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README > CUTLASS 3.0 GEMM Backwards Compatibility

CUTLASS 3.0 GEMM Backwards Compatibility

Although CUTLASS 3.0 restructures the GEMM hierarchy and introduces new types for the threadblock layer and below, we intend the entire source code to be usable in user applications. We expect users to be able to #include any source file from CUTLASS 3.0, whether they implement the 2.x or the 3.x API, without breaking user builds. This means that a single translation unit should be able to contain any valid kernel regardless of its API version. The sections below discuss how device and kernel layer type names are made compatible across the two API versions, and what the users can expect out of the threadblock layer API going forward.

Compatible Device API

The entry point for CUTLASS's Device GEMM API is the class cutlass::gemm::device::GemmUniversalAdapter. This class lives in the header file include/cutlass/gemm/device/gemm_universal_adapter.h.

GemmUniversalAdapter is a "universal adapter" and serves as a common device interface for both CUTLASS 3.x and CUTLASS 2.x kernels. Its template parameter GemmKernel, the GEMM kernel type, can be any of the following:

  • cutlass::gemm::kernel::GemmUniversal, implementing CUTLASS 3.x API kernels;
  • cutlass::gemm::kernel::GemmUniversal, implementing CUTLASS 2.x API kernels;
  • Any valid CUTLASS 2.x kernel layer GEMM that was previously composable with device::GemmUniversalAdapter

Users implementing new kernels in either API should prefer using kernel::GemmUniversal as the kernel type and compose it with device::GemmUniversalAdapter. Users with existing kernel::Gemm kernels can continue to use them as template arguments of device::GemmUniversalAdapter. They can adopt GemmUniversal as a gradual migration path, since GemmUniversal accepts either 3.0 or 2.x collectives. Please see the next section for kernel::GemmUniversal for details.

GemmUniversalAdapter presents a single host-side interface to both 3.0 and 2.x kernels. CUTLASS accomplishes this by specializing GemmUniversalAdapter's implementation on either 2.x API implementing kernel layer GEMMs, or 3.x API implementing kernel layer GEMMs (as detected by gemm::detail::IsCutlass3GemmKernel discussed below). As a result, GemmUniversalAdapter's behavior might differ between the two specializations.

Device API design differences

In CUTLASS 2.x, the Device API was more closely tied to the Kernel API. In CUTLASS 3.0, the Device API accepts any kernel type that meets the Kernel API interface requirements. CUTLASS 3.0's Device API code is parameterized by the kernel type, but this code is generic; the same code works for any kernel type.

The device layer compatibility interface, device::GemmUniversalAdapter, also provides reflective mappings from 3.0-specific types back to the closest possible 2.x equivalent types. This is discussed further in the section below.

CUTLASS 3.0's device::GemmUniversalAdapter also exposes some new APIs that the 2.x device::GemmUniversalAdapter implementation does not. Most notably, this includes the ability to bypass the GemmKernel::Arguments to GemmKernel::Params lowering.

// Primary run() entry point API that is static allowing users to create and manage their own params.
static Status
run(Params& params, cudaStream_t stream = nullptr);

This new API is useful for the following scenarios.

  • Running again does not require reinvoking GemmKernel::to_underlying_arguments()
  • Manual control over construction of GemmKernel::Params for custom kernels with custom stride types
  • Fully static problem shapes and strides for bespoke kernels where no argument mapping needs to take place

Compatible Kernel API

CUTLASS 3.x API shares the kernel layer API with CUTLASS 2.x through the single entry point type cutlass::gemm::kernel::GemmUniversal. All kernel layer GEMMs are viewed as a composition of a collective mainloop and a collective epilogue.

kernel::GemmUniversal implements both 2.x and 3.x APIs

The entry point for CUTLASS's kernel API is the class cutlass::gemm::kernel::GemmUniversal. This class' declaration lives in the header file include/cutlass/gemm/kernel/gemm_universal.hpp.

