fix loop end checking

src/fh_runtime.c

@@ -133,7 +133,7 @@ enum fh_error w_user_word(struct fh_thread_s *fh, const struct fh_word_s *w0)
 
   uint32_t strl;
   uint32_t val;
-  uint32_t limit, index, index0;
+  uint32_t limit, index;
 
   LOG("0x%08x: Instr %s, 0x%08x", fh->execptr, instr_name(instr->kind), instr->data);
 
@@ -271,20 +271,28 @@ enum fh_error w_user_word(struct fh_thread_s *fh, const struct fh_word_s *w0)
       TRY(rs_peek(fh, &limit));
 
       LOG("+LOOP, i=%d, step %d, limit %d", fh->loop_i, val, limit);
-
-      index0 = fh->loop_i;
-      fh->loop_i += val;
-
-      LOG("after add: %d", fh->loop_i);
-
-      // FIXME this is probably wrong
-      // FIXME yes it actually is wrong
-      if (((int32_t)index0 < (int32_t)limit) == ((int32_t)fh->loop_i < (int32_t)limit) && fh->loop_i != limit) { // boundary not crossed, continue
-        fh->execptr = instr->data; // go to beginning
-      } else {
+      
+      const int32_t vi = (int32_t)val;
+      const int32_t bdr = (int32_t)limit - (int32_t)1;
+      const int32_t i0 = (int32_t)fh->loop_i;      
+      fh->loop_i += val; // this can overflow
+      const int32_t i1 = (int32_t)fh->loop_i;
+      
+      // TODO this can probably be optimized
+      if (
+        (vi > 0 && i0 <= bdr && i1 > bdr) 
+        || (vi > 0 && i0 > 0 && i1 < 0 && (bdr >= i0 || bdr <= i1))
+        || (vi < 0 && i0 > bdr && i1 <= bdr)
+        || (vi < 0 && i0 < 0 && i1 > 0 && (bdr <= i0 || bdr >= i1))
+      ) {    
+        //LOGE("end of loop");
         // end of loop
         TRY(rs_pop(fh, &limit));
         TRY(fh_loop_unnest(fh));
+      } else {
+        //LOGE("continue loop");
+        // continue the loop
+        fh->execptr = instr->data;
       }
       goto instr;
 

testfiles/ovfloop.f

@@ -0,0 +1,3 @@
+: x 2147483647 -2147483648 DO I . -1 +LOOP ;
+x
+