C51 Program Optimization

　　1. Pointer:

　　For most compilers, the code generated by using pointers is shorter and more efficient than that generated by using arrays. However, in Keil, the opposite is true. The code generated by using arrays is shorter than that generated by using pointers. Usually, the use of self-increment and self-decrement instructions and compound assignment expressions (such as a-=1 and a+=1, etc.) can generate high-quality program code. The compiler can usually generate instructions such as inc and dec. When using instructions such as a=a+1 or a=a-1, many C compilers will generate two to three bytes of instructions.

　　2. Remainder operation:

　　a=a%8;　　-->　　a=a&7;

　　Note: Bit operations can be completed in just one instruction cycle, while most C compilers use subroutines to complete the "%" operation, which results in long code and slow execution. Usually, if you only need to find the remainder of a square of 2n, you can use bit operations instead.

　　3. Square operation:

     a=pow(a,2.0);　　-->　　a=a*a;

　　Note: In microcontrollers with built-in hardware multipliers (such as the 51 series), multiplication is much faster than squaring because the squaring of floating-point numbers is achieved by calling a subroutine. In AVR microcontrollers with built-in hardware multipliers, such as ATMega163, multiplication can be completed in just 2 clock cycles. Even in AVR microcontrollers without built-in hardware multipliers, the subroutine for multiplication is shorter in code and faster in execution than the subroutine for squaring.

　　If we are looking for the cube, such as: a=pow(a,3.0); --> a=a*a*a; the improvement in efficiency is more obvious.

　　4. Multiplication and division:

　　a=a*4;　　-->　　a=a<<2;

　　b=b/4;　　-->　　b=b>>2;

In fact, as long as you multiply or divide by an integer, you can use the shift method to get the result, such as:

     a=a*9　　-->　　a=(a<<3)+a

　　5. Delay function:

　　The commonly used delay functions are in the form of self-increment:

void delay (void)

{

    unsigned int i;

    for (i=0;i<1000;i++)

    ;

}  

　　Change it to a self-decrementing delay function

void delay (void)

{

    unsigned int i;

    for (i=0;i<1000;i--)

    ;

}   

　　The delay effects of the two functions are similar, but the code generated by almost all C compilers for the latter function is 1 to 3 bytes less than the former code, because almost all MCUs have instructions for transferring to 0, and the latter method can generate such instructions.

　　6. While loop and do...while loop

　　Of the two loops, the length of the code generated by the do...while loop after compilation is shorter than that of the while loop. To write good C language, beautiful macro definitions are very important. Using macro definitions can prevent errors, improve portability, readability, convenience, etc.