[Bug libgomp/81698] New: expand_omp_sections generates unusual switch

["vries at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81698

            Bug ID: 81698
           Summary: expand_omp_sections generates unusual switch
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgomp
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
                CC: jakub at gcc dot gnu.org
  Target Milestone: ---

I.

A switch has the default label as its first label:
...
  switch (cD.2303) <default: <L2> [INV] [count: INV], 
                    case 1: <L1> [INV] [count: INV], 
                    case 2: <L0> [INV] [count: INV]>
...

And because build_cfg makes edges iterating over the switch labels, the default
edge will have index '0'.

In expand_case, we seem to be relying on this (it seems fragile to me though):
...
  edge default_edge = EDGE_SUCC (bb, 0);
...


II.

The switch resulting from expand_omp_sections does not have the default edge as
first edge (in fact, it's the last):
...
  switch (.section.0D.1925) <default: <L6> [INV] [count: INV],
                            case 0: <L0> [INV] [count: INV], 
                            case 1: <L7> [INV] [count: INV], 
                            case 2: <L8> [INV] [count: INV], 
                            case 3: <L9> [INV] [count: INV], 
                            case 4: <L10> [INV] [count: INV],
                            case 5: <L11> [INV] [count: INV]>
;;    succ:       8
;;                7
;;                6
;;                5
;;                4
;;                11
;;                9

;;   basic block 8, loop depth 1, freq 0, maybe hot
;;    prev block 7, next block 9, flags: (NEW, REACHABLE)
;;    pred:       3
<L11>:
  # USE = anything
  # CLB = anything
  barD.1917 (5);
  goto <bb 10>; [INV] [count: INV]
;;    succ:       10 (FALLTHRU)


;;   basic block 9, loop depth 0, freq 0, maybe hot
;;    prev block 8, next block 10, flags: (NEW)
;;    pred:       3
<L6>:
  # USE = anything
  # CLB = anything
  __builtin_trapD.1087 ();
;;    succ:
...


III.

So we need either to:
- fix expand_omp_sections to emit a normal-form switch, or
- stop relying on the first edge being the default edge.