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336 lines
10 KiB
Plaintext
336 lines
10 KiB
Plaintext
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How to generate the most effective code with cc65.
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1. Use prototypes.
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This will not only help to find errors between separate modules, it will
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also generate better code, since the compiler must not assume that a
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variable sized parameter list is in place and must not pass the argument
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count to the called function. This will lead to shorter and faster code.
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2. Don't declare auto variables in nested function blocks.
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Variable declarations in nested blocks are usually a good thing. But with
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cc65, there is a drawback: Since the compiler generates code in one pass,
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it must create the variables on the stack each time the block is entered
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and destroy them when the block is left. This causes a speed penalty and
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larger code.
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3. Remember that the compiler does not optimize.
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The compiler needs hints from you about the code to generate. When
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accessing indexed data structures, get a pointer to the element and
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use this pointer instead of calculating the index again and again.
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If you want to have your loops unrolled, or loop invariant code moved
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outside the loop, you have to do that yourself.
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4. Longs are slow!
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While long support is necessary for some things, it's really, really slow
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on the 6502. Remember that any long variable will use 4 bytes of memory,
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and any operation works on double the data compared to an int.
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5. Use unsigned types wherever possible.
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The CPU has no opcodes to handle signed values greater than 8 bit. So
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sign extension, test of signedness etc. has to be done by hand. The
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code to handle signed operations is usually a bit slower than the same
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code for unsigned types.
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6. Use chars instead of ints if possible.
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While in arithmetic operations, chars are immidiately promoted to ints,
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they are passed as chars in parameter lists and are accessed as chars
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in variables. The code generated is usually not much smaller, but it
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is faster, since accessing chars is faster. For several operations, the
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generated code may be better if intermediate results that are known not
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to be larger than 8 bit are casted to chars.
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When doing
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unsigned char a;
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...
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if ((a & 0x0F) == 0)
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the result of the & operator is an int because of the int promotion
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rules of the language. So the compare is also done with 16 bits. When
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using
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unsigned char a;
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...
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if ((unsigned char)(a & 0x0F) == 0)
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the generated code is much shorter, since the operation is done with
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8 bits instead of 16.
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7. Make the size of your array elements one of 1, 2, 4, 8.
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When indexing into an array, the compiler has to calculate the byte
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offset into the array, which is the index multiplied by the size of
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one element. When doing the multiplication, the compiler will do a
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strength reduction, that is, replace the multiplication by a shift
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if possible. For the values 2, 4 and 8, there are even more specialized
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subroutines available. So, array access is fastest when using one of
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these sizes.
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8. Expressions are evaluated from left to right.
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Since cc65 is not building an explicit expression tree when parsing an
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expression, constant subexpressions may not be detected and optimized
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properly if you don't help. Look at this example:
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#define OFFS 4
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int i;
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i = i + OFFS + 3;
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The expression is parsed from left to right, that means, the compiler sees
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'i', and puts it contents into the secondary register. Next is OFFS, which
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is constant. The compiler emits code to add a constant to the secondary
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register. Same thing again for the constant 3. So the code produced
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contains a fetch of 'i', two additions of constants, and a store (into
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'i'). Unfortunately, the compiler does not see, that "OFFS + 3" is a
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constant for itself, since it does it's evaluation from left to right.
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There are some ways to help the compiler to recognize expression like
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this:
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a. Write "i = OFFS + 3 + i;". Since the first and second operand are
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constant, the compiler will evaluate them at compile time reducing the
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code to a fetch, one addition (secondary + constant) and one store.
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b. Write "i = i + (OFFS + 3)". When seeing the opening parenthesis, the
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compiler will start a new expression evaluation for the stuff in the
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braces, and since all operands in the subexpression are constant, it
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will detect this and reduce the code to one fetch, one addition and
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one store.
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9. Case labels in a switch statments are checked in source order.
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Labels that appear first in a switch statement are tested first. So,
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if your switch statement contains labels that are selected most of
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the time, put them first in your source code. This will speed up the
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code.
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10. Use the preincrement and predecrement operators.
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The compiler not always smart enough to figure out, if the rvalue of an
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increment is used or not. So it has to save and restore that value when
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producing code for the postincrement and postdecrement operators, even if
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this value is never used. To avoid the additional overhead, use the
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preincrement and predecrement operators if you don't need the resulting
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value. That means, use
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...
