diff --git a/htdocs/gcc-13/porting_to.html b/htdocs/gcc-13/porting_to.html
index 0ee58802..7d733b16 100644
--- a/htdocs/gcc-13/porting_to.html
+++ b/htdocs/gcc-13/porting_to.html
@@ -203,11 +203,20 @@ class Alloc
 <p>
 Since C++20, there is no <code>rebind</code> member in
 <code>std::allocator</code>, so deriving your own allocator types from
-<code>std::allocator</code> is simpler and doesn't require the derived
+<code>std::allocator</code> is simpler and does not require the derived
 allocator to provide its own <code>rebind</code>.
 For compatibility with previous C++ standards, the member should still be
 provided. The converting constructor is still required even in C++20.
 </p>
 
+<h2 id="fortran">Fortran language issues</h2>
+<h3 id="overflow">Behavior on integer overflow</h3>
+<p> GCC 13 includes new optimizations which expose reliance on
+  non-standard behavior for integer overflow, which was often used
+  for linear congruential pseudo-random number generators in old
+  programs.  It is recommended to use the intrinsic
+  subroutine <code>RANDOM_NUMBER</code> for random number generators
+  or, if the old behavior is desired, to use the <code>-fwrapv</code>
+  option.  Note that this option can impact performance.
 </body>
 </html>