mirror of
https://github.com/cc65/cc65.git
synced 2024-08-19 17:29:37 +00:00
309 lines
9.7 KiB
Plaintext
309 lines
9.7 KiB
Plaintext
<!doctype linuxdoc system>
|
|
|
|
<article>
|
|
<title>cc65 coding hints
|
|
<author>Ullrich von Bassewitz, <htmlurl url="mailto:uz@cc65.org" name="uz@cc65.org">
|
|
<date>2000-12-03, 2009-09-01
|
|
|
|
<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
|
|
it's 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 [] 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>
|
|
|