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README > CUTLASS 3.0 GEMM API

CUTLASS 3.0 GEMM API

CUTLASS presents a uniform programming model for matrix multiply-accumulate (MMA) operations at different levels of the GPU system hierarchy. CUTLASS 3.0 has GEMM APIs corresponding to the following levels in order of highest to the lowest level.

  1. Device
  2. Kernel
  3. Collective
  4. Tiled MMA and Copy
  5. Atom

This document will cover the first three levels in detail: Device, Kernel, and Collective. It also briefly discusses the Tiled MMA/Copy and Atom level, and then refers readers to CuTe's tutorial for more information.

CUTLASS GEMM Model

CUTLASS implements algorithms that express the classical "triply nested loop" GEMM algorithm with a tiled structure mirroring the above hierarchy.

The following pseudocode describes the model for a GEMM kernel targeting a warp-synchronous matrix multiply instruction like mma.sync. The entire operation is referred to as "Gemm," as it is assumed that an epilogue operation performs the general matrix update similar to BLAS. This is pseudocode and is only meant to illustrate which parts of the layers correspond to the inner or outer loops of the GEMM.

// cutlass::gemm::kernel::GemmUniversal: ClusterTileM and ClusterTileN loops
//   are either rasterized by the hardware or scheduled by the kernel in persistent kernels.
// Parallelism over thread block clusters
for (int cluster_m = 0; cluster_m < GemmM; cluster_m += ClusterTileM) {
  for (int cluster_n = 0; cluster_n < GemmN; cluster_n += ClusterTileN) {

    // cutlass::gemm::collective::CollectiveMma: mainloop that iterates over all k-tiles
    // No loop unrolling is performed at this stage
    for (int k_tile = 0; k_tile < size<2>(gmem_tensor_A); k_tile++) {

      // loops inside cute::gemm(tiled_mma, a, b, c); Dispatch 5: (V,M,K) x (V,N,K) => (V,M,N)
      // TiledMma uses the hardware instruction provided through its Mma_Atom
      // TiledMma's atom layout, value layout, and permutations define the iteration order
      for (int tiled_mma_k = 0; tiled_mma_k < size<2>(A); tiled_mma_k++) {
        for (int tiled_mma_m = 0; tiled_mma_m < size<1>(A); tiled_mma_m++) {
          for (int tiled_mma_n = 0; tiled_mma_n < size<1>(B); tiled_mma_n++) {

            // TiledMma's vector mode dispatches to the underlying instruction.
            mma.call(d, a, b, c);
          } // tiled_mma_n
        } // tiled_mma_m
      } // tiled_mma_k
    } // k_tile mainloop
  } // cluster_m
} // cluster_n

The first three nested for loops correspond to parallelism over thread block clusters. The code does not actually express them as explicit for loops. Instead, the parallelization scheme over tiles is implied by CUDA grid launch semantics. However, for persistent kernels, these three loops are expressed in the source code as a single while loop that queries the work tile scheduler for problem tiles on which to compute.

Inside the three nested for loops, one finds code that pulls matrix tiles from global memory into more "local" memory (like shared memory or registers) and computes MMAs. These tiled copy and tiled mma iterations are generally fully static and get fully unrolled.

CUTLASS GEMM Components

CUTLASS expresses the above loop nest with the following components which are specialized for data type, layout, and math instruction.

API level API Class and/or function names
Device cutlass::gemm::device::GemmUniversalAdapter
Kernel cutlass::gemm::kernel::GemmUniversal
Collective cutlass::gemm::collective::CollectiveMma
cutlass::epilogue::collective::DefaultEpilogue
cutlass::epilogue::collective::Epilogue
Tiled (MMA and Copy) cute::TiledMma and cute::TiledCopy
cute::gemm() and cute::copy()
Atom cute::Mma_Atom and cute::Copy_Atom

In CUTLASS 3.0, we assemble kernels by first composing a collective mainloop and collective epilogue together at the kernel layer, and then wrapping them with a host-side adapter to form a GEMM handle to that kernel.

The following sections describe these components in the order a user should instantiate them in order to assemble a kernel. This order is

  1. assemble the required collective mainloop and epilogues,

  2. compose them together to build a kernel type, and

  3. wrap up the kernel with a device layer adapter.

