bslathi19/cc65
1
0
Fork 0
Code Issues Pull Requests Actions 1 Packages Projects Releases Wiki Activity

This commit was generated by cvs2svn to compensate for changes in r2,

Browse Source 
which included commits to RCS files with non-trunk default branches.


git-svn-id: svn://svn.cc65.org/cc65/trunk@3 b7a2c559-68d2-44c3-8de9-860c34a00d81
This commit is contained in:
uz
2000-05-28 13:40:48 +00:00
parent 579491e8a4
commit 53dd513176
847 changed files with 91345 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files

340
doc/coding.txt Normal file
View File

@@ -0,0 +1,340 @@
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 are several drawbacks:
a. The compiler has only one symbol table (there's no true nesting).
This means that your variables must not have the same names as
variables in the enclosing block.
b. Since the compiler generates code in one pass, it must create the
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 is currently not 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.