[CUDA] Use GEMM with epilogue instead of AddMM (#2569)

2025-10-31 07:58:14 +08:00 · 2025-09-09 13:18:49 +09:00
parent 17310d91a6
commit dde3682b69
3 changed files with 128 additions and 69 deletions
--- a/mlx/backend/cuda/gemms/cublas_gemm.cpp
+++ b/mlx/backend/cuda/gemms/cublas_gemm.cpp
@@ -85,10 +85,10 @@ cublasLtMatrixLayout_t create_matrix_layout(
    int32_t batch_count,
    int64_t batch_stride) {
  cublasLtMatrixLayout_t desc;
+  if (transposed) {
+    std::swap(rows, cols);
+  }
  CHECK_CUBLAS_ERROR(cublasLtMatrixLayoutCreate(&desc, type, rows, cols, ld));
-  cublasLtOrder_t order = transposed ? CUBLASLT_ORDER_COL : CUBLASLT_ORDER_ROW;
-  CHECK_CUBLAS_ERROR(cublasLtMatrixLayoutSetAttribute(
-      desc, CUBLASLT_MATRIX_LAYOUT_ORDER, &order, sizeof(cublasLtOrder_t)));
  if (batch_count > 1) {
    CHECK_CUBLAS_ERROR(cublasLtMatrixLayoutSetAttribute(
        desc,
@@ -138,25 +138,34 @@ CublasGemm::CublasGemm(
      CUBLASLT_MATMUL_DESC_POINTER_MODE,
      &pointer_mode,
      sizeof(int32_t)));
-  cublasOperation_t op = CUBLAS_OP_N;
+
+  // In cublasLt matrices use column-major layout, while it is possible to use
+  // the CUBLASLT_ORDER_ROW option to switch to row-major layout, the bias
+  // epilogue does not work with the option. So instead we swap A and B to make
+  // cublasLt return the row-major result, which works because:
+  // - the data of a matrix in row-major layout is identical to its transpose in
+  //   column-major layout
+  // - C^T = (A @ B)^T = B^T @ A^T
+  cublasOperation_t a_op = b_transposed ? CUBLAS_OP_T : CUBLAS_OP_N;
  CHECK_CUBLAS_ERROR(cublasLtMatmulDescSetAttribute(
      matmul_desc_,
      CUBLASLT_MATMUL_DESC_TRANSA,
-      &op,
+      &a_op,
      sizeof(cublasOperation_t)));
+  cublasOperation_t b_op = a_transposed ? CUBLAS_OP_T : CUBLAS_OP_N;
  CHECK_CUBLAS_ERROR(cublasLtMatmulDescSetAttribute(
      matmul_desc_,
      CUBLASLT_MATMUL_DESC_TRANSB,
-      &op,
+      &b_op,
      sizeof(cublasOperation_t)));

  auto type = dtype_to_cublas_type(dtype);
  a_desc_ = create_matrix_layout(
-      type, a_rows, a_cols, a_transposed, lda, batch_count, a_batch_stride);
+      type, b_cols, b_rows, b_transposed, ldb, batch_count, b_batch_stride);
  b_desc_ = create_matrix_layout(
-      type, b_rows, b_cols, b_transposed, ldb, batch_count, b_batch_stride);
+      type, a_cols, a_rows, a_transposed, lda, batch_count, a_batch_stride);
  out_desc_ = create_matrix_layout(
-      type, a_rows, b_cols, false, b_cols, batch_count, a_rows * b_cols);
+      type, b_cols, a_rows, false, b_cols, batch_count, a_rows * b_cols);
 }

 CublasGemm::CublasGemm(
@@ -191,7 +200,7 @@ CublasGemm::CublasGemm(
          b_batch_stride) {
  auto type = dtype_to_cublas_type(dtype);
  c_desc_ = create_matrix_layout(
-      type, a_rows, b_cols, false, ldc, batch_count, c_batch_stride);
+      type, b_cols, a_rows, false, ldc, batch_count, c_batch_stride);
 }

