Avoid underflow in f2reduce

[gflegar]

When running convert_blocked1d_to_slice0 Triton ends up computing a rank of a matrix with 0 columns during linear layout lowering, which trips up f2reduce, and causes undefined behavior, detectable through UBSAN.

Fix this by returning the rank (0) early in these cases, without calling f2reduce.

Stack trace

third_party/triton/third_party/f2reduce/f2reduce.cpp:421:30: runtime error: shift exponent 18446744073709551615 is too large for 64-bit type 'unsigned long long'
    #0 0x556ee2fea3be in inplace_rref_small third_party/triton/third_party/f2reduce/f2reduce.cpp:421:30
    #1 0x556ee2fea3be in f2reduce::inplace_rref_strided(unsigned long*, unsigned long, unsigned long, unsigned long) third_party/triton/third_party/f2reduce/f2reduce.cpp:470:9
    #2 0x556ee2ea70da in getMatrixRank third_party/triton/lib/Tools/LinearLayout.cpp:125:3
    #3 0x556ee2ea70da in mlir::triton::LinearLayout::checkInvariants(bool) third_party/triton/lib/Tools/LinearLayout.cpp:299:7
    #4 0x556ee2ea656d in mlir::triton::LinearLayout::tryCreate(llvm::MapVector<mlir::StringAttr, std::__u::vector<std::__u::vector<int, std::__u::allocator<int>>, std::__u::allocator<std::__u::vector<int, std::__u::allocator<int>>>>, llvm::DenseMap<mlir::StringAttr, unsigned int, llvm::DenseMapInfo<mlir::StringAttr, void>, llvm::detail::DenseMapPair<mlir::StringAttr, unsigned int>>, llvm::SmallVector<std::__u::pair<mlir::StringAttr, std::__u::vector<std::__u::vector<int, std::__u::allocator<int>>, std::__u::allocator<std::__u::vector<int, std::__u::allocator<int>>>>>, 0u>>, llvm::ArrayRef<std::__u::pair<mlir::StringAttr, int>>, bool) third_party/triton/lib/Tools/LinearLayout.cpp:190:41
    #5 0x556ee2eb2150 in mlir::triton::LinearLayout::divideRight(mlir::triton::LinearLayout const&) third_party/triton/lib/Tools/LinearLayout.cpp:654:51
    #6 0x556ee2ee1c39 in mlir::cvtNeedsSharedMemory(mlir::RankedTensorType, mlir::RankedTensorType) third_party/triton/lib/Analysis/Utility.cpp:652:14
    #7 0x556ee2cf38fd in mlir::triton::getRepShapeForCvtLayout(mlir::triton::gpu::ConvertLayoutOp) third_party/triton/lib/Analysis/Allocation.cpp:66:8
    #8 0x556ee2cf3efa in mlir::triton::getScratchConfigForCvtLayout(mlir::triton::gpu::ConvertLayoutOp, unsigned int&, unsigned int&) third_party/triton/lib/Analysis/Allocation.cpp:95:19
    #9 0x556ee2cf6057 in mlir::triton::AllocationAnalysis::getScratchValueSize(mlir::Operation*) third_party/triton/lib/Analysis/Allocation.cpp:272:24
    #10 0x556ee2cf5499 in operator() third_party/triton/lib/Analysis/Allocation.cpp:343:7
    #11 0x556ee2cf5499 in void llvm::function_ref<void (mlir::Operation*)>::callback_fn<mlir::triton::AllocationAnalysis::getValuesAndSizes()::'lambda'(mlir::Operation*)>(long, mlir::Operation*) third_party/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:45:12
    #12 0x556edeeee7a9 in operator() third_party/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:68:12
    #13 0x556edeeee7a9 in void mlir::detail::walk<mlir::ForwardIterator>(mlir::Operation*, llvm::function_ref<void (mlir::Operation*)>, mlir::WalkOrder) third_party/llvm/llvm-project/mlir/include/mlir/IR/Visitors.h:174:5
    #14 0x556edeeee87c in void mlir::detail::walk<mlir::ForwardIterator>(mlir::Operation*, llvm::function_ref<void (mlir::Operation*)>, mlir::WalkOrder) third_party/llvm/llvm-project/mlir/include/mlir/IR/Visitors.h:182:9