/*
 * Stateless universal device GEMM kernel type that treats GEMM as
 * a composition of a collective mainloop and a collective epilogue.
 * SFIANE shims both 2.x and 3.0 API kernels based on ProblemShapeOrThreadblockMma_.
**/
template <
  class ProblemShapeOrThreadblockMma_,
  class CollectiveMainloopOrEpilogue_,
  class CollectiveEpilogueOrThreadblockSwizzle_,
  class GridSwizzle_ = void,
  class Enable = void
>
class GemmUniversal;

We call this class "universal" because it can be built using either the CUTLASS 3.0 or the 2.x mainloops and epilogues. If GemmUniversal's first template argument (ProblemShapeOrThreadblockMma_) is a cute::tuple, then GemmUniversal assumes that the remaining three template arguments (the mainloop, epilogue, and grid swizzle) implement the 3.0 APIs. Otherwise, GemmUniversal assumes that the remaining three template arguments implement the 2.x APIs. All the template arguments must be either CUTLASS 3.0 or CUTLASS 2.x types. For example, GemmUniversal does not permit using a 2.x mainloop with a 3.0 collective epilogue.

CUTLASS 3.x implements various embodiments of kernel::GemmUniversal. Each kernel layer schedule is specialized for a GEMM scheduling algorithm and GPU architecture. Specializations of kernel::GemmUniversal for 3.0 APIs live in any of various gemm_*.hpp files in the directory include/cutlass/gemm/kernel/. The specialization to which to dispatch is decided through the dispatch policy's Schedule type.

Specializations for 2.x APIs live in the header file include/cutlass/gemm/kernel/gemm_universal.h.

Kernel API design differences

The CUTLASS 2.x Kernel API was more closely tied to the Device API, as we mentioned above. In particular, the 2.x Device API specified the grid shape used to launch the Kernel API. In CUTLASS 3.0, the Kernel API controls its own grid shape, while the device adapter simply queries the kernel with which it needs to be launched.

This change is required to support various kernel schedules that may need their own schedule specific grid planning logic. For example, persistent kernel schedules generally only launch with as many threadblocks as the number of multiprocessors on the GPU.

All CUTLASS 3 kernel::GemmUniversal specializations expose the following (static) API:

// Returns true if the kernel can execute the provided GEMM arguments.
static bool
can_implement(Arguments const& args);

// Returns a dim3 representing the threadblock shape. 
static constexpr dim3
get_block_shape();

// Returns a dim3 representing the grid shape in terms of threadblocks.
static constexpr dim3
get_grid_shape(Params const& params);

The device adapter simply queries the kernel for these three before launching it on the device. CUTLASS 3.0 provides a meta-function to detect whether a cutlass::gemm::kernel::* implements the 3.x API or 2.x API:

// include/cutlass/gemm/gemm.h

namespace cutlass:gemm::detail {
  
// The following metafunction is used to detect whether a
// `kernel::Gemm` or `kernel::GemmUniversal` implements the CUTLASS 3.x API,
// by checking whether the problem shape type is aliased within.
template <class GemmKernel, class = void>
struct IsCutlass3GemmKernel;

} // namespace cutlass:gemm::detail

Users can dispatch their generic code against 2.x and 3.x specializations with this as a type trait for the kernel API version.

Threadblock API and Inner Loops

Much of the CUTLASS 3 GEMM hierarchy for mainloops and inner loops diverges from that of CUTLASS 2.x. With that also comes the introduction of the cutlass::gemm::collective layer as a direct replacement and a superset of the 2.x cutlass::gemm::threadblock layer. Going forward, CUTLASS 3.x will discontinue new developments in the following namespaces.

  • cutlass::*::threadblock::*
  • cutlass::*::warp::*
  • cutlass::gemm::thread::*
  • cutlass::arch::* (except barrier.h)

cutlass::gemm::collectives are a superset of the threadblock layer where all new mainloops will be developed. Users should look to the CollectiveMma type if they wish to author custom mainloop code in the 3.x API.

Similarly, for the GEMM inner loops, cute::MMA_Atoms replace the gemm::warp and gemm::thread layer code. Going forward, all new PTX instructions and associated metadata development will occur directly inside cute/arch/*.hpp and cute/atom/*.hpp.

