[Kernel] Add swapAB support for SM120 CUTLASS blockwise FP8 GEMM (#38325)

csrc/libtorch_stable/quantization/w8a8/cutlass/c3x/scaled_mm_blockwise_sm120_fp8_dispatch.cuh

@@ -26,8 +26,10 @@ using namespace cute;
 template <class OutType, int ScaleGranularityM,
           int ScaleGranularityN, int ScaleGranularityK,
           class MmaTileShape, class ClusterShape,
-          class EpilogueScheduler, class MainloopScheduler>
+          class EpilogueScheduler, class MainloopScheduler,
+          bool swap_ab_ = false>
 struct cutlass_3x_gemm_fp8_blockwise {
+  static constexpr bool swap_ab = swap_ab_;
   using ElementAB = cutlass::float_e4m3_t;
 
   using ElementA = ElementAB;
@@ -55,9 +57,13 @@ struct cutlass_3x_gemm_fp8_blockwise {
   using ElementCompute = float;
   using ElementBlockScale = float; 
 
-  using ScaleConfig = cutlass::detail::Sm120BlockwiseScaleConfig<
+  using ScaleConfig = conditional_t<swap_ab,
+      cutlass::detail::Sm120BlockwiseScaleConfig<
         ScaleGranularityM, ScaleGranularityN, ScaleGranularityK,
-        cute::UMMA::Major::MN, cute::UMMA::Major::K>;
+        cute::UMMA::Major::K, cute::UMMA::Major::MN>,
+      cutlass::detail::Sm120BlockwiseScaleConfig<
+        ScaleGranularityM, ScaleGranularityN, ScaleGranularityK,
+        cute::UMMA::Major::MN, cute::UMMA::Major::K>>;
 
   // layout_SFA and layout_SFB cannot be swapped since they are deduced.
   using LayoutSFA = decltype(ScaleConfig::deduce_layoutSFA());
@@ -78,17 +84,32 @@ struct cutlass_3x_gemm_fp8_blockwise {
       ElementAccumulator,
       ElementCompute,
       ElementC,
-      LayoutC,
+      conditional_t<swap_ab, LayoutC_Transpose, LayoutC>,
       AlignmentC,
       ElementD,
-      LayoutD,
+      conditional_t<swap_ab, LayoutD_Transpose, LayoutD>,
       AlignmentD,
       EpilogueScheduler,
       DefaultOperation
   >::CollectiveOp;
  
   using StageCountType = cutlass::gemm::collective::StageCountAuto; 
-  using CollectiveMainloop = 
+  using CollectiveMainloop = conditional_t<swap_ab,
+      typename cutlass::gemm::collective::CollectiveBuilder<
+          ArchTag,
+          OperatorClass,
+          ElementB,
+          cute::tuple<LayoutB_Transpose, LayoutSFA>,
+          AlignmentB,
+          ElementA,
+          cute::tuple<LayoutA_Transpose, LayoutSFB>,
+          AlignmentA,
+          ElementAccumulator,
+          MmaTileShape,
+          ClusterShape,
+          cutlass::gemm::collective::StageCountAutoCarveout<static_cast<int>(sizeof(typename CollectiveEpilogue::SharedStorage))>,
+          MainloopScheduler
+      >::CollectiveOp,
       typename cutlass::gemm::collective::CollectiveBuilder<
           ArchTag,
           OperatorClass,
@@ -103,7 +124,7 @@ struct cutlass_3x_gemm_fp8_blockwise {
           ClusterShape,
           cutlass::gemm::collective::StageCountAutoCarveout<static_cast<int>(sizeof(typename CollectiveEpilogue::SharedStorage))>,
           MainloopScheduler
-      >::CollectiveOp;
+      >::CollectiveOp>;
 
   // SM12x family to support both SM120 (RTX 5090) and SM121 (DGX Spark)
   using KernelType = enable_sm120_family<cutlass::gemm::kernel::GemmUniversal<
@@ -115,7 +136,7 @@ struct cutlass_3x_gemm_fp8_blockwise {
 // Tile configurations for different M ranges
 template <typename OutType>
 struct sm120_blockwise_fp8_config_default {
-  // M > 256: use 128x128x128 tile with Cooperative (Auto) schedule
+  // use 128x128x128 tile with Cooperative (Auto) schedule
   using KernelSchedule = cutlass::gemm::collective::KernelScheduleAuto;
   using EpilogueSchedule = cutlass::epilogue::collective::EpilogueScheduleAuto;
   using TileShape = Shape<_128, _128, _128>;
@@ -127,8 +148,8 @@ struct sm120_blockwise_fp8_config_default {
 };
 
 template <typename OutType>
-struct sm120_blockwise_fp8_config_M64 {
-  // M in [1, 256]: use 64x128x128 tile with Pingpong schedule
+struct sm120_blockwise_fp8_config_pingpong {
+  // use 64x128x128 tile with Pingpong schedule
   using KernelSchedule = cutlass::gemm::KernelTmaWarpSpecializedBlockwisePingpongSm120;
   using EpilogueSchedule = cutlass::epilogue::collective::EpilogueScheduleAuto;
   using TileShape = Shape<_64, _128, _128>;
@@ -139,11 +160,24 @@ struct sm120_blockwise_fp8_config_M64 {
       EpilogueSchedule, KernelSchedule>;
 };
 
