LLVM ERROR: Instruction Combining did not reach a fixpoint after 1 iterations

[chfast]

define i1 @f(i16 %0) {
  br label %BB1

BB1:                                              ; preds = %BB1, %BB
  %C = icmp ult i16 256, %0
  br i1 true, label %BB1, label %BB2

BB2:                                              ; preds = %BB1
  ret i1 %C
}

https://godbolt.org/z/h7o9sbEKP

LLVM ERROR: Instruction Combining did not reach a fixpoint after 1 iterations
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.	Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/opt -o /app/output.s -S -passes instcombine <source>
 #0 0x00000000034a5a58 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x34a5a58)
 #1 0x00000000034a318c SignalHandler(int) Signals.cpp:0:0
 #2 0x00007f8a50d47420 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x14420)
 #3 0x00007f8a5080a00b raise (/lib/x86_64-linux-gnu/libc.so.6+0x4300b)
 #4 0x00007f8a507e9859 abort (/lib/x86_64-linux-gnu/libc.so.6+0x22859)
 #5 0x00000000007834af (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x7834af)
 #6 0x000000000300c8d8 combineInstructionsOverFunction(llvm::Function&, llvm::InstructionWorklist&, llvm::AAResults*, llvm::AssumptionCache&, llvm::TargetLibraryInfo&, llvm::TargetTransformInfo&, llvm::DominatorTree&, llvm::OptimizationRemarkEmitter&, llvm::BlockFrequencyInfo*, llvm::ProfileSummaryInfo*, unsigned int, bool, llvm::LoopInfo*) InstructionCombining.cpp:0:0
 #7 0x000000000300cf4a llvm::InstCombinePass::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x300cf4a)
 #8 0x00000000036ba32e llvm::detail::PassModel<llvm::Function, llvm::InstCombinePass, llvm::PreservedAnalyses, llvm::AnalysisManager<llvm::Function>>::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x36ba32e)
 #9 0x0000000000d3fd8d llvm::detail::PassModel<llvm::Function, llvm::PassManager<llvm::Function, llvm::AnalysisManager<llvm::Function>>, llvm::PreservedAnalyses, llvm::AnalysisManager<llvm::Function>>::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0xd3fd8d)
#10 0x0000000002e0ebde llvm::ModuleToFunctionPassAdaptor::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x2e0ebde)
#11 0x0000000000d247de llvm::detail::PassModel<llvm::Module, llvm::ModuleToFunctionPassAdaptor, llvm::PreservedAnalyses, llvm::AnalysisManager<llvm::Module>>::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0xd247de)
#12 0x0000000002e0cbdc llvm::PassManager<llvm::Module, llvm::AnalysisManager<llvm::Module>>::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x2e0cbdc)
#13 0x000000000088448a llvm::runPassPipeline(llvm::StringRef, llvm::Module&, llvm::TargetMachine*, llvm::TargetLibraryInfoImpl*, llvm::ToolOutputFile*, llvm::ToolOutputFile*, llvm::ToolOutputFile*, llvm::StringRef, llvm::ArrayRef<llvm::PassPlugin>, llvm::opt_tool::OutputKind, llvm::opt_tool::VerifierKind, bool, bool, bool, bool, bool, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x88448a)
#14 0x000000000078e621 main (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x78e621)
#15 0x00007f8a507eb083 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24083)
#16 0x000000000087980e _start (/opt/compiler-explorer/clang-assertions-trunk/bin/opt+0x87980e)
Program terminated with signal: SIGSEGV
Compiler returned: 139

[arpilipe]

According to our autotriage this is a regression caused/triggered by 72ec2c0 commit.

@nikic, could you please take a look?

[ParkHanbum]

after 1st running combining :

define i1 @f(i16 %0) {
  br label %BB1

BB1:                                              ; preds = %BB1, %1
  br i1 true, label %BB1, label %BB2

BB2:                                              ; preds = %BB1
  %C = icmp ugt i16 %0, 256
  ret i1 %C
}

try to remove %C = icmp ugt i16 %0, 256 in prepareWorklist.

    std::tie(NumDeadInstInBB, NumDeadDbgInstInBB) =
        removeAllNonTerminatorAndEHPadInstructions(&BB);

Breakpoint 9, llvm::removeAllNonTerminatorAndEHPadInstructions (BB=0x55555cc7fba0) at /home/m/llvm-origin/llvm/lib/Transforms/Utils/Local.cpp:2800
2800	    Inst->eraseFromParent();
(gdb) p Inst->dump()
  %C = icmp ugt i16 %0, 256
(gdb) p Inst->getParent()->dump()

BB2:                                              ; preds = %BB1
  %C = icmp ugt i16 %0, 256
  ret i1 poison

after this, prepareWorklist return true

    MadeIRChange |= NumDeadInstInBB + NumDeadDbgInstInBB > 0;
    NumDeadInst += NumDeadInstInBB;

MadeChangeInThisIteration setted to true make it reach to report_fatal_error

    IC.MaxArraySizeForCombine = MaxArraySize;
    bool MadeChangeInThisIteration = IC.prepareWorklist(F, RPOT);
    MadeChangeInThisIteration |= IC.run();
    if (!MadeChangeInThisIteration)
      break;

    MadeIRChange = true;
    if (Iteration > Opts.MaxIterations) {
      report_fatal_error(
          "Instruction Combining did not reach a fixpoint after " +
          Twine(Opts.MaxIterations) + " iterations");

so, it print LLVM ERROR as final result

[ParkHanbum]

can I try to this issue?
any advices is welcome.

[EugeneZelenko]

@ParkHanbum: Just prepare pull request and mention it on this page.

[ParkHanbum]

@EugeneZelenko got it

[nikic]

@ParkHanbum In

llvm-project/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

Line 3085 in 8e8c954

void InstCombinerImpl::handleUnreachableFrom(

and inside removeAllNonTerminatorAndEHPadInstructions, replace operands in the terminator instruction with poison.