getfd.o: new object file

[cc65] / doc / coding.txt

How to generate the most effective code with cc65.


1.  Use prototypes.

    This will not only help to find errors between separate modules, it will
    also generate better code, since the compiler must not assume that a
    variable sized parameter list is in place and must not pass the argument
    count to the called function. This will lead to shorter and faster code.



2.  Don't declare auto variables in nested function blocks.

    Variable declarations in nested blocks are usually a good thing. But with
    cc65, there is a drawback: Since the compiler generates code in one pass,
    it must create the variables on the stack each time the block is entered
    and destroy them when the block is left. This causes a speed penalty and
    larger code.



3.  Remember that the compiler does not optimize.

    The compiler needs hints from you about the code to generate. When
    accessing indexed data structures, get a pointer to the element and
    use this pointer instead of calculating the index again and again.
    If you want to have your loops unrolled, or loop invariant code moved
    outside the loop, you have to do that yourself.



4.  Longs are slow!

    While long support is necessary for some things, it's really, really slow
    on the 6502. Remember that any long variable will use 4 bytes of memory,
    and any operation works on double the data compared to an int.



5.  Use unsigned types wherever possible.

    The CPU has no opcodes to handle signed values greater than 8 bit. So
    sign extension, test of signedness etc. has to be done by hand. The
    code to handle signed operations is usually a bit slower than the same
    code for unsigned types.



6.  Use chars instead of ints if possible.

    While in arithmetic operations, chars are immidiately promoted to ints,
    they are passed as chars in parameter lists and are accessed as chars
    in variables. The code generated is usually not much smaller, but it
    is faster, since accessing chars is faster. For several operations, the
    generated code may be better if intermediate results that are known not
    to be larger than 8 bit are casted to chars.

    When doing

        unsigned char a;
        ...
        if ((a & 0x0F) == 0)

    the result of the & operator is an int because of the int promotion
    rules of the language. So the compare is also done with 16 bits. When
    using

        unsigned char a;
        ...
        if ((unsigned char)(a & 0x0F) == 0)

    the generated code is much shorter, since the operation is done with
    8 bits instead of 16.



7.  Make the size of your array elements one of 1, 2, 4, 8.

    When indexing into an array, the compiler has to calculate the byte
    offset into the array, which is the index multiplied by the size of
    one element. When doing the multiplication, the compiler will do a
    strength reduction, that is, replace the multiplication by a shift
    if possible. For the values 2, 4 and 8, there are even more specialized
    subroutines available. So, array access is fastest when using one of
    these sizes.



8.  Expressions are evaluated from left to right.

    Since cc65 is not building an explicit expression tree when parsing an
    expression, constant subexpressions may not be detected and optimized
    properly if you don't help. Look at this example:

      #define OFFS   4
      int  i;
      i = i + OFFS + 3;

    The expression is parsed from left to right, that means, the compiler sees
    'i', and puts it contents into the secondary register. Next is OFFS, which
    is constant. The compiler emits code to add a constant to the secondary
    register. Same thing again for the constant 3. So the code produced
    contains a fetch of 'i', two additions of constants, and a store (into
    'i'). Unfortunately, the compiler does not see, that "OFFS + 3" is a
    constant for itself, since it does it's evaluation from left to right.
    There are some ways to help the compiler to recognize expression like
    this:

     a. Write "i = OFFS + 3 + i;". Since the first and second operand are
        constant, the compiler will evaluate them at compile time reducing the
        code to a fetch, one addition (secondary + constant) and one store.

     b. Write "i = i + (OFFS + 3)". When seeing the opening parenthesis, the
        compiler will start a new expression evaluation for the stuff in the
        braces, and since all operands in the subexpression are constant, it
        will detect this and reduce the code to one fetch, one addition and
        one store.



9.  Case labels in a switch statments are checked in source order.

    Labels that appear first in a switch statement are tested first. So,
    if your switch statement contains labels that are selected most of
    the time, put them first in your source code. This will speed up the
    code.



10. Use the preincrement and predecrement operators.

    The compiler not always smart enough to figure out, if the rvalue of an
    increment is used or not. So it has to save and restore that value when
    producing code for the postincrement and postdecrement operators, even if
    this value is never used. To avoid the additional overhead, use the
    preincrement and predecrement operators if you don't need the resulting
    value. That means, use

        ...
        ++i;
        ...

    instead of

        ...
        i++;
        ...



