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 6d52d2683c2ceb9ab75810730c3ced2509c32bc5. The crash is assertion `DT && "DT should be available to update LoopInfo!"' failed. https://github.com/llvm/llvm-project/blob/ad8d5349d46734826aaeae4a2ebdc6f427a5bad8/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 https://github.com/llvm/llvm-project/blob/ad8d5349d46734826aaeae4a2ebdc6f427a5bad8/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
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