Consider test case:
...
$ cat small.c
#include <string.h>

volatile int one = 1;

int main() {
  char a1[] = {(char)one, 2, 3, 4};
  char a2[] = {1, (char)(2*one), 3, 4};
  int res = memcmp (a1, a2, 5 + one);
  return res;
}
...

When stepping using step Line 7 is not hit by gdb.
...
$ gcc -O2 -g small.c; gdb -q a.out
Reading symbols from a.out...
(gdb) b main
Breakpoint 1 at 0x401040: file small.c, line 6.
(gdb) r
Starting program: /home/yibiao/Debugger/a.out 

Breakpoint 1, main () at small.c:6
6	  char a1[] = {(char)one, 2, 3, 4};
(gdb) step
8	  int res = memcmp (a1, a2, 5 + one);
(gdb) 
...

When stepping using stepi, Line 7 is hit after Line 6 by gdb.
...
$ gcc -O2 -g small.c; gdb -q a.out
Reading symbols from a.out...
(gdb) b main
Breakpoint 1 at 0x401040: file small.c, line 6.
(gdb) si
The program is not being run.
(gdb) r
Starting program: /home/yibiao/Debugger/a.out 

Breakpoint 1, main () at small.c:6
6	  char a1[] = {(char)one, 2, 3, 4};
(gdb) si
0x0000000000401044	6	  char a1[] = {(char)one, 2, 3, 4};
(gdb) si
0x000000000040104a	7	  char a2[] = {1, (char)(2*one), 3, 4};
(gdb) 



$ gcc --version
gcc (GCC) 10.0.1 20200419 (experimental)
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gdb --version
GNU gdb (GDB) 10.0.50.20200517-git
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.