Pics/overhead - angiescreation.com

PIC, or Position-Independent Code, is a crucial concept in modern software development, particularly for shared libraries and operating systems. It refers to code that can be executed correctly regardless of its absolute address in memory. The core principle behind PIC is to eliminate direct references to absolute memory addresses within the code itself.

Without PIC, a shared library would have to be loaded at a fixed address every time it's used. This is impractical because multiple libraries might conflict if they all demand the same memory range. Additionally, it hinders address space layout randomization (ASLR), a security technique that makes it harder for attackers to predict the location of crucial data and code in memory. ASLR is a cornerstone of modern operating system security.

So, how does PIC achieve address independence? It generally relies on a Global Offset Table (GOT). The GOT is a data structure that resides in memory and contains the absolute addresses of global variables and functions used by the PIC code. Instead of directly referencing these addresses, the PIC code references offsets within the GOT.

When the shared library is loaded into memory, the dynamic linker updates the GOT with the actual runtime addresses of the global variables and functions. This "relocation" process only happens once, when the library is loaded. After that, the PIC code can access global data and functions through the GOT without knowing their absolute addresses. This allows the shared library to be loaded at any memory location.

Now, let's discuss the overhead associated with PIC. It's important to acknowledge that PIC does introduce some performance cost, although the impact is usually relatively small on modern hardware. The overhead comes primarily from the extra indirection involved in accessing global data and functions through the GOT. Instead of a single direct memory access, the code first has to access the GOT to retrieve the address, and then access the actual data or function using that address.

This extra indirection can lead to increased instruction count and potentially more cache misses, as the GOT itself needs to be accessed. The performance penalty can vary depending on the architecture and the specific code being executed. Code that heavily relies on global variables and function calls will likely experience a greater impact than code that primarily operates on local data.

However, the benefits of PIC often outweigh the performance costs. The flexibility and security enhancements enabled by PIC, particularly the ability to use ASLR, are often considered more valuable. Furthermore, compilers and linkers have become increasingly sophisticated in optimizing PIC code, reducing the overhead to a minimum. Techniques like lazy binding (only resolving function addresses when they are actually called) can also mitigate the performance impact.

In summary, PIC is a fundamental technique for creating shared libraries that can be loaded at arbitrary addresses, enabling ASLR and preventing memory conflicts. While it introduces some performance overhead due to the indirection of GOT access, the security and flexibility advantages typically justify its use. Modern compilers and linkers continuously strive to minimize the overhead, making PIC a standard practice in most software development environments.