    #15 0x556ee2cf49e7 in walk<(mlir::WalkOrder)0, mlir::ForwardIterator, (lambda at third_party/triton/lib/Analysis/Allocation.cpp:341:42), mlir::Operation *, void> third_party/llvm/llvm-project/mlir/include/mlir/IR/Visitors.h:313:10
    #16 0x556ee2cf49e7 in walk<(mlir::WalkOrder)0, mlir::ForwardIterator, (lambda at third_party/triton/lib/Analysis/Allocation.cpp:341:42), void> third_party/llvm/llvm-project/mlir/include/mlir/IR/Operation.h:794:12
    #17 0x556ee2cf49e7 in mlir::triton::AllocationAnalysis::getValuesAndSizes() third_party/triton/lib/Analysis/Allocation.cpp:341:16
    #18 0x556ee2cf4852 in run third_party/triton/lib/Analysis/Allocation.cpp:182:5
    #19 0x556ee2cf4852 in AllocationAnalysis third_party/triton/lib/Analysis/Allocation.cpp:169:5
    #20 0x556ee2cf4852 in mlir::Allocation::run(llvm::DenseMap<mlir::FunctionOpInterface, mlir::Allocation, llvm::DenseMapInfo<mlir::FunctionOpInterface, void>, llvm::detail::DenseMapPair<mlir::FunctionOpInterface, mlir::Allocation>>&) third_party/triton/lib/Analysis/Allocation.cpp:627:3
    #21 0x556ee1677402 in operator() third_party/triton/include/triton/Analysis/Allocation.h:227:26
    #22 0x556ee1677402 in void mlir::CallGraph<mlir::Allocation>::doWalk<(mlir::WalkOrder)0, (mlir::WalkOrder)1, mlir::ModuleAllocation::ModuleAllocation(mlir::ModuleOp)::'lambda'(mlir::CallOpInterface, mlir::FunctionOpInterface), mlir::ModuleAllocation::ModuleAllocation(mlir::ModuleOp)::'lambda'(mlir::FunctionOpInterface)>(mlir::FunctionOpInterface, llvm::DenseSet<mlir::FunctionOpInterface, llvm::DenseMapInfo<mlir::FunctionOpInterface, void>>&, mlir::ModuleAllocation::ModuleAllocation(mlir::ModuleOp)::'lambda'(mlir::CallOpInterface, mlir::FunctionOpInterface), mlir::ModuleAllocation::ModuleAllocation(mlir::ModuleOp)::'lambda'(mlir::FunctionOpInterface)) third_party/triton/include/triton/Analysis/Utility.h:350:7
    #23 0x556ee16756b3 in walk<(mlir::WalkOrder)0, (mlir::WalkOrder)1, (lambda at third_party/triton/include/triton/Analysis/Allocation.h:222:9), (lambda at third_party/triton/include/triton/Analysis/Allocation.h:224:9)> third_party/triton/include/triton/Analysis/Utility.h:242:7
    #24 0x556ee16756b3 in mlir::ModuleAllocation::ModuleAllocation(mlir::ModuleOp) third_party/triton/include/triton/Analysis/Allocation.h:220:5
    #25 0x556ee2c2bf18 in (anonymous namespace)::AllocateSharedMemory::runOnOperation() third_party/triton/lib/Conversion/TritonGPUToLLVM/AllocateSharedMemory.cpp:26:22
...
UndefinedBehaviorSanitizer: invalid-shift-exponent third_party/triton/third_party/f2reduce/f2reduce.cpp:421:30 

• I am not making a trivial change, such as fixing a typo in a comment.

• I have written a PR description following these
  rules.

• I have run pre-commit run --from-ref origin/main --to-ref HEAD.

• Select one of the following.

  • I have added tests.
    • /test for lit tests
    • /unittest for C++ tests
    • /python/test for end-to-end tests
  • This PR does not need a test because it is fixing a preexisting undefined behavior in a lit test.

• Select one of the following.

  • I have not added any lit tests.
  • The lit tests I have added follow these best practices,
    including the "tests should be minimal" section. (Usually running Python code
    and using the instructions it generates is not minimal.)

[jlebar]

Thanks, Goran.

[gflegar]

@jlebar is there anything else needed here, or can we merge it?

[jlebar]

Nope, good to merge.

[gflegar]

Thank you! 😄