diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -9586,7 +9586,13 @@
 """""""""
 
 The '``frem``' instruction returns the remainder from the division of
-its two operands.
+its two operands. 
+
+.. note::
+
+The instruction is implemented as a call to libm's '``fmod``'
+for some targets, and using the instruction may thus require linking libm.
+
 
 Arguments:
 """"""""""