A brief discussion on literal pools in ARM assembly

      Chinese description:      

               The essence of the literal pool is a memory block in the ARM assembly language code section that is used to store constant data rather than executable code.

       English description:

              Literal pools are areas of constant data in a code section.

            (From ARM official document "ARM Compiler armcc User Guide")

2) Reasons for using literal pool

       When you want to use a 4-byte constant data in an instruction (this data can be a memory address or a numeric constant)

       Since the ARM instruction set is fixed-length (ARM instruction 4 bytes or Thumb instruction 2 bytes), it is impossible to convert this 4-byte

       The constant data is encoded in a compiled instruction. At this time, the ARM compiler (compiles the C source program)/assembler (compiles the assembly program)

       A block of memory will be allocated in the code section and the 4-byte data constant will be saved there. Then, an instruction will be used to store the 4-byte data constant.

       Byte numeric constants are loaded into registers for operation.

       In C source code, the allocation of literal pools is arranged by the compiler at compile time. When designing assembly programs, developers can

       The literal pool is allocated by the developer. If the developer does not arrange the literal pool, the assembler will do it for you.

3) Practical examples:

       Figure 1 is a specific example of the compiler automatically allocating a literal pool when compiling a C function.

figure 1

   4) Alternatives to literal pools

          The literal pool is not necessary. FIG. 2 is a specific example of using MOVW and MOVT to replace the literal pool.

figure 2

5) Principles for using the text pool

       In most cases, using a literal pool can improve program performance, but in certain cases, using a literal pool (that is, using a literal pool) can improve program performance.

       The most ideal alternative is