diff --git a/racket/src/cs/primitive/flfxnum.ss b/racket/src/cs/primitive/flfxnum.ss
index 182278d306..a76514e6df 100644
--- a/racket/src/cs/primitive/flfxnum.ss
+++ b/racket/src/cs/primitive/flfxnum.ss
@@ -5,7 +5,7 @@
   [fl+ (known-procedure -1)]
   [fl- (known-procedure -2)]
   [fl->exact-integer (known-procedure 2)]
-  [fl->fx (known-procedure 2)]
+  [fl->fx (known-procedure/has-unsafe 2 'unsafe-fl->fx)]
   [fl/ (known-procedure -2)]
   [fl< (known-procedure -2)]
   [fl<= (known-procedure -2)]
@@ -40,7 +40,7 @@
   [fx* (known-procedure -1)]
   [fx+ (known-procedure -1)]
   [fx- (known-procedure -2)]
-  [fx->fl (known-procedure 2)]
+  [fx->fl (known-procedure/has-unsafe 2 'unsafe-fx->fl)]
   [fx< (known-procedure -2)]
   [fx<= (known-procedure -2)]
   [fx= (known-procedure -2)]
diff --git a/racket/src/cs/primitive/kernel.ss b/racket/src/cs/primitive/kernel.ss
index e4c621849d..0e7c7a016e 100644
--- a/racket/src/cs/primitive/kernel.ss
+++ b/racket/src/cs/primitive/kernel.ss
@@ -42,7 +42,7 @@
   [bitwise-xor (known-procedure -1)]
   [boolean? (known-procedure/pure 2)]
   [box (known-procedure/pure 2)]
-  [box-cas! (known-procedure 8)]
+  [box-cas! (known-procedure/has-unsafe 8 'unsafe-box*-cas!)]
   [box-immutable (known-procedure/pure 2)]
   [box? (known-procedure/pure 2)]
   [break-enabled (known-procedure 3)]
@@ -71,10 +71,10 @@
   [bytes-copy (known-procedure 2)]
   [bytes-copy! (known-procedure 56)]
   [bytes-fill! (known-procedure 4)]
-  [bytes-length (known-procedure 2)]
+  [bytes-length (known-procedure/has-unsafe 2 'unsafe-bytes-length)]
   [bytes-open-converter (known-procedure 4)]
-  [bytes-ref (known-procedure 4)]
-  [bytes-set! (known-procedure 8)]
+  [bytes-ref (known-procedure/has-unsafe 4 'unsafe-bytes-ref)]
+  [bytes-set! (known-procedure/has-unsafe 8 'unsafe-bytes-set!)]
   [bytes-utf-8-index (known-procedure 28)]
   [bytes-utf-8-length (known-procedure 30)]
   [bytes-utf-8-ref (known-procedure 28)]
@@ -755,7 +755,7 @@
   [semaphore-wait/enable-break (known-procedure 2)]
   [semaphore? (known-procedure/pure 2)]
   [set-box! (known-procedure 4)]
-  [set-box*! (known-procedure 4)]
+  [set-box*! (known-procedure/has-unsafe 4 'unsafe-set-box*!)]
   [set-mcar! (known-procedure 4)]
   [set-mcdr! (known-procedure 4)]
   [set-phantom-bytes! (known-procedure 4)]
@@ -930,7 +930,7 @@
   [true-object? (known-procedure/pure 2)]
   [truncate (known-procedure 2)]
   [unbox (known-procedure 2)]
-  [unbox* (known-procedure 2)]
+  [unbox* (known-procedure/has-unsafe 2 'unsafe-unbox*)]
   [uncaught-exception-handler (known-constant)]
   [unquoted-printing-string (known-procedure 2)]
   [unquoted-printing-string-value (known-procedure 2)]
@@ -942,7 +942,7 @@
   [vector->pseudo-random-generator (known-procedure 2)]
   [vector->pseudo-random-generator! (known-procedure 4)]
   [vector->values (known-procedure 14)]
-  [vector-cas! (known-procedure 16)]
+  [vector-cas! (known-procedure/has-unsafe 16 'unsafe-vector*-cas!)]
   [vector-copy! (known-procedure 56)]
   [vector-fill! (known-procedure 4)]
   [vector-immutable (known-procedure/pure -1)]
@@ -951,9 +951,9 @@
   [vector-set! (known-procedure 8)]
   [vector-set-performance-stats! (known-procedure 6)]
   [vector? (known-procedure/pure 2)]
-  [vector*-length (known-procedure 2)]
-  [vector*-ref (known-procedure 4)]
-  [vector*-set! (known-procedure 8)]
+  [vector*-length (known-procedure/has-unsafe 2 'unsafe-vector*-length)]
+  [vector*-ref (known-procedure/has-unsafe 4 'unsafe-vector*-ref)]
+  [vector*-set! (known-procedure/has-unsafe 8 'unsafe-vector*-set!)]
   [version (known-procedure/pure 1)]
   [void (known-procedure/pure -1)]
   [void? (known-procedure/pure 2)]
diff --git a/racket/src/schemify/known.rkt b/racket/src/schemify/known.rkt
index f18386c5ab..d6f570cf45 100644
--- a/racket/src/schemify/known.rkt
+++ b/racket/src/schemify/known.rkt
@@ -13,6 +13,7 @@
          known-procedure/can-inline/need-imports-needed
          known-procedure/succeeds known-procedure/succeeds?
          known-procedure/pure known-procedure/pure?
+         known-procedure/has-unsafe known-procedure/has-unsafe? known-procedure/has-unsafe-alternate
          known-struct-type known-struct-type? known-struct-type-type
          known-struct-type-field-count known-struct-type-pure-constructor?
          known-constructor known-constructor? known-constructor-type
@@ -57,6 +58,10 @@
 ;; procedure that accepts any arguments and is functional so that it can be reordered
 (struct known-procedure/pure () #:prefab #:omit-define-syntaxes #:super struct:known-procedure/succeeds)
 
+;; procedure with an unsafe variant, especially ones that won't get substituted
+;; simply by compiling in unsafe mode
+(struct known-procedure/has-unsafe (alternate) #:prefab #:omit-define-syntaxes #:super struct:known-procedure)
+
 (struct known-struct-type (type field-count pure-constructor?) #:prefab #:omit-define-syntaxes #:super struct:known-consistent)
 
 ;; procedures with a known connection to a structure type:
diff --git a/racket/src/schemify/schemify.rkt b/racket/src/schemify/schemify.rkt
index 687a0994cd..f4561de5c2 100644
--- a/racket/src/schemify/schemify.rkt
+++ b/racket/src/schemify/schemify.rkt
@@ -758,6 +758,12 @@
                           (known-field-mutator? k)
                           (inline-field-mutate k s-rator im args))
                      => (lambda (e) e)]
+                    [(and unsafe-mode?
+                          (known-procedure/has-unsafe? k))
+                     (left-to-right/app (known-procedure/has-unsafe-alternate k)
+                                        args
+                                        #t for-cify?
+                                        prim-knowns knowns imports mutated)]
                     [else
                      (define plain-app? (or (known-procedure? k)
                                             (lambda? rator)))