[tail-merge, PR84956] Don't merge bbs with bb_has_abnormal_pred

[Tom de Vries]

Hi,

This fixes a 6/7/8 regression, a P3 ICE in the tail-merge pass.


Consider the test-case from the patch.

When compiling at -O2, duplicates are found in tail-merge:
...
find_duplicates: <bb 6> duplicate of <bb 7>
find_duplicates: <bb 6> duplicate of <bb 9>
...
So, tail-merge calls replace_block_by (bb7, bb6).

However, bb7 has an abnormal edge bb5 -> bb7 as predecessor (bb6 has an 
abnormal edge as predecessor as well, but that doesn't look necessary to 
trigger the ICE):
...
;;   basic block 9, loop depth 0
;;    prev block 3, next block 4, flags: (NEW, REACHABLE)
;;    pred:       3 [50.0% (guessed)] (FALSE_VALUE,EXECUTABLE)
   goto <bb 8>; [100.00%]
;;    succ:       8 [always]  (FALLTHRU,EXECUTABLE)

;;   basic block 6, loop depth 0
;;    prev block 5, next block 7, flags: (NEW, REACHABLE, VISITED)
;;    pred:       5 [33.3% (guessed)] (ABNORMAL,EXECUTABLE)
   goto <bb 8>; [100.00%]
;;    succ:       8 [always] (FALLTHRU,EXECUTABLE)

;;   basic block 7, loop depth 0
;;    prev block 6, next block 8, flags: (NEW, REACHABLE, VISITED)
;;    pred:       5 [33.3% (guessed)]  (ABNORMAL,EXECUTABLE)
;;    succ:       8 [always]  (FALLTHRU,EXECUTABLE)
...

So when replace_block_by calls redirect_edge_and_branch (bb5 -> bb7, 
bb6) it lands in gimple_redirect_edge_and_branch, and fails here:
...
6024	  if (e->flags & EDGE_ABNORMAL)
6025	    return NULL;
...

which causes this assert to trigger:
...
       gcc_assert (pred_edge != NULL);
...


The patch fixes the ICE conservatively by skipping bbs with 
bb_has_abnormal_preds in find_clusters_1.

[ A more optimal fix could be to detect in this example that there's an 
abnormal edge bb5 -> bb6, and skip redirecting bb5 -> bb7 to bb6. ]

Bootstrapped and reg-tested on x86_64.

OK for stage4, gcc-6-branch and gcc-7-branch?

Thanks,
- Tom
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