[llvm] [BOLT] a local out-of-range stub might lead to infinite loop in LongJmp (PR #73918)

[via llvm-commits]

linsinan1995 wrote:

More detail for the infinite loop:

BB has a local stub .LStub1111 that is out of range, and the execution path entered the problematic [code piece](https://github.com/llvm/llvm-project/blob/main/bolt/lib/Passes/LongJmp.cpp#L203-L209).
```
.LFT2057:
    00019e80: 	ldr	x0, [x25]
    00019e84: 	mov	x1, x24
    00019e88: 	ldr	x8, [x0]
    00019e8c: 	ldr	x8, [x8, #0x10]
    00019e90: 	blr	x8 # handler: 0; action: 0
    00019e94: 	tbz	w0, #0x0, .LStub1111
    00019e98: 	b	.Ltmp22
preds: .LStub1110
succs: .LStub1111 .Ltmp22
```

After the problematic code, `TgtSym` and `TgtBBwere` changed from both `.LStub1111` into `.Ltmp2459` and `null`. Then a new local stub `.LStub1370` was created to replace the out-of-range stub.

```
TgtSym .LStub1111 => .Ltmp2459
TgtBB  .LStub1111 => null

** createNewStub .LStub1370 **

.LFT2057:
    00019e80: 	ldr	x0, [x25]
    00019e84: 	mov	x1, x24
    00019e88: 	ldr	x8, [x0]
    00019e8c: 	ldr	x8, [x8, #0x10]
    00019e90: 	blr	x8 # handler: 0; action: 0
    00019e94: 	tbz	w0, #0x0, .LStub1370
    00019e98: 	b	.Ltmp22
preds: .LStub1110
succs: .LStub1111 .Ltmp22
```

likely because of we did not remove the relation between BB `.LFT2057` and `.LStub1111`, so the target of branch instruction was changed back to `.LStub1111` via `BinaryFunction::fixBranches` 

```
  During fixBranch
    00000000: 	tbz	w0, #0x0, .LStub1370
                  =>
    00000000: 	tbz	w0, #0x0, .LStub1111
```

Then Bolt was stuck in the loop of back-and-forth conversions between `.LStub1111` and `.LStub1370` .

https://github.com/llvm/llvm-project/pull/73918