When safety level is above 2, calls to fdefinition and access to global variables is compiled even if the value is discarded. This is done for the side effect of checking whether the function or the variable is bound.

src/cmp/cmploc.lsp

@@ -234,10 +234,22 @@
          (wt-nl "value0=") (wt-coerce-loc :object loc)
          (wt "; cl_env_copy->nvalues=1;"))))
 
-(defun set-trash-loc (loc)
-  (cond ((uses-values loc) (wt-nl "(void)" loc ";"))
-        ((and (consp loc)
-              (eq (first loc) 'C-INLINE)
-              (fifth loc)) ; side effects?
-         (wt-nl loc ";"))))
+(defun loc-with-side-effects-p (loc &aux name)
+  (cond ((var-p loc)
+	 (and (global-var-p loc)
+	      (policy-global-var-checking)))
+	((atom loc)
+	 nil)
+	((member (setf name (first loc)) '(CALL CALL-NORMAL CALL-INDIRECT)
+		 :test #'eq)
+	 t)
+	((eq name 'FDEFINITION)
+	 (policy-global-function-checking))
+	((eq name 'C-INLINE)
+	 (or (eq (sixth loc) 'VALUES) ;; Uses VALUES
+	     (fifth loc))))) ;; or side effects
 
+(defun set-trash-loc (loc)
+  (when (loc-with-side-effects-p loc)
+    (wt-nl loc ";")
+    t))

src/cmp/cmppolicy.lsp

@@ -257,10 +257,10 @@
 (define-policy array-bounds-check :on safety 1
   "Check out of bounds access to arrays")
 
-(define-policy global-var-checking :alias assume-no-errors
+(define-policy global-var-checking :on safety 3
   "Read the value of a global variable even if it is discarded, ensuring it is bound")
 
-(define-policy global-function-checking :alias assume-no-errors
+(define-policy global-function-checking :on safety 3
   "Read the binding of a global function even if it is discarded")
 
 (define-policy check-nargs :on safety 1 :only-on ext:check-arguments-type 1