This order is also reflected in the CUTLASS 3.0 Hopper kernel examples as seen in the excerpt below.

// Step 1: Generate the required collective layer mainloop specialization
using CollectiveMainloop = typename cutlass::gemm::collective::CollectiveBuilder<
    ArchTag, OperatorClass,
    ElementA, LayoutA, AlignmentA,
    ElementB, LayoutB, AlignmentB,
    ElementAccumulator,
    TilesShape, ClusterShape,
    cutlass::gemm::collective::StageCountAuto,
    cutlass::gemm::collective::KernelScheduleAuto
  >::CollectiveOp;

// Step 2: Specify the collective layer epilogue type
using CollectiveEpilogue = cutlass::epilogue::collective::DefaultEpilogue<
    cutlass::gemm::TagToStrideC_t<LayoutC>,
    cutlass::gemm::TagToStrideC_t<LayoutC>,
    cutlass::epilogue::thread::LinearCombination<ElementC, 1, ElementAccumulator, ElementAccumulator>>;

// Step 3: Compose the mainloop and epilogue together at the kernel layer
using GemmKernel = cutlass::gemm::kernel::GemmUniversal<
    cute::Shape<int,int,int,int>, // ProblemShape [M,N,K,L]
    CollectiveMainloop,
    CollectiveEpilogue
>;

// Step 4: Wrap up the kernel::GemmUniversal kernel class
// with the device adapter to obtain a host-side handle to the kernel
using GemmHandle = cutlass::gemm::device::GemmUniversalAdapter<GemmKernel>;

Towards the end, we also briefly cover CuTe's tiled mma and copy as well as the atom layer APIs, before redirecting users to CuTe-specific documentation for further details.

Collective API

A Collective is "the largest collection of threads onto which mma atoms and copy atoms are tiled." That is, it is the largest number of threads in a grid that can cooperate by leveraging hardware features for accelerated communication and synchronization. These hardware features include

  • asynchronous array copy (e.g., from global memory to shared memory);

  • MMA instructions for small tiles that live in shared memory;

  • synchronization operations for clusters, thread blocks, and/or warps; and/or

  • hardware acceleration (such as barriers) for ensuring that data dependencies between asynchronous operations are met.

A Collective uses the TiledMma and TiledCopy API (see below) to access operations that copy and perform MMA on tiles.

Different units of parallelism (e.g., threads, warps, or thread blocks) in a Collective might have different roles. For example, in "warp-specialized" algorithms, some warps may be responsible for copying data, while others may be responsible for computation. Nevertheless, the different units of parallelism still need to share data and coordinate access to the shared data. For example, the producer warps in a warp-specialized algorithm that copy input matrix tiles into shared memory need to let the consumer MMA warp(s) know that their MMA inputs are ready. We contrast this with the kernel:: layer API, which schedules the collectives over independent tiles in the grid.

The Collective API includes both the "mainloop" of matrix multiply-accumulate, and the epilogue. This API is the composition point for optimizations such as mainloop fusions and epilogue fusions. It is responsible for implementing the k_tile loop in the above triply nested loop pseudocode.

Collective Mainloops

The cutlass::gemm::collective::CollectiveMma class is the primary interface to the collective matrix multiply-accumulate (MMA) mainloops. "Mainloop" refers to the "main loop" over tiles -- the "cluster tile k" loop in the pseudocode near the top of this document. Any looping over multiple tiles that the algorithm might need to do would happen here.

The CollectiveMma class is declared in the header cutlass/gemm/collective/collective_mma.hpp.

namespace cutlass::gemm::collective {

template <
  class DispatchPolicy,
  class TileShape,
  class ElementA,
  class StrideA,
  class ElementB,
  class StrideB,
  class TiledMma,
  class GmemTiledCopyA,
  class SmemLayoutAtomA,
  class SmemCopyAtomA,
  class TransformA,
  class GmemTiledCopyB,
  class SmemLayoutAtomB,
  class SmemCopyAtomB,
  class TransformB
>
struct CollectiveMma {
  static_assert(sizeof(ElementA) == 0, "Could not find a mainloop specialization.");
};

} // namespace cutlass::gemm::collective

  • DispatchPolicy is the most important type for a collective, and is covered in more detail below.

