[XLA] Improve comment about why we can't unroll loops with static trip count of 1...

PiperOrigin-RevId: 185034032

diff --git a/tensorflow/compiler/xla/service/while_loop_simplifier.cc b/tensorflow/compiler/xla/service/while_loop_simplifier.cc
index 87a7f86..981de9b 100644 (file)
--- a/tensorflow/compiler/xla/service/while_loop_simplifier.cc
+++ b/tensorflow/compiler/xla/service/while_loop_simplifier.cc
@@ -564,9 +564,11 @@ static StatusOr<bool> TryRemoveWhileLoop(HloInstruction* while_op) {
   //
   // This is not a fundamental limitation.  The control operands can be moved
   // onto the new HLOs after simplification, and any side-effecting ops inside
-  // the loop aren't removed, just cloned and added back to the loop.
-  // Nevertheless our infrastructure sees loop simplification as removal of
-  // these nodes and currently doesn't allow it.
+  // the loop aren't removed, just cloned and added back to the loop.  But
+  // moving an op out of the loop also removes implicit control dependencies
+  // between the op and the ops outside the loop, so we'd have to add those back
+  // for things like infeed/outfeed.  It gets complicated.  So for now we just
+  // avoid it.
   if (!while_op->parent()->IsRemovable(while_op) || while_op->HasSideEffect()) {
     VLOG(2) << "Not attempting to remove while loop it is not removable: "
             << while_op->ToShortString();