diff --git a/src/frontc/cabs2cil.ml b/src/frontc/cabs2cil.ml
index 2f4dea79b..19c54a6f6 100644
--- a/src/frontc/cabs2cil.ml
+++ b/src/frontc/cabs2cil.ml
@@ -4752,6 +4752,22 @@ and doExp (asconst: bool)   (* This expression is used as a constant *)
                           prestype := intType
                   | _ -> ignore (warn "Invalid call to builtin_types_compatible_p");
                 end
+                else if fv.vname = "__builtin_clzll" && asconst && isEmpty (!prechunk ()) then
+                  begin
+                  (* Constant-fold the argument and see if it is a constant *)
+                    let countLeadingZeros (arg: cilint) pos = pos - Z.numbits arg
+                    in
+                    match !pargs with
+                      [ arg ] -> begin
+                        match constFold true arg with
+                          (Const CInt (arg, kind, _)) ->
+                            piscall := false;
+                            pres := integer (countLeadingZeros arg 64);
+                            prestype := intType
+                        | _ -> ()
+                      end
+                    | _ -> ignore (warn "Invalid call to __builtin_clzll");
+                  end
             end
           | _ -> ()
         );
diff --git a/test/small1/builtin_clzll.c b/test/small1/builtin_clzll.c
new file mode 100644
index 000000000..d3c749964
--- /dev/null
+++ b/test/small1/builtin_clzll.c
@@ -0,0 +1,11 @@
+struct S1 {
+    int x : __builtin_choose_expr(__builtin_clzll(1) == 63, 1, -1),
+          : __builtin_choose_expr(__builtin_clzll(2) == 62, 1, -1),
+          : __builtin_choose_expr(__builtin_clzll(4) == 61, 1, -1),
+          : __builtin_choose_expr(__builtin_clzll(1024) == 53, 1, -1),
+          : __builtin_choose_expr(__builtin_clzll(1024 * 1024) == 43, 1, -1);
+} s1 = {0};
+
+int main() {
+    return 0;
+}
\ No newline at end of file
diff --git a/test/testcil.pl b/test/testcil.pl
index 151a12652..9de7c7f2e 100644
--- a/test/testcil.pl
+++ b/test/testcil.pl
@@ -557,6 +557,7 @@ sub addToGroup {
 addTest("testrun/builtin4 ");
 addTest("test/builtin5 ");
 addTest("test/builtin6 ");
+addTest("testrun/builtin_clzll ");
 addTest("test/sync-1 _GNUCC=1");
 addTest("test/sync-2 _GNUCC=1");
 addTest("test/sync-3 _GNUCC=1");