Register

Modifier

The register modifier implies that the corresponding variables of the compiler will be used frequently. If possible, they should be stored in the CPU's registers to speed up their storage. For example, the following memory block copy code,

/* Procedure for the assignment of structures, */

/* if the C compiler doesn't support this feature */

#ifdef NOSTRUCTASSIGN

memcpy (d, s, i)

{register char *d;

register char *s;

register int i;

while (i--)

*d++ = *s++;

}

#endif

However, there are several restrictions on using the register modifier.

First, the register variable must be of a type that can be accepted by the CPU. This usually means that the register variable must be a single value, and the length should be less than or equal to the length of the integer. However, some machine registers can also store floating-point numbers.

Second, because the register variable may not be stored in memory, you cannot use "&" to get the address of the register variable.

Because the number of registers is limited, and some registers can only accept certain types of data (such as pointers and floating-point numbers), the number and type of register modifiers that really work depends on the machine running the program, and any redundant register Modifiers will be ignored by the compiler.

In some cases, saving variables in registers will actually slow down the program. Because the occupied register can no longer be used for other purposes; or the variable is not used many times, not enough to load and store the additional overhead caused by the variable.

Early C compilers did not save variables in registers unless you ordered it to do so, then the register modifier is a valuable addition to the C language. However, with advances in compiler programming technology, the C compilation environment can make better decisions than programmers when deciding which variables should be stored in registers. In fact, many compilers ignore the register modifier, because although it is completely legal, it is only a hint, not a command.