Register keyword

Preface

Compiler optimization is largely through data flow analysis 、 Adjust the order of reading memory , Reduce CPU Read and write to memory . Because the running speed of memory is relative to CPU Is very slow , Add cache Just to solve it CPU And memory running speed is too different . In terms of running speed ： register >cache> Memory > External storage ; In terms of price, it's just the opposite ： register >cache> Memory > External storage . This is also register and cache Is the fastest to read , But in the computer, registers and cache The reason why the capacity of is very small , You can buy 1T But I can't buy it 1T Memory module of , The price is too high , I can't afford it . The number and function of registers are Soc It's fixed , It cannot be expanded later , Register resources are scarce , Be careful with register operation .

register The role of keywords

register： This keyword asks the compiler to make variables exist as much as possible CPU Internal registers instead of memory addressing access to improve efficiency . Note that as much as possible , Not absolute . Do you think , One CPU There are only a few or dozens of registers , If you define a lot register Variable , It's too tired to put all these variables into registers , It may not be your turn .

register Keyword application scenarios

When a variable is called frequently , It can be used register To modify variables , Save variables in registers , It can improve the speed of accessing variables .

register Key words

1.register Variables must be able to be CPU The type accepted . Variables are stored in registers , That means that the degree of variables cannot exceed the number of bits of the register and the type should be consistent with the data type accepted by the register . General situation register The decorated variable must be a single value , And the length should be less than or equal to the length of an integer . If the register allows floating point numbers , that register Decorated variables can also be floating-point numbers .

2. because register Variables may not be stored in memory , So it can't be used “&” To get register The address of the variable . use register Decorated variables are not necessarily stored in registers , use register Decoration just suggests saving the variable to a register , It's not mandatory .

3.register Keywords can only be used for local variables （ That is, the variables in the code block ）, Cannot be used to decorate global variables .

4. Local static variables cannot be defined as register variables . Can not write ：register static int a, b, c. I guess the reason is ： The life cycle of static variables is the same as that of the whole program , You can't always allocate a register to be occupied by a variable , Register resources are very valuable , It's too wasteful .

5. Cannot define any number of register variables , The reason is simple Soc There are too few registers in . Even if you use register Embellishments are not necessarily placed in registers , Let alone define multiple . Personally feel , These special keywords , Don't use it without full confidence , Now the compiler is very intelligent , The compiler knows how to optimize the code . For ordinary people like me , Know the meaning of these keywords , See Linux Kernel code , Just know what these keyword modifiers do .