Don't need to reduce row_sum during online softmax

Author: tridao

csrc/flash_attn/src/softmax.h

@@ -55,10 +55,10 @@ __device__ __forceinline__ void reduce_max(Tensor<Engine0, Layout0> const& tenso
     reduce_<zero_init>(tensor, max, max_op);
 }
 
-template<typename Engine0, typename Layout0, typename Engine1, typename Layout1>
+template<bool zero_init=true, typename Engine0, typename Layout0, typename Engine1, typename Layout1>
 __device__ __forceinline__ void reduce_sum(Tensor<Engine0, Layout0> const& tensor, Tensor<Engine1, Layout1> &sum){
     SumOp<float> sum_op;
-    reduce_(tensor, sum, sum_op);
+    thread_reduce_<zero_init>(tensor, sum, sum_op);
 }
 
 // Apply the exp to all the elements.
@@ -133,7 +133,7 @@ struct Softmax {
         if (Is_first) {
             flash::template reduce_max</*zero_init=*/true>(scores, row_max);
             flash::scale_apply_exp2(scores, row_max, softmax_scale_log2);
-            flash::reduce_sum(scores, row_sum);
+            flash::reduce_sum</*zero_init=*/true>(scores, row_sum);
         } else {
             Tensor scores_max_prev = make_fragment_like(row_max);
             cute::copy(row_max, scores_max_prev);
@@ -152,15 +152,16 @@ struct Softmax {
                 for (int ni = 0; ni < size<1>(acc_o_rowcol); ++ni) { acc_o_rowcol(mi, ni) *= scores_scale; }
             }
             flash::scale_apply_exp2(scores, row_max, softmax_scale_log2);
-            Tensor scores_sum_cur = make_fragment_like(row_sum);
-            flash::reduce_sum(scores, scores_sum_cur);
-            #pragma unroll
-            for (int mi = 0; mi < size(row_sum); ++mi) { row_sum(mi) += scores_sum_cur(mi); }
+            // We don't do the reduce across threads here since we don't need to use the row_sum.
+            // We do that reduce at the end when we need to normalize the softmax.
+            flash::reduce_sum</*zero_init=*/false>(scores, row_sum);
         }
     };
 
     template<bool Is_dropout=false, bool Split=false, typename Tensor0>
     __forceinline__ __device__ TensorT normalize_softmax_lse(Tensor0 &acc_o, float softmax_scale, float rp_dropout=1.0) {
+        SumOp<float> sum_op;
+        quad_allreduce_(row_sum, row_sum, sum_op);
         TensorT lse = make_fragment_like(row_sum);
         Tensor acc_o_rowcol = make_tensor(acc_o.data(), flash::convert_layout_acc_rowcol(acc_o.layout()));
         static_assert(decltype(size<0>(acc_o_rowcol))::value == kNRows);