  • StrideA and StrideB are instances of type cute::Stride that represent the global memory layout of A and B tensors. These strides are required to be rank-3, representing the modes [outer, inner, batch]. Each of the 3 ranks can be a multi-modal hierarchical stride; this would apply if implementing a tensor contraction.

  • TiledMma is an instance of cute::TiledMma.

  • GmemTiledCopyA and GmemTiledCopyB are instances of cute::TiledCopy types. Both tiled operation types are covered in more detail below.

  • SmemLayoutAtomA and SmemLayoutAtomB are instances of type cute::Layout and represent the smallest layout that will get tiled over the entire collective's shared memory. This layout does not include the pipeline mode, and therefore, both are expected to be rank 2 layouts of shape [outer, inner].

  • SmemCopyAtomA and SmemCopyAtomB are Copy_Atoms to be used for moving data from shared memory into register memory.

Notice that CUTLASS 3.0 mainloops do not accept a dedicated accumulator element type. We obtain the accumulator type from the typename TiledMma::ValTypeC. Note also that top level API's ElementA and ElementB can differ from those of the MMA facing typename TiledMma::ValTypeA and typename TiledMma::ValTypeB, allowing TMA or user supplied transform operations to perform type conversions.

Collective Dispatch Policies

CollectiveMma implementations are not generic. Instead, they must be specialized for each algorithm and GPU architecture. Users can dispatch to a CollectiveMma specialization by picking template arguments matching that specialization. CUTLASS 3.0 adopts a tag-based dispatch policy type to specialize mainloop implementations and add tuning knobs to them.

Below is an example of one of the dispatch policies that is used to dispatch to a Hopper TMA warp-specialized mainloop implementation:

// n-buffer in smem (Hopper TMA),
// pipelined with Hopper GMMA and TMA,
// warp-specialized dynamic schedule
template<
  int Stages_,
  class ClusterShape_ = Shape<_1,_1,_1>,
  class KernelSchedule = KernelTmaWarpSpecializedCooperative
>
struct MainloopSm90TmaGmmaWarpSpecialized {
  constexpr static int Stages = Stages_;
  using ClusterShape = ClusterShape_;
  using ArchTag = arch::Sm90;
  using Schedule = KernelSchedule;
};

The Stages_ template parameter lets the user freely vary the number of pipeline stages, while the ClusterShape_ type allows for parameterization over the shape of the threadblock cluster over which TMA multicast will take place.

The collective dispatch policy is also the primary point of composing various kernel schedules freely with any mainloop. Each mainloop policy either prescribes a Schedule with which it needs to be run, or exposes a template API that lets the user pick a subset of the following schedules:

struct KernelCpAsyncWarpSpecialized { };
struct KernelCpAsyncWarpSpecializedPingpong { };
struct KernelCpAsyncWarpSpecializedCooperative { };
struct KernelTma { };
struct KernelTmaWarpSpecialized { };
struct KernelTmaWarpSpecializedPingpong { };
struct KernelTmaWarpSpecializedCooperative { };

  • A single kernel schedule can support multiple mainloop implementations. For example, KernelMultistage can be composed with many different mainloop implementations across GPU architectures such as MainloopSm70TwoStage, MainloopSm80CpAsyncUnpredicated, and many more.

  • A single mainloop can be composed with multiple possible kernel schedules. For example, the MainloopSm90TmaGmmaWarpSpecialized can be composed with any of the KernelTmaWarpSpecialized, KernelTmaWarpSpecializedPingpong or KernelTmaWarpSpecializedCooperative kernel schedules.

As discussed in the CUTLASS 3.0 design documentation, adopting tag dispatch policies for our core vocabulary types allows us to maintain a single type name for all operations that conceptually belong to the same class. This design has the following benefits.

  • It avoids code duplication in cases where mainloops can be composed with multiple kernels or vice versa.
  • It makes writing generic code easier, as the primary type name CollectiveMma does not change across any implementation.
  • It provides a clear, singular extension point for users to plug in new, custom mainloops implementations specialized on their own dispatch policies.

