Level 3 optimization includes level 2 optimizations, plus
full optimization across all subprograms within a single file. Level
3 also inlines certain subprograms within the input file. Use +O3
to get level 3 optimization.
Level 3 optimization produces faster run-time code than level
2 on code that frequently calls small functions within a file. Level
3 links faster than level 4.
Inlining within a Single Source File |
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Inlining substitutes functions calls with copies of the function's
object code. Only functions that meet the optimizer's criteria are
inlined. This may result in slightly larger executable files. However,
this increase in size is offset by the elimination of time-consuming
procedure calls and procedure returns.
Example of Inlining |
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The following is an example of inlining at the source code
level. Before inlining, the source file looks like this:
/* Return the greatest common divisor of two positive integers, */
/* int1 and int2, computed using Euclid's algorithm. (Return 0 */
/* if either is not positive.) */
int gcd(int1,int2)
int int1;
int int2;
{
int inttemp;
if ( ( int1 <= 0 ) || ( int2 <= 0 ) ) {
return(0);
}
do {
if ( int1 < int2 ) {
inttemp = int1;
int1 = int2;
int2 = inttemp;
}
int1 = int1 - int2;
} while (int1 > 0);
return(int2);
}
main()
{
int xval,yval,gcdxy;
/* statements before call to gcd */
gcdxy = gcd(xval,yval);
/* statements after call to gcd */
} |
After inlining, the source file looks like this:
main()
{
int xval,yval,gcdxy;
/* statements before inlined version of gcd */
{
int int1;
int int2;
int1 = xval;
int2 = yval;
{
int inttemp;
if ( ( int1 <= 0 ) || ( int2 <= 0 ) ) {
gcdxy = ( 0 );
goto AA003;
}
do {
if ( int1 < int2 ) {
inttemp = int1;
int1 = int2;
int2 = inttemp;
}
int1 = int1 - int2;
} while ( int1 > 0 );
gcdxy = ( int2 );
}
}
AA003 : ;
/* statements after inlined version of gcd */
} |
