Index: llvm/docs/CommandGuide/llvm-mca.rst
===================================================================
--- llvm/docs/CommandGuide/llvm-mca.rst
+++ llvm/docs/CommandGuide/llvm-mca.rst
@@ -254,6 +254,11 @@
     return a;
   }
 
+However, this interferes with optimizations like loop vectorization and may have
+an impact on the code generated.  This is because the ``__asm`` statements are
+seen as real code having important side effects, which limits how the code
+around them can be transformed.
+
 HOW LLVM-MCA WORKS
 ------------------