assigned to @Nuullll

opt -S -lowerswitch t.ll

define i32 @​f(i32 %c) {
chk65:
%cmp = icmp sgt i32 %c, 65
br i1 %cmp, label %return, label %chk0

chk0: ; preds = %chk65
%cmp1 = icmp slt i32 %c, 0
br i1 %cmp, label %return, label %bb_if

bb_if: ; preds = %chk0
%ashr.val = ashr exact i32 %c, 2
%cmp2 = icmp sgt i32 %ashr.val, 14
br i1 %cmp2, label %bb_switch, label %return

bb_switch: ; preds = %bb_if
br label %LeafBlock

LeafBlock: ; preds = %bb_switch
%SwitchLeaf = icmp eq i32 %ashr.val, 16
br i1 %SwitchLeaf, label %another_ret, label %NewDefault

successor_of_unreachable: ; No predecessors!
br label %return

another_ret: ; preds = %LeafBlock
br label %return

NewDefault: ; preds = %LeafBlock
br label %return

return: ; preds = %NewDefault, %another_ret, %successor_of_unreachable, %bb_if, %chk0, %chk65
%retval = phi i32 [ -1, %chk0 ], [ -1, %chk65 ], [ -1, %bb_if ], [ 42, %NewDefault ], [ 42, %another_ret ], [ -42, %successor_of_unreachable ]
ret i32 %retval
}

Another simpler example: there's only one non-default case of the switch inst.

define i32 @​g(i32 %c) {
bb_switch:
  switch i32 %c, label %out_of_switch_range [
    i32 16, label %return
  ]

out_of_switch_range:
  unreachable

return:
  ret i32 %c
}

opt -S -lowerswitch t.ll

define i32 @​g(i32 %c) {
bb_switch:
  br label %return

out_of_switch_range:                              ; No predecessors!
  unreachable

return:                                           ; preds = %bb_switch
  ret i32 %c
}

The default block isn't deleted.

I think we need to use Dominator Tree to be able to delete new dead blocks accurately.

https://reviews.llvm.org/D113081

We should delegate the dead block elimination to subsequent passes (such as DCE), to avoid unnecessary expense on compilation time.