The desired inner loop MMA iteration order and tiling can be achieved through careful selection of the atom layout, value layout, and permutations of the cute::TiledMma.

For epilogues, the cutlass::epilogue::collective layer replaces cutlass::threadblock::collective. However, the thread-level epilogue elementwise operations in cutlass::epilogue::thread will continue to be used in 3.x kernels as well, albeit, with a more idiomatic epilogue vectorization strategy. Example 50 shows how to use 2.x epilogue thread operators with 3.0 API kernels.

Porting from 2.x to 3.0 API

CUTLASS 2.x layout tags and CUTLASS 3.0 major modes

CUTLASS 2.x and CUTLASS 3.0 use both different wording and different types to describe the permitted layouts of GEMM's input matrices A and B.

CUTLASS 3.0 does not use the terms "column major" or "row major" to describe matrix layouts. Starting with CUTLASS 3.0, adoption of CuTe allows us to decouple

  • the coordinate mode order (logical shape) of layouts from

  • the index space stride order of the backing storage.

In line with our switch to a conceptual GEMM hierarchy, we view the major modes not from a BLAS-3 perspective. Rather, we divide the modes into two categories.

  • "Inner modes" or "K-modes" are contracted over during the GEMM. Therefore, they are not present in the output tensor.

  • "Outer modes" or "MN-modes" are preserved in the output.

Now, instead of RowMajor or ColumnMajor, whose major stride depends on whether we are referring to the A or the B matrix, we uniformly employ the "K major" or "MN major" terminology and enforce the convention of all tensors having the shape [M/N, K, L] regardless of which mode is major. That is,

  • the input matrix A has shape M x K,
  • the input matrix B has shape N x K, and
  • the input/output matrices C/D have shape M x N.

Note that this convention for B differs from the BLAS's GEMM interface, which specifies that B has shape K x N.

CUTLASS 3.0 uses these names of the modes to specify which mode of a matrix has stride 1. For the matrix A,

  • "M major" means that the matrix is stride 1 in the M mode, and
  • "K major" means that the matrix is stride 1 in the K mode.

For the matrix B,

  • "N major" means that the matrix is stride 1 in the N mode (which for B is mode 0, because the convention is that B is N x K); and
  • "K major" means that the matrix is stride 1 in the K mode (which for B is mode 1).

CUTLASS 2.x defines "layout tag" classes cutlass::layout::ColumnMajor and cutlass::layout::RowMajor, that live in the header file cutlass/layout/matrix.h. The interpretation of these layouts in GEMM depends on whether they are applied to the input matrix A or B. For the matrix A, "column major" means that mode corresponding to M extent has stride 1, and "row major" means that mode corresponding to K extent has stride 1. This is the usual computer science definition of column major and row major for a rank-2 array. For the matrix B, the opposite holds: "column major" means that mode corresponding to N extent has stride 1, and "row major" means that mode corresponding to K extent has stride 1.

Using the convention of [outer, inner, batch] mode order for tensor logical shapes avoids potential confusion with the meaning of column major and row major changing depending on whether they are applied to A or B.

The table below summarizes our mode order convention and mapping of 2.x layout tags to corresponding M-major, N-major, or K-major strides.

Matrix CUTLASS 2.x layout 2.x Shape Logical major mode 3.x Shape/Stride Major ordinal
A ColumnMajor M x K M major M x K x L 0 (outer)
A RowMajor M x K K major M x K x L 1 (inner)
B RowMajor K x N N major N x K x L 0 (outer)
B ColumnMajor K x N K major N x K x L 1 (inner)
C ColumnMajor M x N M major M x N x L 0 (outer)
C RowMajor M x N N major M x N x L 1 (inner)

Notice that in CUTLASS 3.0, interpretation of layouts no longer changes based on whether we are talking about the A or B matrix. M and N major inputs always have a static size-1 stride in their 0th (outer) mode. Similarly, K major inputs always contain the static size-1 stride in their 1st mode. This uniformity in stride order allows us to represent tensor layouts much more cleanly and treat both A and B equally in our interfaces. See for example the following snippet from our kernel/sm70_gemm.hpp for Ampere kernel schedules.

