diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -3683,11 +3683,19 @@
 The number of elements is a constant integer value larger than 0;
 elementtype may be any integer, floating-point or pointer type. Vectors
 of size zero are not allowed. For scalable vectors, the total number of
-elements is a constant multiple (called vscale) of the specified number
-of elements; vscale is a positive integer that is unknown at compile time
-and the same hardware-dependent constant for all scalable vectors at run
-time. The size of a specific scalable vector type is thus constant within
-IR, even if the exact size in bytes cannot be determined until run time.
+elements is a multiple (called ``vscale``) of the specified number of elements;
+
+.. _vscale
+
+``vscale`` is a positive integer that is unknown at compile time but has the
+same hardware-dependent value for all scalable vectors in a function. ``vscale``
+can change at function call boundaries but such a change is tightly controlled
+by function attributes and should not be observable to the calling function.
+
+The size of a specific scalable vector type is thus constant within a function,
+even if the exact size in bytes cannot be determined until run time. However,
+care must be taken to limit cross function optimisations if ``vscale`` might
+change.
 
 :Examples:
 
@@ -24745,11 +24753,9 @@
 Semantics:
 """"""""""
 
-``vscale`` is a positive value that is constant throughout program
-execution, but is unknown at compile time.
-If the result value does not fit in the result type, then the result is
-a :ref:`poison value <poisonvalues>`.
-
+:ref:`vscale <vscale>` is a positive value that is constant throughout the
+calling function. If the result value does not fit in the result type, then the
+result is a :ref:`poison value <poisonvalues>`.
 
 Stack Map Intrinsics
 --------------------