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Tip of the Day
Language: C++
Expertise: All
Apr 17, 1998

Use Register variables to enhance performance

The C/C++ register keyword can be used as a hint to the compiler that the declared variable is to be accessed very often during program execution, and hence, should be stored on a machine register instead of RAM, in order to enhance performance. A good example is a loop control variable. When not stored in a register, a significant amount of the loop's execution time is dedicated to dealing with fetching the variable from memory, assigning a new value to it, and storing it back in memory over and over again. Storing it in a machine register can improve performance significantly:
 
void my_memset(void * pbuf, size_t bufsize, char assigned_val = `\0') 
{
	char *p = static_cast<char *> (pbuf);
	if  (pbuf  && bufsize)
	{ 
		for (register int i  = 0; i < bufsize; i++) 
			* p++ = assigned_val;
	}
	return; 
};
When using register variables please note:
  1. A register declaration is only a "recommendation" to the compiler, it may be ignored.
  2. The address of a register variable may not be taken.
  3. The register keyword can be used for types other than int. In that case, it serves as a hint to the compiler to store the variable in the fastest memory location (for example, cache memory).
  4. Some compilers ignore the register recommendation and automatically store variables in registers according to a set of built-in optimization rules.
Danny Kalev
 
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