1, choose the appropriate algorithm and data structure
Should be familiar with the algorithm language, know the advantages and disadvantages of various algorithms, specific information, please refer to the appropriate reference materials, there are many computer books are introduced. The slower sequential search method is replaced by a faster binary search or out-of-order search. Insertion sorting or bubble sorting is replaced by quick sorting, merge sorting, or root sorting, which can greatly improve the efficiency of program execution. It is also important to choose a suitable data structure. For example, if you use a lot of insert and delete instructions in a bunch of randomly-stored numbers, then using a linked list is much faster.
Arrays and pointers have very strong password relationships. In general, pointers are more flexible and concise, while arrays are more intuitive and easier to understand. For most compilers, using pointers is shorter than using array-generated code, which is more efficient. But in Keil, on the other hand, using arrays produces shorter code than using pointers.
2, use the smallest possible data type
You can use a variable defined by a char, do not define it with an int variable, a variable defined with an integer variable, you can't use a long int, and you can't use a float. Do not use floating-point variables for variables. Of course, do not exceed the scope of the variable after defining the variable. If you exceed the scope of the variable assignment, the C compiler does not report an error, but the program execution result is wrong, and such an error is difficult to find.
In ICCAVR, you can set the use of printf parameters in Options, try to use the basic parameters (% c,% d,% x,% X,% u, and % s format specifiers), use less long integer parameters (% The ld, %lu, %lx, and %lX format specifiers), as far as the floating-point parameter (%f) is concerned, should be used as far as possible, as are other C compilers. Using other parameters, the %f parameter will increase the amount of code generated and reduce the execution speed.
3, use self-increment and decrement instructions
Normally, self-incrementing and decrementing instructions and compound assignment expressions (such as a-=1 and a+=1) can generate high-quality program code. Compilers can usually generate instructions such as inc and dec and use For instructions such as a=a+1 or a=a-1, many C compilers generate two to three byte instructions. In the AVR single-chip ICCAVR, GCCAVR, IAR and other C compilers generate the same code in several ways, but also can generate high-quality code such as inc and dec.
4, I reduce the intensity of the operation, I feel deeply
You can replace the original complex expression with an expression with a small amount of computation but the same function. as follows:
(1) Calculate remainder.
a=a%8;
Can be changed to:
a=a&7;
Note: The bit operation can be completed with only one instruction cycle, and most of the C compiler's "%" operations are called by subroutines. The code length is long and the execution speed is slow. In general, only the remainder of 2n is required, and the bit operation method can be used instead.
(2) Square operation
a=pow(a,2.0);
Can be changed to:
a=a*a;
Note: In microcontrollers with built-in hardware multipliers (such as the 51 series), multiplication operations are much faster than squaring operations, because the square of floating-point numbers is achieved by calling subroutines, and AVRs with hardware multipliers In the microcontroller, such as ATMega163, multiplication can be completed in only 2 clock cycles. Even in AVR MCUs without built-in hardware multipliers, the subroutines for multiplication are shorter than the subroutine code for squaring operations and perform faster.
If it is for the third power, such as:
a=pow(a,3.0);
change to:
a=a*a*a;
The improvement in efficiency is even more pronounced.
(3) Using shift to implement multiplication and division
a=a*4;
b=b/4;
Can be changed to:
a=a<<2;
b=b>>2;
Note: Usually if you need to multiply or divide by 2n, you can use the shift method instead. In ICCAVR, if it is multiplied by 2n, it can generate the left-shifted code and multiply it by other integers or divide by any number to call the multiplication and division subroutine. It is more efficient to use the shift method to get the code than to call the multiplication and subroutine. In fact, as long as it is multiplied by or divided by an integer, you can use the shift method to get the result, such as:
a=a*9
Can be changed to:
a=(a<<3)+a
5, cycle
(1), circulation language
For some tasks that do not need to participate in the operation of the loop variables can be put outside the loop, the tasks here include expressions, function calls, pointer operations, array access, etc., should not need to perform multiple operations all together, Put it into an init initialization routine.
(2) Delay function:
The commonly used delay functions are self-adding:
Void delay (void)
{
Unsigned int i;
For (i=0;i<1000;i++)
;
}
Change it to self-delayed function:
Void delay (void)
{
Unsigned int i;
For (i=1000;i>0;i--)
;
}
The delay effects of the two functions are similar, but almost all C compilers generate code that is 1 to 3 bytes shorter than the previous code, because almost all MCUs have zero-shifting instructions. The latter method can be used to generate such instructions.
The same is true when using while loops. Control loops using the decrement instruction will have 1 to 3 fewer letters than the code generated using the auto-instruction control loop.
However, when there is an instruction that reads or writes an array through the loop variable "i" in the loop, it may cause the array to be out of bounds when using the pre-reduction loop.
(3) while loop and do...while loop
There are two types of loops when using the while loop:
Unsigned int i;
i=0;
While (i<1000)
{
i++;
//User program
}
or:
Unsigned int i;
i=1000;
Do
I--;
//User program
While (i>0);
In both loops, the code generated after compiling with the do...while loop is shorter than the while loop.
6, check the table
In the program, generally do not perform very complex operations, such as the multiplication and division of floating-point numbers and square root, as well as some complex mathematical model of interpolation operations, these operations that consume time and consume resources, should try to use the look-up table And place the data table in the program memory area. If it is more difficult to directly generate the required tables, it is also as simple as possible to reduce the amount of repetitive calculations during program execution.
7, other
For example, using on-line assembly and saving strings and some constants in program memory are all conducive to optimization.
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