Writing efficient and simple C language code, many software engineers are pursuing. This article is for the programming work of some of the experience and experience to do the relevant elaboration. The first trick: to space for time Computer program the greatest paradox in space and time the contradiction, then, from the perspective of reverse thinking to consider the efficiency of the program problems, we have to solve the problem of 1 trick-in the space for time. For instance the string assignment: Method A: The usual approach | #define LEN 3 2 char string1 [LEN]; memset (string1,0,LEN); strcpy (string1,"This is a example!!"）; --- Method B: const char string2[LEN] ="This is a example!"; char * cp; cp = string2 ; --- Use when you can directly use the pointer to operate. From the above examples it can be seen, A and B the efficiency is not comparable. In the same storage space, B directly using the pointer can be operated, and A need to call the two characters function to complete. B the disadvantage is that flexibility is not A good. In the need to frequently change a string content, A has A better flexibility; if using method B, you'll need to pre-stored number strings, although takes up a lot of memory, but to get the program execution efficiency. If the system is of high real-time requirements, memory there are some, then I recommend you use the tricks. The tricks of becomes trick-use a macro function instead of a function. For example as follows: Method C: #define bwMCDR2_ADDRESS 4 #define bsMCDR2_ADDRESS 1 7 int BIT_MASK(int __bf) { return ((1U << (bw ## __bf)) - 1)<< (bs ## __bf); } void SET_BITS(int __dst, int __bf, int __val) { __dst = ((__dst) & ~(BIT_MASK(__bf))) | \ (((__val) << (bs ## __bf)) & (BIT_MASK(__bf)))) } SET_BITS(MCDR2, MCDR2_ADDRESS,ReGISterNumber); --- Method D: #define bwMCDR2_ADDRESS 4 #define bsMCDR2_ADDRESS 1 7 #define bmMCDR2_ADDRESS BIT_MASK(MCDR2_ADDRESS) #define BIT_MASK(__bf) (((1U << (bw ## __bf)) - 1) << (bs ## __bf)) #define SET_BITS(__dst, __bf, __val) \ ((__dst) = ((__dst) & ~(BIT_MASK(__bf))) | \ (((__val) << (bs ## __bf)) & (BIT_MASK(__bf)))) SET_BITS(MCDR2, MCDR2_ADDRESS, RegisterNumber); --- Function and macro function of the difference, the macro function takes up a lot of space, and the function takes up time. We want to know is, the function call is to use the system stack to save the data, if the compiler has stack checking option, generally in the function of the head will be embedded in some of the compilation of statements on the current stack inspection; the same time, the CPU also in function calls save and restore the current scene, press the stack and flicking the stack to operate, so, the function call requires some CPU time. And the macro function does not have this problem. The macro function only as pre-written code embedded into the current program, does not produce the function call, it is just taking up space, in Frequent calls to the same macro function of time, the phenomenon is particularly prominent. D method is I see the best of the set bit operation functions, is ARM the company source part, in three short rows to achieve a lot of features, covering almost all of the bit operation function. C method is a variant thereof, wherein the taste also need everyone carefully experience. Second trick: a mathematical method to solve the problem Now we deduce efficient C language prepared by the second trick-the use of mathematical methods to solve problems. Math is the computer of the Mother, there is no mathematical basis and Foundation, it is not the development of computers, so writing a program, using some mathematical methods for the implementation of the program efficiency with orders of magnitude improved. The example below, require 1 to 1 0 0 and. Method E: int I , j; for (I = 1 ;I<=1 0 0; I ++） { j += I; } --- Method F int I; I= (1 0 0 * (1+1 0 0)) / 2 --- This example is my impression the deepest of a mathematical case, is my computer teacher test me. I was only third grade, but I didn't know with the equation N×（N+1）/ 2 to solve this problem. Method E cycle 1 0 0 times before solving the problem, that is to say the least with the 1 0 0 Value 1 0 0 judge, 2 0 0 a adder I and j; method F using only 1 addition, 1 multiplication, 1 division. The effect of natural self-evident. So now I'm making the program, more brains to find patterns that maximize the mathematical power to improve the program running efficiency. Third trick: use bit operation To achieve efficient C language to write the third trick--using bit operations. Reduce the division and modulo operations. In the computer program data bit is to be operation of the minimum data unit, in theory you can use the"bit computing"to complete all of the arithmetic and operation. General bit manipulation is used to control the hardware, or do the data conversion to use, however, flexible bit manipulation can effectively improve the program running efficiency. For example as follows: Method G int I,J; I = 2 5 7 /8; J = 4 5 6% a 3 2; --- Method H int I,J; I = 2 5 7 >>3; J= 4 5 6 - (4 5 6 >> 4 << 4); --- In literally something like H than G trouble a lot, but, look carefully at the generated Assembly code will understand, the method G calls the basic modulo function and division function, both the function call, there is a lot of Assembly code and registers involved in the operation; and the method H is only a few words related to the compilation, the code is more concise, more efficient. Of course, since the compiler is different, the possible efficiency of the gap is not, however, in my current experience of MS C ,the ARM C of view, the efficiency gap is still not small. The relevant Assembly code is not listed here. Use this trick to note is that because the CPU is different. For example, on the PC use this trick to write a program, and in the PC, the Debug passed, transplant to a 1 6-bit machine on the platform, might produce code risk. So only in a certain Technology Advanced the basis of the following before you can use this trick. Fourth trick: to compile embedded Efficient C programming language Nirvana, fourth strokes--the embedded compilation." In the familiar language of the human eye, the C language program is garbage." This argument, although extreme, but it is the truth. Assembly language is the most efficient computer language, however, it is impossible to rely on it to write one[operating system]()? So, in order to get the program efficient, we had to use a workaround approach-embed the compilation of mixed programming. The example below, the array A is assigned to the array di,the requirements of each byte are consistent. char string1[1 0 2 4],string2[1 0 2 4]; The method I int I; for (I =0 ;I<1 0 2 4;I++) *(string2 + I) = *(string1 + I) --- Method J #ifdef _PC_ int I; for (I =0 ;I<1 0 2 4;I++) *(string2 + I) = *(string1 + I); #else #ifdef _ARM_ __asm { MOV R0,string1 MOV R1,string2 MOV R2,#0 loop: WHEN R0!, [R3-R11] STMIA R1!, [R3-R11] ADD R2,R2,#8 CMP R2, #4 0 0 BNE loop } #endif --- Method I is the most common method, using a 1 0 2 4 cycles; method J then depending on the platform made a distinction, in ARM platform, the embedded compiled only with 1 2 8 cycles to complete the same operation. Here are friends would say, Why not use a standard memory Copy Function? This is because the source data may contain data of 0 bytes, in this case, the standard library function will be in advance of the end and will not complete our requirements for the operation. This routine is typically applied to the LCD data copy process. According to different CPU, skilled use of the corresponding embedded compilation, can greatly improve the program execution efficiency. Although it is Nirvana, but if used lightly will pay a heavy price. This is because, using the embedded assembler, it limits the program's portability, so that programs on different platforms porting the process, crouching, treacherous it! While the tricks also with modern software engineering ideas contrary toOnly in compelling circumstances only may be employed.

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