diff --git a/tools/gen-expr/.gitignore b/tools/gen-expr/.gitignore
new file mode 100644
index 0000000000000000000000000000000000000000..90b7a4a7f44ac419c5b1ea44b7a1447aa0d55ae8
--- /dev/null
+++ b/tools/gen-expr/.gitignore
@@ -0,0 +1 @@
+.code.c
diff --git a/tools/gen-expr/Makefile b/tools/gen-expr/Makefile
new file mode 100644
index 0000000000000000000000000000000000000000..ddddfaf85f361225a3551faa6d020fd2de208ea5
--- /dev/null
+++ b/tools/gen-expr/Makefile
@@ -0,0 +1,2 @@
+gen-expr: gen-expr.c
+	gcc -O2 -Wall -Werror -o $@ $<
diff --git a/tools/gen-expr/gen-expr.c b/tools/gen-expr/gen-expr.c
new file mode 100644
index 0000000000000000000000000000000000000000..eb829e15ccf51638de21d7a3ceb6aea28cb9ff21
--- /dev/null
+++ b/tools/gen-expr/gen-expr.c
@@ -0,0 +1,116 @@
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+#include <assert.h>
+#include <string.h>
+
+// this should be enough
+static char buf[65536];
+static char *pbuf;
+
+#define format_buf(fmt, ...) pbuf += sprintf(pbuf, fmt, ##__VA_ARGS__)
+
+static inline uint32_t choose(uint32_t max) {
+  return rand() % max;
+}
+
+static inline void gen_rand_op() {
+  char op_list[4] = {'+', '-', '*', '/'};
+  format_buf("%c", op_list[choose(4)]);
+}
+
+static inline void gen_num() {
+  format_buf("%uu", choose(64));
+}
+
+static inline void gen_space() {
+  char *space_list[3] = {
+    "",
+    " ",
+    "  ",
+  };
+  format_buf("%s", space_list[choose(3)]);
+}
+
+static int nr_op = 0;
+
+static inline void gen_rand_expr() {
+  gen_space();
+  switch (choose(4)) {
+    case 0:
+      if (nr_op == 0) gen_rand_expr();
+      else gen_num();
+      break;
+    case 1:
+      format_buf("(");
+      gen_rand_expr();
+      format_buf(")");
+      break;
+    default:
+      nr_op ++;
+      if (choose(2)) gen_rand_expr();
+      else gen_num();
+      gen_space();
+      gen_rand_op();
+      gen_space();
+      if (choose(2)) gen_rand_expr();
+      else gen_num();
+      break;
+  }
+  gen_space();
+}
+
+void remove_u(char *p) {
+  char *q = p;
+  while ((q = strchr(q, 'u')) != NULL) {
+    // reuse code_buf
+    strcpy(code_buf, q + 1);
+    strcpy(q, code_buf);
+  }
+}
+
+static char code_buf[65536];
+static char *code_format =
+"#include <stdio.h>\n"
+"int main() { "
+"  unsigned result = %s; "
+"  printf(\"%%u\", result); "
+"  return 0; "
+"}";
+
+int main(int argc, char *argv[]) {
+  int seed = time(0);
+  srand(seed);
+  int loop = 1;
+  if (argc > 1) {
+    sscanf(argv[1], "%d", &loop);
+  }
+  int i;
+  for (i = 0; i < loop; i ++) {
+    nr_op = 0;
+    pbuf = buf;
+
+    gen_rand_expr();
+
+    sprintf(code_buf, code_format, buf);
+
+    FILE *fp = fopen(".code.c", "w");
+    assert(fp != NULL);
+    fputs(code_buf, fp);
+    fclose(fp);
+
+    int ret = system("gcc .code.c -o .expr");
+    if (ret != 0) continue;
+
+    fp = popen("./.expr", "r");
+    assert(fp != NULL);
+
+    int result;
+    fscanf(fp, "%d", &result);
+    pclose(fp);
+    remove_u(buf);
+    printf("%u %s\n", result, buf);
+  }
+  return 0;
+}