[llvm-bugs] [Bug 52383] New: LowerSwitch should remove all dead blocks introduced by switch processing

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52383

            Bug ID: 52383
           Summary: LowerSwitch should remove all dead blocks introduced
                    by switch processing
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Transformation Utilities
          Assignee: unassignedbugs at nondot.org
          Reporter: yilong.guo at intel.com
                CC: llvm-bugs at lists.llvm.org

Consider the following ll:

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

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

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

bb_switch:
; %ashr.val must be 15 or 16 here (analyzed by LazyValueInfo)
  switch i32 %ashr.val, label %out_of_switch_range [
    i32 15, label %return
    i32 16, label %another_ret
  ]

; unreachable default case
out_of_switch_range:
  br label %successor_of_unreachable

; unreachable successors
successor_of_unreachable:
  br label %return

another_ret:
  br label %return

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

Both "out_of_switch_range" and "successor_of_unreachable" should be deleted as
they become dead after switch lowering.

But LowerSwitch only deletes the "out_of_switch_range" -- it didn't delete dead
block's dominatees which also become unreachable.

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