 CublasGemm::~CublasGemm() {
@@ -213,14 +222,25 @@ void CublasGemm::set_out(
  CHECK_CUBLAS_ERROR(cublasLtMatrixLayoutDestroy(out_desc_));
  out_desc_ = create_matrix_layout(
      dtype_to_cublas_type(dtype),
-      rows,
      cols,
+      rows,
      transposed,
      ld,
      batch_count,
      batch_stride);
 }

+void CublasGemm::set_bias(void* bias) {
+  cublasLtEpilogue_t epilogue = CUBLASLT_EPILOGUE_BIAS;
+  CHECK_CUBLAS_ERROR(cublasLtMatmulDescSetAttribute(
+      matmul_desc_,
+      CUBLASLT_MATMUL_DESC_EPILOGUE,
+      &epilogue,
+      sizeof(epilogue)));
+  CHECK_CUBLAS_ERROR(cublasLtMatmulDescSetAttribute(
+      matmul_desc_, CUBLASLT_MATMUL_DESC_BIAS_POINTER, &bias, sizeof(bias)));
+}
+
 void CublasGemm::run(
    cu::CommandEncoder& encoder,
    array& out,
@@ -330,9 +350,9 @@ void CublasGemm::execute(
      handle_,
      matmul_desc_,
      &alpha,
-      a,
+      b, // a and b are swapped
      a_desc_,
-      b,
+      a,
      b_desc_,
      &beta,
      c ? c : out,
--- a/mlx/backend/cuda/gemms/cublas_gemm.h
+++ b/mlx/backend/cuda/gemms/cublas_gemm.h
@@ -55,6 +55,8 @@ class CublasGemm {
      int32_t batch_count,
      int64_t batch_stride);

+  void set_bias(void* bias);
+
  void run(
      cu::CommandEncoder& encoder,
      array& out,
--- a/mlx/backend/cuda/matmul.cpp
+++ b/mlx/backend/cuda/matmul.cpp
@@ -11,6 +11,7 @@
 #include <numeric>

 namespace mlx::core {
+
 namespace {

 std::tuple<bool, int64_t, array>
@@ -28,6 +29,74 @@ check_transpose(cu::CommandEncoder& enc, const Stream& s, const array& arr) {
  }
 }

+void gemm_and_bias(
+    cu::CommandEncoder& encoder,
+    int M,
+    int N,
+    int K,
+    bool a_transposed,
+    int64_t lda,
+    bool b_transposed,
+    int64_t ldb,
+    array& out,
+    const array& a,
+    const array& b,
+    void* bias = nullptr) {
+  // Check and collapse batch dimensions
+  auto [batch_shape, a_batch_strides, b_batch_strides] = collapse_batches(a, b);
+
+  auto batch_count = out.size() / (M * N);
+
+  // Collapse batches into M if needed
+  if (batch_count > 1 && !a_transposed && batch_shape.size() == 1 &&
+      a.strides()[a.ndim() - 2] == K && a_batch_strides.back() == M * K &&
+      b_batch_strides.back() == 0) {
+    M *= batch_shape.back();
+    batch_count = 1;
+
+    a_batch_strides = {0};
+    b_batch_strides = {0};
+    batch_shape = {1};
+  }
+
+  // Use gemmv when possible
+  if (!bias && cu::can_use_gemv(M, N, K, a_transposed, b_transposed)) {
+    cu::gemv(
+        a,
+        b,
+        out,
+        M,
+        N,
+        K,
+        batch_count,
+        batch_shape,
+        a_batch_strides,
+        b_batch_strides,
+        encoder);
+    return;
+  }
+
+  // Invoke cublasLt
+  CublasGemm gemm(
+      encoder.device(),
+      a.dtype(),
+      a_transposed,
+      M,
+      K,
+      lda,
+      b_transposed,
+      K,
+      N,
+      ldb,
+      batch_shape.back(),
+      a_batch_strides.back(),
+      b_batch_strides.back());
+  if (bias) {
+    gemm.set_bias(bias);
+  }
+  gemm.run(encoder, out, a, b, batch_shape, a_batch_strides, b_batch_strides);
+}
+
 } // namespace

 void Matmul::eval_gpu(const std::vector<array>& inputs, array& out) {
@@ -48,9 +117,6 @@ void Matmul::eval_gpu(const std::vector<array>& inputs, array& out) {

  out.set_data(allocator::malloc(out.nbytes()));

