Mingjie Xu 28a0cfa946 Reland "[BasicBlockUtils] Fix dominator tree update for entry block in splitBlockBefore() (#178895) (#179392) ...

https://github.com/llvm/llvm-project/pull/178895 caused a clang
crash(see https://lab.llvm.org/buildbot/#/builders/210/builds/8229),
reverted in 6d52d2683c.

The crash is assertion `DT && "DT should be available to update
LoopInfo!"' failed.

ad8d5349d4/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp (L1106)

```
 #7 0x00007f5a380254e3 __assert_perror_fail (/usr/lib/libc.so.6+0x254e3)
 #8 0x0000563df5d8fde1 UpdateAnalysisInformation(llvm::BasicBlock*, llvm::BasicBlock*, llvm::ArrayRef<llvm::BasicBlock*>, llvm::DomTreeUpdater*, llvm::DominatorTree*, llvm::LoopInfo*, llvm::MemorySSAUpdater*, bool, bool&) BasicBlockUtils.cpp:0:0
 #9 0x0000563df5d8f3bb llvm::splitBlockBefore(llvm::BasicBlock*, llvm::ilist_iterator_w_bits<llvm::ilist_detail::node_options<llvm::Instruction, true, false, void, true, llvm::BasicBlock>, false, false>, llvm::DomTreeUpdater*, llvm::LoopInfo*, llvm::MemorySSAUpdater*, llvm::Twine const&)
#10 0x0000563df5d8cb08 llvm::SplitEdge(llvm::BasicBlock*, llvm::BasicBlock*, llvm::DominatorTree*, llvm::LoopInfo*, llvm::MemorySSAUpdater*, llvm::Twine const&)
#11 0x0000563df4ff5b59 (anonymous namespace)::CodeGenPrepare::splitLargeGEPOffsets()::$_1::operator()(long, llvm::Value*, llvm::GetElementPtrInst*) const CodeGenPrepare.cpp:0:0
#12 0x0000563df4fc0ec8 (anonymous namespace)::CodeGenPrepare::_run(llvm::Function&) CodeGenPrepare.cpp:0:0
#13 0x0000563df4fbb36c (anonymous namespace)::CodeGenPrepareLegacyPass::runOnFunction(llvm::Function&) CodeGenPrepare.cpp:0:0
```

I think this happened when we get DominatorTree with `DT.get()` in
`splitLargeGEPOffsets()` but `DT.reset()` already setting it to nullptr
in
ad8d5349d4/llvm/lib/CodeGen/CodeGenPrepare.cpp (L660).
To fix this assertion failure, use `getDT()` for
`splitLargeGEPOffsets()` to build the DominatorTree if it is set to
nullptr by `DT.reset()`.

I don't have a RSIC-V environment, so no reproducer. Checked that the
crash is fixed by rerunning buildbot with this patch
https://lab.llvm.org/buildbot/#/builders/210/builds/8248

2026-02-03 22:38:41 +08:00

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