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Updated and clarified the coding hints.

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uz 2009-09-01 10:19:20 +00:00
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@ -3,12 +3,14 @@
<article>
<title>cc65 coding hints
<author>Ullrich von Bassewitz, <htmlurl url="mailto:uz@cc65.org" name="uz@cc65.org">
<date>03.12.2000
<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
@ -28,13 +30,14 @@ code.
<sect>Remember that the compiler does not optimize<p>
<sect>Remember that the compiler does no high level optimizations<p>
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.
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.
@ -48,10 +51,10 @@ operation works on double the data compared to an int.
<sect>Use unsigned types wherever possible<p>
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.
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.
@ -64,25 +67,8 @@ 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
<tscreen><verb>
unsigned char a;
...
if ((a & 0x0F) == 0)
</verb></tscreen>
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
<tscreen><verb>
unsigned char a;
...
if ((unsigned char)(a & 0x0F) == 0)
</verb></tscreen>
the generated code is much shorter, since the operation is done with 8 bits
instead of 16.
You should especially use unsigned chars for loop control variables if the
loop is known not to execute more than 255 times.
@ -180,7 +166,7 @@ subscript is a constant. So
<tscreen><verb>
#define VDC ((unsigned char*)0xD600)
#define STATUS 0x01
VDC [STATUS] = 0x01;
VDC[STATUS] = 0x01;
</verb></tscreen>
will also work.
@ -191,7 +177,7 @@ compiler does not know anything about the contents of the variable.
<sect>Use initialized local variables - but use it with care<p>
<sect>Use initialized local variables<p>
Initialization of local variables when declaring them gives shorter and faster
code. So, use
@ -234,44 +220,6 @@ The latter will work, but will create larger and slower code.
<sect>When using the ternary operator, cast values that are not ints<p>
The result type of the <tt/?:/ 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 <tt/int/ &rarr; <tt/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:
<tscreen><verb>
long f (long a)
{
return (a != 0)? 1 : a;
}
</verb></tscreen>
When the compiler sees the literal "1", it does not know, that the result type
of the <tt/?:/ operator is a long, so it will emit code to load a integer
constant 1. After parsing "a", which is a long, a <tt/int/ &rarr; <tt/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:
<tscreen><verb>
long f (long a)
{
return (a != 0)? 1L : a;
}
</verb></tscreen>
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.
<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
@ -302,11 +250,12 @@ instead.
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.
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
@ -324,43 +273,18 @@ And remember: Register variables must be enabled with <tt/-r/ or <tt/-Or/.
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:
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>
unsigned val;
...
if (val < 65535) {
...
}
test.c(7): Warning: Constant is long
</verb></tscreen>
Here, the compare is evaluated using 32 bit precision. This makes the code
larger and a lot slower.
Use the <tt/U/, <tt/L/ or <tt/UL/ suffixes to tell the compiler the desired
type of a numeric constant.
Using
<tscreen><verb>
unsigned val;
...
if (val < 0xFFFF) {
...
}
</verb></tscreen>
or
<tscreen><verb>
unsigned val;
...
if (val < 65535U) {
...
}
</verb></tscreen>
instead will give shorter and faster code.
<sect>Access to parameters in variadic functions is expensive<p>