fix JIT bug

The bug happens with n-ary uses of arithmetic operations that
have constant arguments but couldn't be constant-folded ---
maybe due to a divide-by-zero.

collects/tests/racket/optimize.rktl

@@ -1924,6 +1924,11 @@
                             (lambda ()
                               (set! a 'v)))))))
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Check compilation of an n-ary `/' that isn't
+;; constant folded due to a divide-by-zero:
+
+(err/rt-test (call/cc (lambda (k) (/ 1 2 0))))
 
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 

src/racket/src/jitarith.c

@@ -1903,9 +1903,10 @@ int scheme_generate_nary_arith(mz_jit_state *jitter, Scheme_App_Rec *app,
 
   __START_SHORT_JUMPS__(c < 100);
 
-  if (trigger_arg > c) {
-    /* we don't expect this to happen, since constant-folding would
-       have collapsed it */
+  if (trigger_arg >= c) {
+    /* we don't expect this to happen, since constant-folding normally
+       would have collapsed it --- but a division by zero, for example,
+       might block constant folding */
     trigger_arg = 0;
   }