path: root/apps/factorial3.subx

## compute the factorial of 5, and return the result in the exit code
#
# Uses syntax sugar for:
#   rm32 operands
#   function calls
#
# 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 and argv[1] == "test", then return run_tests()
    # if (argc <= 1) goto run-main
    81 7/subop/compare *ebp 1/imm32
    7e/jump-if-<= $run-main/disp8
    # if (!kernel-string-equal?(argv[1], "test")) goto run-main
    (kernel-string-equal? *(ebp+8) "test")  # => eax
    # . if (eax == false) goto run-main
    3d/compare-eax-and 0/imm32/false
    74/jump-if-= $run-main/disp8
    #
    (run-tests)
    # syscall(exit, *Num-test-failures)
    8b/-> *Num-test-failures 3/r32/ebx
    eb/jump $main:end/disp8
$run-main:
    # - otherwise
    (factorial 5)  # => eax
    # syscall(exit, eax)
    89/<- %ebx 0/r32/eax
$main:end:
    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
    b8/copy-to-eax 1/imm32
    81 7/subop/compare *(ebp+8) 1/imm32
    7e/jump-if-<= $factorial:end/disp8
    # var ebx: int = n-1
    8b/-> *(ebp+8) 3/r32/ebx
    4b/decrement-ebx
    #
    (factorial %ebx)  # => eax
    # return n * factorial(n-1)
    f7 4/subop/multiply-into-eax *(ebp+8)
    # TODO: check for overflow
$factorial:end:
    # 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