the CODE_LABEL from within the jump table
have kept the GC system from garbage collecting the CODE_LABEL node?


Jeff