Or, in one line of shell:

  echo 'int square(int num) { return num*num; }' | gcc -S -x c - -o -

How about this: open an editor on "test.c". Then in another terminal window, run this script:

  #!/bin/sh

  rm -f test_copy.c

  while true ; do
    if [ test.c -nt test_copy.c ] ; then
      cp test.c test_copy.c
      clear
      gcc -S test_copy.c -o -
    fi
    sleep 0.3
  done

Every time you save test.c, the compiler output updates in the other shell window (within a fraction of a second).