11. Use constants to access absolute memory locations.

    The compiler produces optimized code, if the value of a pointer is a
    constant. So, to access direct memory locations, use

        #define VDC_DATA   0xD601
        *(char*)VDC_STATUS = 0x01;

    That will be translated to

        lda     #$01
        sta     $D600

    The constant value detection works also for struct pointers and arrays,
    if the subscript is a constant. So

        #define VDC     ((unsigned char*)0xD600)
        #define STATUS  0x01
        VDC [STATUS] = 0x01;

    will also work.

    If you first load the constant into a variable and use that variable to
    access an absolute memory location, the generated code will be much
    slower, since the compiler does not know anything about the contents of
    the variable.



12. Use initialized local variables - but use it with care.

    Initialization of local variables when declaring them gives shorter
    and faster code. So, use

        int i = 1;

    instead of

        int i;
        i = 1;

    But beware: To maximize your savings, don't mix uninitialized and
    initialized variables. Create one block of initialized variables and
    one of uniniitalized ones. The reason for this is, that the compiler
    will sum up the space needed for uninitialized variables as long as
    possible, and then allocate the space once for all these variables.
    If you mix uninitialized and initialized variables, you force the
    compiler to allocate space for the uninitialized variables each time,
    it parses an initialized one. So do this:

        int i, j;
        int a = 3;
        int b = 0;

    instead of

        int i;
        int a = 3;
        int j;
        int b = 0;

    The latter will work, but will create larger and slower code.



13. When using the ?: operator, cast values that are not ints.

    The result type of the ?: operator is a long, if one of the second or
    third operands is a long. If the second operand has been evaluated and
    it was of type int, and the compiler detects that the third operand is
    a long, it has to add an additional int->long conversion for the
    second operand. However, since the code for the second operand has
    already been emitted, this gives much worse code.

    Look at this:

        long f (long a)
        {
            return (a != 0)? 1 : a;
        }

    When the compiler sees the literal "1", it does not know, that the
    result type of the ?: operator is a long, so it will emit code to load
    a integer constant 1. After parsing "a", which is a long, a int->long
    conversion has to be applied to the second operand. This creates one
    additional jump, and an additional code for the conversion.

    A better way would have been to write:

        long f (long a)
        {
            return (a != 0)? 1L : a;
        }

    By forcing the literal "1" to be of type long, the correct code is
    created in the first place, and no additional conversion code is
    needed.



14. Use the array operator [] even for pointers.

    When addressing an array via a pointer, don't use the plus and
    dereference operators, but the array operator. This will generate
    better code in some common cases.

    Don't use

        char* a;
        char b, c;
        char b = *(a + c);

    Use

        char* a;
        char b, c;
        char b = a[c];

    instead.



15. Use register variables with care.

    Register variables may give faster and shorter code, but they do also
    have an overhead. Register variables are actually zero page
    locations, so using them saves roughly one cycle per access. Since
    the old values have to be saved and restored, there is an overhead of
    about 70 cycles per 2 byte variable. It is easy to see, that - apart
    from the additional code that is needed to save and restore the
    values - you need to make heavy use of a variable to justify the
    overhead.

    An exception are pointers, especially char pointers. The optimizer
    has code to detect and transform the most common pointer operations
    if the pointer variable is a register variable. Declaring heavily
    used character pointers as register may give significant gains in
    speed and size.

    And remember: Register variables must be enabled with -Or.



16. Decimal constants greater than 0x7FFF are actually long ints

    The language rules for constant numeric values specify that decimal
    constants without a type suffix that are not in integer range must be
    of type long int or unsigned long int. This means that a simple
    constant like 40000 is of type long int, and may cause an expression
    to be evaluated with 32 bits.

    An example is:

        unsigned val;
        ...
        if (val < 65535) {
            ...
        }

    Here, the compare is evaluated using 32 bit precision. This makes the
    code larger and a lot slower.

    Using

        unsigned val;
        ...
        if (val < 0xFFFF) {
            ...
        }

    or

        unsigned val;
        ...
        if (val < 65535U) {
            ...
        }

    instead will give shorter and faster code.