Tips to Encourage IPO
x = p
Code that discourages IPO
Use static variables and static functions, and avoid assigning function
addresses or variable addresses to globals. Unless the compiler can
detect the whole program, it has no knowledge about the complete use
of global variables, external functions, or static variables and static
functions whose addresses are taken and assigned to a global variable
or function pointer.
If IPO doesnt inline automatically, use the inline
keyword in C++, and __inline in C.
Avoid passing pointers into a function as a parameter and then assigning
them to a global variable. The code on the right discourages IPO.
x is a global variable and p
is a pointer.
Try to provide the whole program to the compiler when using IPO.
Most of the IPO optimizations are either not triggered or severely
weakened unless the compiler sees the whole program. Although inlining
is an exception to this rule, the compiler can inline better in the
presence of the whole program.
Even in the case of inlining, you should refrain from building libraries
out of frequently called small functions when using IPO.