remote TABs in doc/ and test/

[cc65] / doc / coding.sgml

<!doctype linuxdoc system>

<article>
<title>cc65 coding hints
<author><url url="mailto:uz@cc65.org" name="Ullrich von Bassewitz">

<abstract>
How to generate the most effective code with cc65.
</abstract>



<sect>Use prototypes<p>

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.



<sect>Don't declare auto variables in nested function blocks<p>

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.



<sect>Remember that the compiler does no high level optimizations<p>

The compiler needs hints from you about the code to generate. It will try to
optimize the generated code, but follow the outline you gave in your C
program. So for example, 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.



<sect>Longs are slow!<p>

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.



<sect>Use unsigned types wherever possible<p>

The 6502 CPU has no opcodes to handle signed values greater than 8 bit. So
sign extension, test of signedness etc. has to be done with extra code. As a
consequence, the code to handle signed operations is usually a bit larger and
slower than the same code for unsigned types.



<sect>Use chars instead of ints if possible<p>

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.

You should especially use unsigned chars for loop control variables if the
loop is known not to execute more than 255 times.



<sect>Make the size of your array elements one of 1, 2, 4, 8<p>

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.



<sect>Expressions are evaluated from left to right<p>

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:

<tscreen><verb>
      #define OFFS   4
      int  i;
      i = i + OFFS + 3;
</verb></tscreen>

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
its evaluation from left to right. There are some ways to help the compiler
to recognize expression like this:

<enum>

<item>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.

<item>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.

</enum>


<sect>Use the preincrement and predecrement operators<p>

The compiler is 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

<tscreen><verb>
        ...
        ++i;
        ...
</verb></tscreen>

    instead of

<tscreen><verb>
        ...
        i++;
        ...
</verb></tscreen>



<sect>Use constants to access absolute memory locations<p>

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

<tscreen><verb>
        #define VDC_STATUS 0xD601
        *(char*)VDC_STATUS = 0x01;
</verb></tscreen>

That will be translated to

<tscreen><verb>
        lda     #$01
        sta     $D601
</verb></tscreen>

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

<tscreen><verb>
        #define VDC     ((unsigned char*)0xD600)
        #define STATUS  0x01
        VDC[STATUS] = 0x01;
</verb></tscreen>

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.



<sect>Use initialized local variables<p>

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

<tscreen><verb>
        int i = 1;
</verb></tscreen>

instead of

<tscreen><verb>
        int i;
        i = 1;
</verb></tscreen>

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:

<tscreen><verb>
        int i, j;
        int a = 3;
        int b = 0;
</verb></tscreen>

instead of

<tscreen><verb>
        int i;
        int a = 3;
        int j;
        int b = 0;
</verb></tscreen>

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



<sect>Use the array operator &lsqb;&rsqb; even for pointers<p>

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

<tscreen><verb>
        char* a;
        char b, c;
        char b = *(a + c);
</verb></tscreen>

Use

<tscreen><verb>
        char* a;
        char b, c;
        char b = a[c];
</verb></tscreen>

instead.



<sect>Use register variables with care<p>

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. The calling routine may also use register
variables, so the old values have to be saved on function entry and restored
on exit. Saving an d restoring has 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.

As a general rule: Use register variables only for pointers that are
dereferenced several times in your function, or for heavily used induction
variables in a loop (with several 100 accesses).

When declaring register variables, try to keep them together, because this
will allow the compiler to save and restore the old values in one chunk, and
not in several.

And remember: Register variables must be enabled with <tt/-r/ or <tt/-Or/.



<sect>Decimal constants greater than 0x7FFF are actually long ints<p>

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. So a simple constant like 40000 is of type long int!
This is often unexpected and may cause an expression to be evaluated with 32
bits. While in many cases the compiler takes care about it, in some places it
can't. So be careful when you get a warning like

<tscreen><verb>
        test.c(7): Warning: Constant is long
</verb></tscreen>

Use the <tt/U/, <tt/L/ or <tt/UL/ suffixes to tell the compiler the desired
type of a numeric constant.



<sect>Access to parameters in variadic functions is expensive<p>

Since cc65 has the "wrong" calling order, the location of the fixed parameters
in a variadic function (a function with a variable parameter list) depends on
the number and size of variable arguments passed. Since this number and size
is unknown at compile time, the compiler will generate code to calculate the
location on the stack when needed.

Because of this additional code, accessing the fixed parameters in a variadic
function is much more expensive than access to parameters in a "normal"
function. Unfortunately, this additional code is also invisible to the
programmer, so it is easy to forget.

As a rule of thumb, if you access such a parameter more than once, you should
think about copying it into a normal variable and using this variable instead.


</article>