Index: llvm/docs/CommandGuide/llvm-mca.rst
===================================================================
--- llvm/docs/CommandGuide/llvm-mca.rst
+++ llvm/docs/CommandGuide/llvm-mca.rst
@@ -241,6 +241,9 @@
 Note that multiple anonymous regions cannot overlap. Also, overlapping regions
 cannot have the same name.
 
+USING MARKERS IN SOURCE CODE
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 Inline assembly directives may be used from source code to annotate the
 assembly text:
 
@@ -254,6 +257,17 @@
     return a;
   }
 
+However, currently, `this interferes with optimizations like loop vectorization
+<https://bugs.llvm.org/show_bug.cgi?id=42173>`_ and changes the code that is 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.
+
+It is generally safe to use source-level markers in straight-line source code without
+any control flow, like the example above.  Otherwise, you must take care to make
+sure that the code you are analyzing is the same as the code the compiler actually
+wants to generate.  Unfortunately, the only robust way to do this is to manually
+add the markers to the assembly text.
+
 HOW LLVM-MCA WORKS
 ------------------