optimization/6007: cfg cleanup tremendous performance hog with -O1

[Jan Hubicka]

> [RE: crossjumping]
> 
> > I will try to check whether I can squeze out some more cycles or
> > find way how to limit this.
> 
> My code uses a lot of computed goto's, with many labels.  Is crossjumping
> possibly a win in this case?  Can it ignore these jumps?
> 
> Brad
Hi, here is patch I made as a test.  It simply disables crossjumping
if there is moer than 100 outgoing edges.  Unfortunately I can't benchmark
your testcase as my machine runs out of space before getting there.  Can you
check if this solves your problem?  If so, I will prepare more polished
version of this patch.

Index: cfgcleanup.c
===================================================================
RCS file: /cvs/gcc/egcs/gcc/cfgcleanup.c,v
retrieving revision 1.45
diff -c -3 -p -r1.45 cfgcleanup.c
*** cfgcleanup.c	2002/03/22 11:18:33	1.45
--- cfgcleanup.c	2002/03/28 11:44:48
*************** try_crossjump_bb (mode, bb)
*** 1467,1472 ****
--- 1467,1473 ----
  {
    edge e, e2, nexte2, nexte, fallthru;
    bool changed;
+   int n = 0;
  
    /* Nothing to do if there is not at least two incoming edges.  */
    if (!bb->pred || !bb->pred->pred_next)
*************** try_crossjump_bb (mode, bb)
*** 1475,1483 ****
    /* It is always cheapest to redirect a block that ends in a branch to
       a block that falls through into BB, as that adds no branches to the
       program.  We'll try that combination first.  */
!   for (fallthru = bb->pred; fallthru; fallthru = fallthru->pred_next)
!     if (fallthru->flags & EDGE_FALLTHRU)
!       break;
  
    changed = false;
    for (e = bb->pred; e; e = nexte)
--- 1476,1488 ----
    /* It is always cheapest to redirect a block that ends in a branch to
       a block that falls through into BB, as that adds no branches to the
       program.  We'll try that combination first.  */
!   for (fallthru = bb->pred; fallthru; fallthru = fallthru->pred_next, n++)
!     {
!       if (fallthru->flags & EDGE_FALLTHRU)
! 	break;
!       if (n > 100)
! 	return false;
!     }
  
    changed = false;
    for (e = bb->pred; e; e = nexte)