A function is called
pure function
if it always returns the same result for same argument values and it
has no side effects like modifying an argument (or global variable) or
outputting something.
The only result of
calling a pure function is the return value. Examples of pure functions
are strlen(), pow(), sqrt() etc. Examples of impure functions are
printf(), rand(), time(), etc.
If a function is known as pure to compiler then
Loop optimization and
subexpression elimination can be applied to it. In GCC, we can mark functions as pure using the “pure” attribute.
__attribute__ ((pure)) return-type fun-name(arguments1, …)
{
/* function body */
}
Following is an example pure function that returns square of a passed integer.
__attribute__ _((pure)) int my_square(int val)
{
return val*val;
}Consider the below example
for (len = 0; len < strlen(str); ++len)
printf("%c", toupper(str[len]));
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If “strlen()” function is not marked as pure function then compiler
will invoke the “strlen()” function with each iteration of the loop, and
if function is marked as pure function then compiler knows that value
of “strlen()” function will be same for each call, that’s why compiler
optimizes the for loop and generates code like following.
int len = strlen(str);
for (i = 0; i < len; ++i)
printf("%c", toupper((str[i]));
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