https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120729
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> --- So the code checks _all_ paths through the CFG, visited_flag_phis only prunes backedges during the recursion of of processing one path. To be unlucky one has to have a very deep back-to-back PHI def chain, with each two args you then get 2^N exponential number of paths to check. I have a patch to limit the search, but then you get a diagnostic: /home/harald/llvm-project/main/llvm/utils/TableGen/X86RecognizableInstr.cpp: In static member function ‘static llvm::X86Disassembler::OperandType llvm::X86Disassembler::RecognizableInstr::typeFromString(llvm::StringRef, bool, uint8_t)’: /home/harald/llvm-project/main/llvm/utils/TableGen/X86RecognizableInstr.cpp:1171:12: warning: ‘*(unsigned int*)((char*)&Switch + offsetof(llvm::StringSwitch<llvm::X86Disassembler::OperandType, llvm::X86Disassembler::OperandType>,llvm::StringSwitch<llvm::X86Disassembler::OperandType, llvm::X86Disassembler::OperandType>::Result.std::optional<llvm::X86Disassembler::OperandType>::<unnamed>.std::_Optional_base<llvm::X86Disassembler::OperandType, true, true>::<unnamed>))’ may be used uninitialized [-Wmaybe-uninitialized] /home/harald/llvm-project/main/llvm/utils/TableGen/X86RecognizableInstr.cpp:1026:29: note: ‘*(unsigned int*)((char*)&Switch + offsetof(llvm::StringSwitch<llvm::X86Disassembler::OperandType, llvm::X86Disassembler::OperandType>,llvm::StringSwitch<llvm::X86Disassembler::OperandType, llvm::X86Disassembler::OperandType>::Result.std::optional<llvm::X86Disassembler::OperandType>::<unnamed>.std::_Optional_base<llvm::X86Disassembler::OperandType, true, true>::<unnamed>))’ was declared here Note with GCC 15+ we might simply have optimized enough to avoid this. Even with a very very large amount of work (10000000 pruned path elements allowed) we diagnose this. With an -O0 compiler uninit analysis the takes 30% of the compile-time.