-  /////////////////////////////////////////////////////////////////////////////
-  // Init checks and prep
-
  int M = a_pre.shape(-2);
  int N = b_pre.shape(-1);
  int K = a_pre.shape(-1);
@@ -60,58 +126,8 @@ void Matmul::eval_gpu(const std::vector<array>& inputs, array& out) {
  auto [a_transposed, lda, a] = check_transpose(encoder, s, a_pre);
  auto [b_transposed, ldb, b] = check_transpose(encoder, s, b_pre);

-  /////////////////////////////////////////////////////////////////////////////
-  // Check and collapse batch dimensions
-
-  auto [batch_shape, a_batch_strides, b_batch_strides] = collapse_batches(a, b);
-
-  auto batch_count = out.size() / (M * N);
-
-  // Collapse batches into M if needed
-  if (batch_count > 1 && !a_transposed && batch_shape.size() == 1 &&
-      a.strides()[a.ndim() - 2] == K && a_batch_strides.back() == M * K &&
-      b_batch_strides.back() == 0) {
-    M *= batch_shape.back();
-    batch_count = 1;
-
-    a_batch_strides = {0};
-    b_batch_strides = {0};
-    batch_shape = {1};
-  }
-
-  if (cu::can_use_gemv(M, N, K, a_transposed, b_transposed)) {
-    cu::gemv(
-        a,
-        b,
-        out,
-        M,
-        N,
-        K,
-        batch_count,
-        batch_shape,
-        a_batch_strides,
-        b_batch_strides,
-        encoder);
-    return;
-  }
-
-  /////////////////////////////////////////////////////////////////////////////
-  // Invoke cublasLt
-  CublasGemm gemm(
-      cu::device(s.device),
-      a.dtype(),
-      a_transposed,
-      M,
-      K,
-      lda,
-      b_transposed,
-      K,
-      N,
-      ldb,
-      batch_shape.back(),
-      a_batch_strides.back(),
-      b_batch_strides.back());
-  gemm.run(encoder, out, a, b, batch_shape, a_batch_strides, b_batch_strides);
+  gemm_and_bias(
+      encoder, M, N, K, a_transposed, lda, b_transposed, ldb, out, a, b);
 }

 void AddMM::eval_gpu(const std::vector<array>& inputs, array& out) {
@@ -136,6 +152,27 @@ void AddMM::eval_gpu(const std::vector<array>& inputs, array& out) {
  auto [a_transposed, lda, a] = check_transpose(encoder, s, a_pre);
  auto [b_transposed, ldb, b] = check_transpose(encoder, s, b_pre);

+  /////////////////////////////////////////////////////////////////////////////
+  // Dispatch to GEMM with epilogue or AddMM
+
+  if (beta_ == 1 && c.strides(-1) == 1 && c.data_size() == out.shape(-1)) {
+    out.set_data(allocator::malloc(out.nbytes()));
+    gemm_and_bias(
+        encoder,
+        M,
+        N,
+        K,
+        a_transposed,
+        lda,
+        b_transposed,
+        ldb,
+        out,
+        a,
+        b,
+        c.data<void>());
+    return;
+  }
+
  int64_t ldc;
  {
    auto stx = c.strides()[c.ndim() - 2];
@@ -177,7 +214,7 @@ void AddMM::eval_gpu(const std::vector<array>& inputs, array& out) {
  }

  /////////////////////////////////////////////////////////////////////////////
-  // Invoke cublasLt
+  // Invoke cublasLt with AddMM settings

  CublasGemm gemm(
      cu::device(s.device),