Collective Builder for CollectiveMmas

The primary CollectiveMma is intended to be an expert user interface that allows full control over all the properties of the collective's GPU micro-kernel. However, often a user just wants an off-the-shelf GEMM mainloop implementation parameterized on simple configuration parameters. CUTLASS 3.0 provides cutlass::gemm::collective::CollectiveBuilder for such scenarios.

namespace cutlass::gemm::collective {
template <
  class ArchTag,
  class OpClass,
  class ElementA,
  class GmemLayoutA,
  int AlignmentA,
  class ElementB,
  class GmemLayoutB,
  int AlignmentB,
  class ElementAccumulator,
  class TileShape_MNK,
  class ClusterShape_MNK,
  class StageCountType,
  class KernelScheduleType,
  class Enable = void
>
struct CollectiveBuilder {
  static_assert(sizeof(ElementA) == 0, "Could not build a collective for given parameters.");
};
} // namespace cutlass::gemm::collective

CollectiveBuilder accepts CUTLASS 2.x equivalent input template arguments, and attempts to build the best performing CollectiveMma from the given parameters.

  • ArchTag is one of the SM architectures tags from cutlass::arch::Sm*.
  • OpClass is one of the operator class tags from cutlass::arch::Sm*.
  • ElementA and ElementB are the logical value types of the A resp. B tensors.
  • ElementAccumulator is the accumulator type to be used in the instruction.
  • GmemLayoutA and GmemLayoutB are CUTLASS 2.x layout tags, layout::RowMajor or layout::ColumnMajor.
  • AlignmentA and AlignmentB are global memory alignments of A and B tensors in terms of element count.
  • TileShape_MNK is an instance of cute::Shape that is rank-3, representing the MxNxK collective tile shape.
  • ClusterShape_MNK is an instance of cute::Shape that is rank-3, representing the MxNxK threadblock cluster tile shape.
  • StageCountType is either collective::StageCountAuto or an instance of collective::StageCount<N>.
  • KernelScheduleType is either collective::KernelScheduleAuto or one of the specific kernel schedule tags discussed in the dispatch policy section above.

StageCountAuto allows the collective builder to compute the size of a single stage's size in shared memory and maximize the shared memory usage assuming 1 threadblock / multiprocessor occupancy.

KernelScheduleAuto allows the collective builder to pick the best kernel schedule available for the given set of parameters, or let's the user override this with a specific kernel schedule type.

Note that collective builders are still in beta, and their functionality does not map onto the full design space that the primary expert CollectiveMma API allows for. We expect their supported mainloop types to expand in future releases, but with 3.0, only SM90 tensorop kernels are supported through the builder API. The builder API may also change in the future as we adopt user feedback.

If the builder is able to provide a collective mainloop type for the given set of parameters, it will be aliased within as CollectiveOp. For more information on how to parameterize kernels conveniently with the collective builder, please see example 49_hopper_gemm_with_collective_builder.

Epilogue

The collective epilogue implements element-wise operations involving the output matrix. Users can provide a custom epilogue, or use one of the standard epilogues. These live in the directory include/cutlass/epilogue/collective/, and include classes like cutlass::epilogue::collective::DefaultEpilogue and cutlass::epilogue::collective::Epilogue. CUTLASS's provided collective epilogues do not live under include/cutlass/gemm or in the cutlass::gemm namespace, because they can be used for computations other than GEMM.

Kernel API

The kernel is "a collection of all clusters in the grid." The kernel layer schedules have four main responsibilities.

  • Ordering the execution of collectives within the kernel, performing any synchronization between that may be necessary
  • Marshalling the threads of a warp specialized schedules into their respective roles
  • Performing any necessary grid swizzling logic
  • Tiling the input tensors with the threadblock cluster value tile before invoking the collectives on them

The Kernel API is the entry point for a grid of thread blocks that may or may not be organized in a cluster. It is the composition point for fusing back-to-back GEMMs, epilogues, and/or other operations.