// Represent the full tensors
Tensor mA_mkl = make_tensor(make_gmem_ptr(params.mainloop.ptr_A), make_shape(M,K,L), params.mainloop.dA); // (m,k,l)
Tensor mB_nkl = make_tensor(make_gmem_ptr(params.mainloop.ptr_B), make_shape(N,K,L), params.mainloop.dB); // (n,k,l)

// Get batch slice
Tensor mA_mk = mA_mkl(_,_,get<3>(blk_coord_mnkl)); // (m,k)
Tensor mB_nk = mB_nkl(_,_,get<3>(blk_coord_mnkl)); // (n,k)

// Slice to get the tiles for which this thread block is responsible
Tensor gA = local_tile(mA_mk, blk_shape, take<0,3>(blk_coord_mnkl), Step<_1, X,_1>{}); // (BLK_M,BLK_K,k)
Tensor gB = local_tile(mB_nk, blk_shape, take<0,3>(blk_coord_mnkl), Step< X,_1,_1>{}); // (BLK_N,BLK_K,k)

As seem in this snippet, all input tensors have the logical shape [outer, inner, batch], and the strides could represent either outer or inner (or any other complex hierarchical stride) major storage. CuTe layouts always maintain the logical consistency of the coordinate spaces regardless of the strides.

By convention, in CUTLASS 3.0, we treat the M and N mode as the 0th mode, and K mode as the 1st mode of the stride.

Conversions between 2.x tags and 3.0 types

Starting with CUTLASS 3.0, all layouts are described using cute::Shape and cute::Stride which compose into a cute::Layout<Shape, Stride>. In CUTLASS 2.x, various layout tags such as cutlass::layout::RowMajor are used to specialize template implementations. These tag types only encode information about the tensor strides, as 2.x layouts did not incorporate any concept of tensor shape in the layout tags themselves. Users may find a need to convert between CUTLASS 2.x layout tags, and 3.0 CuTe stride types. CUTLASS 3.0 gemm::collective::CollectiveBuilder interfaces also accept these 2.x layout tags as input parameters in their template API as a convenience for users. At every entry point into CUTLASS 3.0, these tags get converted to their corresponding CuTe Stride type with metafunctions that best approximate their corresponding cute::Stride.

  • cutlass::gemm::detail::TagToStrideA_t<LayoutTag>
  • cutlass::gemm::detail::TagToStrideB_t<LayoutTag>
  • cutlass::gemm::detail::TagToStrideC_t<LayoutTag>

By convention, and to match user expectations, the cute::Stride types that these map onto always contain one static mode corresponding to the layout tag, and two 64-bit dynamic stride modes corresponding to the minor mode and the batch mode. Batch mode is included by default as all CUTLASS 3.0 kernels support packed batch-mode GEMMs out of the box.

The cutlass/gemm/gemm.h#440 header file includes functions that can be useful for converting from CUTLASS 3.0 cute::Strides back to CUTLASS 2.x layout tags.

  • cutlass::gemm::detail::StrideToLayoutTagA_t<CuteStride>
  • cutlass::gemm::detail::StrideToLayoutTagB_t<CuteStride>
  • cutlass::gemm::detail::StrideToLayoutTagC_t<CuteStride>

These metafunctions take the CuTe Stride as a template parameter and attempt to find the size-1 stride in the idiomatic M, N, or K modes to best approximate a corresponding 2.x layout tag type. Note that this may not work in general for any cute::Stride as the mapping between the stride and tag type is not bijective.

These mapping utilities are kept in a detail namespace as we do not guarantee stability of their implementation. Their behavior may change in future releases as we add new features. However, we do expect these type names to remain stable. For users who want these 2.x reflective types from an assembled kernel with a more stable API, the specialization of cutlass::gemm::device::GemmUniversalAdapter for CUTLASS 3.0 kernel provides all aliases for all 2.x type aliases in addition to the layout tags. You can see how they are used in the header file cutlass/gemm/device/gemm_universal_adapter.h. Here is an excerpt.

