Why isn't all code compiled position independent?

Question

When compiling shared libraries in gcc the -fPIC option compiles the code as position independent. Is there any reason (performance or otherwise) why you would not compile

Answer 1

Because implementing completely position independent code adds a constraint to the code generator which can prevent the use of faster operations, or add extra steps to preserve that constraint.

This might be an acceptable trade-off to get multiprocessing without a virtual memory system, where you trust processes to not invade each other's memory and might need to load a particular application at any base address.

In many modern systems the performance trade-offs are different, and a relocating loader is often less expensive (it costs any time code is first loaded) than the best an optimizer can do if it has free reign. Also, the availability of virtual address spaces hides most of the motivation for position independence in the first place.