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

position-independent code has a performance overhead on most architecture, because it requires an extra register.

So, this is for performance purpose.

Answer 2

Yes there are performance reasons. Some accesses are effectively under another layer of indirection to get the absolute position in memory.

There is also the GOT (Global offset table) which stores offsets of global variables. To me, this just looks like an IAT fixup table, which is classified as position dependent by wikipedia and a few other sources.

http://en.wikipedia.org/wiki/Position_independent_code

Answer 3

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.

Answer 4

Nowadays operating system and compiler by default make all the code as position independent code. Try compiling without the -fPIC flag, the code will compile fine but you will just get a warning.OS's like windows use a technique called as memory mapping to achieve this.

Answer 5

Also, virtual memory hardware in most modern processors (used by most modern OSes) means that lots of code (all user space apps, barring quirky use of mmap or the like) doesn't need to be position independent. Every program gets its own address space which it thinks starts at zero.

Answer 6

This article explains how PIC works and compares it to the alternative - load time relocation. I think it's relevant to your question.