  // Map back to 2.x type as best as possible
  using LayoutA = gemm::detail::StrideToLayoutTagA_t<typename GemmKernel::StrideA>;
  using LayoutB = gemm::detail::StrideToLayoutTagB_t<typename GemmKernel::StrideB>;
  using LayoutC = gemm::detail::StrideToLayoutTagC_t<typename GemmKernel::StrideC>;
  using LayoutD = gemm::detail::StrideToLayoutTagC_t<typename GemmKernel::StrideD>;

  // Legacy: Assume MultiplyAdd only since we do not use this tag type in 3.0
  using MathOperator = cutlass::arch::OpMultiplyAdd;

  // If our TiledMMA's instruction thread layout size is larger than 1,
  // we know it's a tensorop
  using OperatorClass = std::conditional_t<
      (cute::size(typename GemmKernel::TiledMma::AtomThrID{}) > 1),
      cutlass::arch::OpClassTensorOp, cutlass::arch::OpClassSimt>;

  // Assume TiledMma's ShapeMNK is the same as 2.x's ThreadblockShape
  using ThreadblockShape = cutlass::gemm::GemmShape<
      cute::size<0>(TileShape{}),
      cute::size<1>(TileShape{}),
      cute::size<2>(TileShape{})>;

  using ClusterShape = cutlass::gemm::GemmShape<
      cute::size<0>(typename GemmKernel::DispatchPolicy::ClusterShape{}),
      cute::size<1>(typename GemmKernel::DispatchPolicy::ClusterShape{}),
      cute::size<2>(typename GemmKernel::DispatchPolicy::ClusterShape{})>;

  // We get the instruction shape directly from our TiledMma's atom shape
  using InstructionShape = cutlass::gemm::GemmShape<
      cute::size<0>(typename CollectiveMainloop::TiledMma::AtomShape_MNK{}),
      cute::size<1>(typename CollectiveMainloop::TiledMma::AtomShape_MNK{}),
      cute::size<2>(typename CollectiveMainloop::TiledMma::AtomShape_MNK{})>;

  static int constexpr kStages = CollectiveMainloop::DispatchPolicy::Stages;
  static int const kThreadCount = GemmKernel::MaxThreadsPerBlock;

  // Warp shape is not a primary API type in 3.x,
  // but we can best approximate it by inspecting the TiledMma::TiledShape_MNK.
  // For this, we make the assumption that we always have 4 warps along M,
  // and the rest along N, with none along K.  We also always round up
  // the warp count to 4 if the tiled mma is smaller than 128 threads.
  static constexpr int WarpsInMma = std::max(4, cute::size(typename GemmKernel::TiledMma{}) / 32);
  static constexpr int WarpsInMmaM = 4;
  static constexpr int WarpsInMmaN = cute::ceil_div(WarpsInMma, WarpsInMmaM);
  using WarpCount = cutlass::gemm::GemmShape<WarpsInMmaM, WarpsInMmaN, 1>;
  using WarpShape = cutlass::gemm::GemmShape<
      cute::size<0>(typename CollectiveMainloop::TiledMma::TiledShape_MNK{}) / WarpsInMmaM,
      cute::size<1>(typename CollectiveMainloop::TiledMma::TiledShape_MNK{}) / WarpsInMmaN,
      cute::size<2>(typename CollectiveMainloop::TiledMma::TiledShape_MNK{})>;

  // Inspect TiledCopy for A and B to compute the alignment size
  static int constexpr kAlignmentA = gemm::detail::get_alignment_count_from_gmem_tiled_copy<
      typename CollectiveMainloop::GmemTiledCopyA, ElementA>();
  static int constexpr kAlignmentB = gemm::detail::get_alignment_count_from_gmem_tiled_copy<
      typename CollectiveMainloop::GmemTiledCopyB, ElementB>();

CUTLASS's library and profiler use these reflective interfaces to obtain the kernel's configuration parameters. Users can use these to approximate the CUTLASS 2.x types for 3.0 API kernels. However, the reflective interfaces cannot always match the types exactly, as the mappings are not always bijective.

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