[llvm-bugs] [Bug 40793] New: X86 ISel gets stuck in infinite loop

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40793

            Bug ID: 40793
           Summary: X86 ISel gets stuck in infinite loop
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: andrew at scheidecker.net
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

Created attachment 21498
  --> https://bugs.llvm.org/attachment.cgi?id=21498&action=edit
IR that triggers bug

Using SVN revision 354401 (from 2019/02/19), llc gets stuck in a loop on the
attached input, when run with the command-line:

  llc llvm-lock.ll -mcpu=skylake-avx512

Using llc from the LLVM 6.0 release does not get stuck in a loop.

Running it with e.g. -mcpu=skylake also does not get stuck in a loop. Based on
this, I've assumed this is an X86/AVX512 target specific bug.

Passing -debug prints the following in a loop:

  Promoting t1036: i16 = and t46, Constant:i16<15>
  Creating new node: t1038: i32 = any_extend t46
  Creating constant: t1039: i32 = Constant<15>
  Creating new node: t1040: i32 = and t1038, Constant:i32<15>
  Creating new node: t1041: i16 = truncate t1040

  Replacing.1 t1036: i16 = and t46, Constant:i16<15>

  With: t1041: i16 = truncate t1040
   and 0 other values

  Combining: t1037: i32 = srl Constant:i32<32733>, t1041
  Creating new node: t1042: i16 = and t46, Constant:i16<15>
  Creating new node: t1043: i32 = srl Constant:i32<32733>, t1042
   ... into: t1043: i32 = srl Constant:i32<32733>, t1042

  Combining: t39: i32 = Constant<32733>

  Combining: t43: i64 = any_extend t1043

  Combining: t1043: i32 = srl Constant:i32<32733>, t1042

  Combining: t1042: i16 = and t46, Constant:i16<15>

Breaking into the debugger shows this call stack:

  `anonymous namespace'::DAGCombiner::CombineTo
  `anonymous namespace'::DAGCombiner::CombineTo
  `anonymous namespace'::DAGCombiner::PromoteIntBinOp
  `anonymous namespace'::DAGCombiner::combine
  `anonymous namespace'::DAGCombiner::Run
  llvm::SelectionDAG::Combine
  llvm::SelectionDAGISel::CodeGenAndEmitDAG
  llvm::SelectionDAGISel::SelectBasicBlock
  llvm::SelectionDAGISel::SelectAllBasicBlocks
  llvm::SelectionDAGISel::runOnMachineFunction
  `anonymous namespace'::X86DAGToDAGISel::runOnMachineFunction
  llvm::MachineFunctionPass::runOnFunction
  llvm::FPPassManager::runOnFunction
  llvm::FPPassManager::runOnModule

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