The entry point API for CUTLASS 3.0 kernel is the class cutlass::gemm::kernel::GemmUniversal, found in the header file include/cutlass/gemm/kernel/gemm_universal.hpp. GemmUniversal is a stateless universal device kernel that implements GEMM as the composition of two parts:

  • a collective mainloop, and
  • a collective epilogue
namespace cutlass::gemm::kernel {
/*
 * Stateless universal device GEMM kernel type that treats GEMM as
 * a composition of a collective mainloop and a collective epilogue.
 *
 * Supports both the 2.x and 3.x APIs based on whether the first type is
 * a cute::tuple<> or not.
 * 2.x API implementation: cutlass/gemm/kernel/gemm_universal.h
 * 3.x API implementation: cutlass/gemm/kernel/gemm_*.hpp
 *
 * In the following declaration, the name preceding the 'Or' refers to
 * 3.x API type argument order, and the name succeeding the 'Or' refers to
 * 2.x API type argument order. Template arguments without two names
 * belong to the 3.x API only.
**/
template <
  class ProblemShapeOrThreadblockMma_, // (m, n, k) or (m, n, k, l)
  class CollectiveMainloopOrEpilogue_,
  class CollectiveEpilogueOrThreadblockSwizzle_,
  class TileScheduler_ = void,
  class Enable = void
>
class GemmUniversal;
} // namespace cutlass::gemm::kernel

Stateless means that the caller -- for example, the Device API described above -- manages the kernel's state. The kernel just takes input and output parameters (Params).

Universal means that GemmUniversal works for both CUTLASS 3.0 and 2.x interfaces and across a broad range of kernel schedules. If GemmUniversal's first template argument is a cute::Shape, then GemmUniversal assumes that the remaining template arguments implement the 3.0 APIs. Otherwise, GemmUniversal assumes that the remaining template arguments implement the 2.x APIs. Starting with CUTLASS 3.0, the problem shape has been promoted to a top-level template API for the GEMM kernel. This supports fully static GEMM instantiations where the user expects to know some or all of the problem shapes at compile time in order to extract even more performance.

The collective mainloop implements MMA on local tiles. The collective epilogue addresses any operations after the MMA, such as applying the beta * C part of C := beta * C + alpha * A * B. We will explain collective in more detail below.

Specializations of kernel::GemmUniversal for 3.0 APIs live in any of various gemm_*.hpp files in the directory include/cutlass/gemm/kernel/. Specializations for 2.x APIs can be found in the header file include/cutlass/gemm/kernel/gemm_universal.h.

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 include/cutlass/gemm/kernel/{arch_tag}*.hpp files in the directory include/cutlass/gemm/kernel/. Which specialization to dispatch to is decided through the dispatch policy's Schedule type.

For example, the header file include/cutlass/gemm/kernel/sm90_gemm_tma_warpspecialized_pingpong.hpp has a specialization of kernel::GemmUniversal for Hopper that uses a warp-specialized mainloop with a persistent scheduling algorithm, while the header file include/cutlass/gemm/kernel/sm90_gemm_tma_warpspecialized.hpp has a specialization of GemmUniversal for Hopper that uses a warp-specialized but non-persistent algorithm.

To support composition between supported kernel schedules and mainloop dispatch policies without having to duplicate collective mainloop implementations, GEMM kernel layer schedules can be composed with any mainloop that specifies their corresponding kernel schedule as their Schedule type in the policy. This is discussed in detail in the collective dispatch policy section above.

// An example of the SM90 KernelMultistage kernel's
// specialization logic that allows it to be composed
// with many mainloops such as `MainloopSm80CpAsync`
// and `MainloopSm70TwoStage`.
template <
  class ProblemShape_,
  class CollectiveMainloop_,
  class CollectiveEpilogue_,
  class TileScheduler_
>
class GemmUniversal<
  ProblemShape_,
  CollectiveMainloop_,
  CollectiveEpilogue_,
  TileScheduler_,
  std::enable_if_t<std::is_base_of_v<KernelMultistage, typename CollectiveMainloop_::DispatchPolicy::Schedule>>>

Device API

The Device API is a universal, kernel-agnostic host interface for kernel launch and managing the lifetime of reusable host-side parameters.

This API is how users' host-side .cu code invokes CUTLASS's single-GPU GEMM kernels. It serves the same purpose as cuBLAS and behaves similarly.

The entry point for the 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 stateful, reusable handle, which is parameterized on the cutlass::gemm::kernel type.