+template <typename OutType>
+struct sm120_blockwise_fp8_config_swapab {
+  // use 128x32x128 tile with Cooperative schedule
+  using KernelSchedule = cutlass::gemm::KernelTmaWarpSpecializedBlockwiseCooperativeSm120;
+  using EpilogueSchedule = cutlass::epilogue::collective::EpilogueScheduleAuto;
+  using TileShape = Shape<_128, _32, _128>;
+  using ClusterShape = Shape<_1, _1, _1>;
+  using Gemm = cutlass_3x_gemm_fp8_blockwise<
+      OutType, 128, 1, 128, TileShape, ClusterShape,
+      EpilogueSchedule, KernelSchedule, true>;
+};
+
 template <typename Gemm>
 void cutlass_gemm_caller_blockwise(torch::stable::Tensor& out, torch::stable::Tensor const& a,
                                    torch::stable::Tensor const& b,
                                    torch::stable::Tensor const& a_scales,
                                    torch::stable::Tensor const& b_scales) {
+  static constexpr bool swap_ab = Gemm::swap_ab;
   using GemmKernel = typename Gemm::GemmKernel;
   using StrideA = typename Gemm::GemmKernel::StrideA;
   using StrideB = typename Gemm::GemmKernel::StrideB;
@@ -167,11 +201,13 @@ void cutlass_gemm_caller_blockwise(torch::stable::Tensor& out, torch::stable::Te
   b_stride =
       cutlass::make_cute_packed_stride(StrideB{}, cute::make_shape(n, k, 1));
   c_stride =
-      cutlass::make_cute_packed_stride(StrideC{}, cute::make_shape(m, n, 1));
+      cutlass::make_cute_packed_stride(StrideC{}, swap_ab ? cute::make_shape(n, m, 1) : cute::make_shape(m, n, 1));
 
-  LayoutSFA layout_SFA = 
+  LayoutSFA layout_SFA = swap_ab ?
+      ScaleConfig::tile_atom_to_shape_SFA(make_shape(n, m, k, 1)) :
       ScaleConfig::tile_atom_to_shape_SFA(make_shape(m, n, k, 1));
-  LayoutSFB layout_SFB = 
+  LayoutSFB layout_SFB = swap_ab ?
+      ScaleConfig::tile_atom_to_shape_SFB(make_shape(n, m, k, 1)) :
       ScaleConfig::tile_atom_to_shape_SFB(make_shape(m, n, k, 1));
 
   auto a_ptr = static_cast<ElementAB const*>(a.data_ptr());
@@ -180,15 +216,24 @@ void cutlass_gemm_caller_blockwise(torch::stable::Tensor& out, torch::stable::Te
   auto b_scales_ptr = static_cast<ElementBlockScale const*>(b_scales.data_ptr());
 
   typename GemmKernel::MainloopArguments mainloop_args{};
+  mainloop_args.layout_SFA = layout_SFA;
+  mainloop_args.layout_SFB = layout_SFB;
+  if (swap_ab) {
+    mainloop_args.ptr_A = b_ptr;
+    mainloop_args.dA = b_stride;
+    mainloop_args.ptr_B = a_ptr;
+    mainloop_args.dB = a_stride;
+    mainloop_args.ptr_SFA = b_scales_ptr;
+    mainloop_args.ptr_SFB = a_scales_ptr;
+  } else {
     mainloop_args.ptr_A = a_ptr;
     mainloop_args.dA = a_stride;
     mainloop_args.ptr_B = b_ptr;
     mainloop_args.dB = b_stride;
     mainloop_args.ptr_SFA = a_scales_ptr;
-  mainloop_args.layout_SFA = layout_SFA;
     mainloop_args.ptr_SFB = b_scales_ptr;
-  mainloop_args.layout_SFB = layout_SFB;
-  auto prob_shape = cute::make_shape(m, n, k, 1);
+  }
+  auto prob_shape = swap_ab ? cute::make_shape(n, m, k, 1) : cute::make_shape(m, n, k, 1);
 
   auto c_ptr = static_cast<ElementD*>(out.data_ptr());
   typename GemmKernel::EpilogueArguments epilogue_args{
@@ -204,8 +249,12 @@ void cutlass_gemm_blockwise_sm120_fp8_dispatch(torch::stable::Tensor& out,
                                                torch::stable::Tensor const& a_scales,
                                                torch::stable::Tensor const& b_scales) {
   int M = a.size(0);
+  // more heuristic tuning can be done here by checking N/K dimensions as well
+  bool swap_ab = (M <= 64) || (M % 4 != 0);
+
+  if (!swap_ab) {
     if (M <= 256) {
-    using Gemm = typename sm120_blockwise_fp8_config_M64<OutType>::Gemm;
+      using Gemm = typename sm120_blockwise_fp8_config_pingpong<OutType>::Gemm;
       return cutlass_gemm_caller_blockwise<Gemm>(
           out, a, b, a_scales, b_scales);
     }
@@ -213,6 +262,13 @@ void cutlass_gemm_blockwise_sm120_fp8_dispatch(torch::stable::Tensor& out,
     using Gemm = typename sm120_blockwise_fp8_config_default<OutType>::Gemm;
     return cutlass_gemm_caller_blockwise<Gemm>(
         out, a, b, a_scales, b_scales);
+  } else {
+    // Swap A/B for small M to improve performance
+    // Use TILE_N=32 as the minimum compatible tile size.
+    using Gemm = typename sm120_blockwise_fp8_config_swapab<OutType>::Gemm;
+    return cutlass_gemm_caller_blockwise<Gemm>(
+        out, a, b, a_scales, b_scales);
+  }
 }
 
 }  // namespace vllm