update comment

tests/test_language.py

@@ -240,9 +240,14 @@ def test_instruction_fusion_multi_deps_mscclpp():
     topology = fully_connected(3)
     collective = AllReduce(3, 1, True)
     prgm = MSCCLPPProgram("allreduce", topology, collective, 1)
-    # last reduce_packet depends on put_packets(write after read)
-    # and first reduce_packet(rank 2 put data to rank 1, data put depends on first reduce_packet).
-    # In this case, we don't need to fuse the operations.
+    # The dependency graph for rank 1 is as follows:
+    # put(0i to 1s) => reduce(1s to 1i) => put(2i to 1s) => reduce(1s to 1i)
+    #                       |  => put(1i to 0s)                      ^
+    #                       |  =>  put(1i to 2s)------------------- -|
+    # put(2i to 1s) => reduce(1s to 1i) for read after write
+    # put(1i to 2s) => reduce(1s to 1i) for write after read
+    # when we try to merge reduce(1s to 1i) => put(2i to 1s) => reduce(1s to 1i),
+    # circular dependency is introduced
     with prgm:
         c0 = chunk(0, Buffer.input, 0)
         c0.put_packet(1, "scratch", 0, sendtb=0)