diff --git a/src/3rd_party/faiss/VectorTransform.cpp b/src/3rd_party/faiss/VectorTransform.cpp
index c6bf9d4a1..22fecbf78 100644
--- a/src/3rd_party/faiss/VectorTransform.cpp
+++ b/src/3rd_party/faiss/VectorTransform.cpp
@@ -20,6 +20,8 @@
 using namespace faiss;
 
 #ifdef ARM
+// we use various AVX/SSE instructions in this file
+// simd_utils translates these into ARM/NEON compatible instructions
 #include "3rd_party/simd_utils/simd_utils.h"
 #endif