[llvm] [SelectionDAG] Introducing a new ISD::POISON SDNode to represent the poison value in the IR. (PR #125883)

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

This crashes with this patch:
```llc -mtriple=riscv32 bbi-106007_riscv.ll -o /dev/null```
It fails like:
```
SoftenFloatResult #0: t3: f64 = poison

LLVM ERROR: Do not know how to soften the result of this operator!
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.	Program arguments: build-all/bin/llc -mtriple=riscv32 bbi-106007_riscv.ll -o /dev/null
1.	Running pass 'Function Pass Manager' on module 'bbi-106007_riscv.ll'.
2.	Running pass 'RISC-V DAG->DAG Pattern Instruction Selection' on function '@b'
 #0 0x0000560f739d8e86 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (build-all/bin/llc+0x7552e86)
 #1 0x0000560f739d696e llvm::sys::RunSignalHandlers() (build-all/bin/llc+0x755096e)
 #2 0x0000560f739d9549 SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
 #3 0x00007fdbd7461d10 __restore_rt (/lib64/libpthread.so.0+0x12d10)
 #4 0x00007fdbd4e0152f raise (/lib64/libc.so.6+0x4e52f)
 #5 0x00007fdbd4dd4e65 abort (/lib64/libc.so.6+0x21e65)
 #6 0x0000560f7393c863 llvm::report_fatal_error(llvm::Twine const&, bool) (build-all/bin/llc+0x74b6863)
 #7 0x0000560f7393c696 (build-all/bin/llc+0x74b6696)
 #8 0x0000560f73851e80 llvm::DAGTypeLegalizer::SoftenFloatRes_EXTRACT_ELEMENT(llvm::SDNode*) LegalizeFloatTypes.cpp:0:0
 #9 0x0000560f737c650b llvm::DAGTypeLegalizer::run() LegalizeTypes.cpp:0:0
#10 0x0000560f737cbf86 llvm::SelectionDAG::LegalizeTypes() (build-all/bin/llc+0x7345f86)
#11 0x0000560f737b1d68 llvm::SelectionDAGISel::CodeGenAndEmitDAG() (build-all/bin/llc+0x732bd68)
#12 0x0000560f737b0672 llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) (build-all/bin/llc+0x732a672)
#13 0x0000560f737ada31 llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) (build-all/bin/llc+0x7327a31)
#14 0x0000560f737ab229 llvm::SelectionDAGISelLegacy::runOnMachineFunction(llvm::MachineFunction&) (build-all/bin/llc+0x7325229)
#15 0x0000560f72a30b87 llvm::MachineFunctionPass::runOnFunction(llvm::Function&) (build-all/bin/llc+0x65aab87)
#16 0x0000560f72f7bfe9 llvm::FPPassManager::runOnFunction(llvm::Function&) (build-all/bin/llc+0x6af5fe9)
#17 0x0000560f72f845e2 llvm::FPPassManager::runOnModule(llvm::Module&) (build-all/bin/llc+0x6afe5e2)
#18 0x0000560f72f7cac8 llvm::legacy::PassManagerImpl::run(llvm::Module&) (build-all/bin/llc+0x6af6ac8)
#19 0x0000560f70a3221d compileModule(char**, llvm::LLVMContext&) llc.cpp:0:0
#20 0x0000560f70a2f900 main (build-all/bin/llc+0x45a9900)
#21 0x00007fdbd4ded7e5 __libc_start_main (/lib64/libc.so.6+0x3a7e5)
#22 0x0000560f70a2ed6e _start (build-all/bin/llc+0x45a8d6e)
Abort (core dumped)
```
[bbi-106007_riscv.ll.gz](https://github.com/user-attachments/files/19663036/bbi-106007_riscv.ll.gz)


https://github.com/llvm/llvm-project/pull/125883