path: root/apps/factorial4.subx

## compute the factorial of 5, and return the result in the exit code
#
# Uses syntax sugar for:
#   rm32 operands
#   function calls
#   control flow
#
# To run:
#   $ ./translate_subx init.linux 0*.subx apps/factorial.subx -o apps/factorial
#   $ ./subx run apps/factorial
# Expected result:
#   $ echo $?
#   120
#
# You can also run the automated test suite:
#   $ ./subx run apps/factorial test
# Expected output:
#   ........
# Every '.' indicates a passing test. Failing tests get a 'F'.

== code

Entry:  # run tests if necessary, compute `factorial(5)` if not
    # . prologue
    89/<- %ebp 4/r32/esp

    # initialize heap
    (new-segment *Heap-size Heap)

    # - if argc > 1, then return run_tests()
    {
      # if (argc <= 1) break
      81 7/subop/compare *ebp 1/imm32
      7e/jump-if-<= break/disp8
      # if (!kernel-string-equal?(argv[1], "test")) break
      (kernel-string-equal? *(ebp+8) "test")  # => eax
      3d/compare-eax-and 0/imm32/false
      74/jump-if-= break/disp8
      #
      (run-tests)
      # eax = *Num-test-failures
      8b/-> *Num-test-failures 3/r32/ebx
    }
    # if (argc <= 1) factorial(5)
    {
      # if (argc > 1) break
      81 7/subop/compare *ebp 1/imm32
      7f/jump-if-> break/disp8
      # eax = factorial(5)
      (factorial 5)
      # syscall(exit, eax)
      89/<- %ebx 0/r32/eax
    }

    b8/copy-to-eax 1/imm32/exit
    cd/syscall 0x80/imm8

factorial:  # n : int -> int/eax
    # . prologue
    55/push-ebp
    89/<- %ebp 4/r32/esp
    # save registers
    53/push-ebx
    # if (n <= 1) return 1
    81 7/subop/compare *(ebp+8) 1/imm32
    {
      7f/jump-if-> break/disp8
      b8/copy-to-eax 1/imm32
    }
    # if (n > 1) return n * factorial(n-1)
    {
      7e/jump-if-<= break/disp8
      # var ebx : int = n-1
      8b/-> *(ebp+8) 3/r32/ebx
      4b/decrement-ebx
      (factorial %ebx)  # => eax
      f7 4/subop/multiply-into-eax *(ebp+8)
    }
    # restore registers
    5b/pop-to-ebx
    # . epilogue
    89/<- %esp 5/r32/ebp
    5d/pop-to-ebp
    c3/return

test-factorial:
    (factorial 5)
    (check-ints-equal %eax 0x78 "F - test-factorial")
    c3/return