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++i;
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...
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instead of
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...
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i++;
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...
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11. Use constants to access absolute memory locations.
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The compiler produces optimized code, if the value of a pointer is a
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constant. So, to access direct memory locations, use
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#define VDC_DATA 0xD601
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*(char*)VDC_STATUS = 0x01;
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That will be translated to
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lda #$01
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sta $D600
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The constant value detection works also for struct pointers and arrays,
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if the subscript is a constant. So
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#define VDC ((unsigned char*)0xD600)
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#define STATUS 0x01
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VDC [STATUS] = 0x01;
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will also work.
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If you first load the constant into a variable and use that variable to
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access an absolute memory location, the generated code will be much
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slower, since the compiler does not know anything about the contents of
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the variable.
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12. Use initialized local variables - but use it with care.
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Initialization of local variables when declaring them gives shorter
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and faster code. So, use
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int i = 1;
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instead of
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int i;
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i = 1;
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But beware: To maximize your savings, don't mix uninitialized and
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initialized variables. Create one block of initialized variables and
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one of uniniitalized ones. The reason for this is, that the compiler
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will sum up the space needed for uninitialized variables as long as
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possible, and then allocate the space once for all these variables.
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If you mix uninitialized and initialized variables, you force the
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compiler to allocate space for the uninitialized variables each time,
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it parses an initialized one. So do this:
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int i, j;
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int a = 3;
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int b = 0;
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instead of
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int i;
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int a = 3;
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int j;
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int b = 0;
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The latter will work, but will create larger and slower code.
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13. When using the ?: operator, cast values that are not ints.
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The result type of the ?: operator is a long, if one of the second or
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third operands is a long. If the second operand has been evaluated and
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it was of type int, and the compiler detects that the third operand is
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a long, it has to add an additional int->long conversion for the
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second operand. However, since the code for the second operand has
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already been emitted, this gives much worse code.
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Look at this:
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long f (long a)
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{
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return (a != 0)? 1 : a;
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}
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When the compiler sees the literal "1", it does not know, that the
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result type of the ?: operator is a long, so it will emit code to load
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a integer constant 1. After parsing "a", which is a long, a int->long
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conversion has to be applied to the second operand. This creates one
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additional jump, and an additional code for the conversion.
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A better way would have been to write:
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long f (long a)
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{
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return (a != 0)? 1L : a;
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}
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By forcing the literal "1" to be of type long, the correct code is
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created in the first place, and no additional conversion code is
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needed.
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14. Use the array operator [] even for pointers.
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When addressing an array via a pointer, don't use the plus and
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dereference operators, but the array operator. This will generate
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better code in some common cases.
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Don't use
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char* a;
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char b, c;
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char b = *(a + c);
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Use
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char* a;
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char b, c;
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char b = a[c];
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instead.
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15. Use register variables with care.
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Register variables may give faster and shorter code, but they do also
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have an overhead. Register variables are actually zero page
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locations, so using them saves roughly one cycle per access. Since
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the old values have to be saved and restored, there is an overhead of
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about 70 cycles per 2 byte variable. It is easy to see, that - apart
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from the additional code that is needed to save and restore the
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values - you need to make heavy use of a variable to justify the
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overhead.
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An exception are pointers, especially char pointers. The optimizer
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has code to detect and transform the most common pointer operations
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if the pointer variable is a register variable. Declaring heavily
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used character pointers as register may give significant gains in
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speed and size.
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And remember: Register variables must be enabled with -Or.
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16. Decimal constants greater than 0x7FFF are actually long ints
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The language rules for constant numeric values specify that decimal
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constants without a type suffix that are not in integer range must be
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of type long int or unsigned long int. This means that a simple
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constant like 40000 is of type long int, and may cause an expression
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to be evaluated with 32 bits.
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An example is:
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unsigned val;
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...
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if (val < 65535) {
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...
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}
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Here, the compare is evaluated using 32 bit precision. This makes the
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code larger and a lot slower.
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Using
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unsigned val;
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...
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if (val < 0xFFFF) {
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...
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}
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or
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unsigned val;
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...
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if (val < 65535U) {
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...
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}
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instead will give shorter and faster code.
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