/*! 
  GemmUniversalAdapter is a stateful, reusable GEMM handle built around a kernel
  of type cutlass::gemm::kernel::*

  It manages the lifetime of the underlying `kernel::Params` struct, and exposes APIs
  to create it from the host facing arguments. For power users, new static methods
  are exposed in 3.x APIs that bypass the stateful methods or args->params lowering.

  It supports kernel types that implement both the 2.x and 3.0 APIs,
  however, this is done by specializing the implementation of GemmUniversalAdapter
  on the two kernel API types, and thus, GemmUniversalAdapter's behavior might
  differ between the two specializations.
*/
template <class GemmKernel_, class Enable = void>
class GemmUniversalAdapter;

Stateful means that the handle instance contains state that the kernel needs to run. This means that the user must initialize the handle first, then use the initialized handle instance to run the kernel. Statefulness also means that the handle can manage the lifetime of the kernel's Params -- the parameters of the kernel itself. An important duty of GemmUniversalAdapter is to map from the user's Arguments -- what the user sees as the kernel's parameters -- to the Params that the kernel actually sees. For power users, the class exposes new static methods in 3.0 APIs that can bypass stateful methods or go directly to Params without intermediate Arguments.

Reusable means that the handle instance can be used to call the kernel multiple times with different arguments (e.g., different matrices). Reusing the handle may be more efficient than just creating a new handle for each kernel invocation.

Parameterized on the kernel type means that the GemmUniversalAdapter class' behavior depends on the GEMM kernel type (see the next section). Specifically, GemmUniversalAdapter has a template parameter GemmKernel, which is the GEMM kernel type. Valid template arguments for GemmKernel are

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

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 the 2.x API implementing kernel layer GEMMs, or on the 3.x API implementing kernel layer GEMMs. The metafunction cutlass::gemm::detail::IsCutlass3GemmKernel is what GemmUniversalAdapter uses to distinguish between 2.x and 3.x kernels.

GemmUniversalAdapter sets up and launches the kernel, using the CUDA extended launch API for threadblock cluster support if required. Note, GemmUniversalAdapter does not specify the grid shape. The kernel controls the grid shape and other kernel-specific launch parameters. This makes it possible for all 3.0 kernels to use the same kernel launch code, thus factoring out kernel launch from the actual kernel.

Tiled MMA and Copy

The Tiled MMA or Copy are tilings of MMA atoms resp. Copy atoms across threads and data, with possible permutations applied to the resulting tiling. This layer is most analogous to the warp level tiling of MMA instructions in CUTLASS 2.x. However, it views the tiling from the perspective of all threads participating in the operation and generalizes the concept to copy operations as well. The purpose of this layer is to build composable GPU micro-kernels out of a plethora of hardware accelerated math and data movement operations, each with their unit layouts in threads and data. The tiled MMA and Copy types present all these various hardware accelerated CuTe Atoms with a single, consistent API.

The resulting tiled operation acts as a single MMA or copy operation that users can invoke in the "inner" loop of the three-nested-loops pseudocode at the top of this document using cute::gemm() or cute::copy().

We call this API "tiled" because it constructs larger operations out of the Atoms provided by CuTe, as if fitting together individual tiles to build a reusable component of a mosaic. For example, CuTe might provide an MMA Atom that users can call on a single warp, for fixed M, N, and K dimensions. CUTLASS can then use CuTe operations like make_tiled_mma to turn this Atom into an operation that works on an entire thread block, for larger M, N, and K dimensions.

Atom API

An "Atom" is the smallest collection of threads and data that must participate in the execution of a hardware-accelerated math or copy operation.

An Atom is "atomic" (indivisible) not in the sense of concurrent memory operations like atomicAdd (which are "indivisible in time (causality)"), but in the sense of indivisibility in "space" -- the number of values and the groups of parallel workers that must participate in the operation together.

An Atom uses CuTe Layouts to express the required dimensions and strides of its input and output arrays. Generally these are fixed at compile time.

The Atom API wraps calls to actual hardware instructions that accelerate MMA or copy operations. Users can ask for GPU architecture-specific implementations, or just pick generic implementations and rely on whatever GPU architectures were enabled.

For more information about Atoms, please refer to CuTe's tutorial, e.g., the sections on

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