One of the larger (but often overlooked) changes to x64 with respect to x86 is that most instructions that previously only referenced data via absolute addressing can now reference data via RIP-relative addressing.
RIP-relative addressing is a mode where an address reference is provided as a (signed) 32-bit displacement from the current instruction pointer. While this was typically only used on x86 for control transfer instructions (call, jmp, and soforth), x64 expands the use of instruction pointer relative addressing to cover a much larger set of instructions.
What’s the advantage of using RIP-relative addressing? Well, the main benefit is that it becomes much easier to generate position independent code, or code that does not depend on where it is loaded in memory. This is especially useful in today’s world of (relatively) self-contained modules (such as DLLs or EXEs) that contain both data (global variables) and the code that goes along with it. If one used flat addressing on x86, references to global variables typically required hardcoding the absolute address of the global in question, assuming the module loads at its preferred base address. If the module then could not be loaded at the preferred base address at runtime, the loader had to perform a set of base relocations that essentially rewrite all instructions that had an absolute address operand component to refer to take into account the new address of the module.
The loader is hardly capable of figuring out what instructions would need to be rewritten in such a form, instead requiring assistance from the compiler and linker (in terms of the base relocation section of a PE image, for Windows) to provide it with a list of addresses that correspond to instruction operands that need to be modified to reflect the new image base after an image has been relocated.
An instruction that uses RIP relative addressing, however, typically does not require any base relocations (otherwise known as “fixups”) at load time if the module containing it is relocated, however. This is because as long as portions of the module are not internally re-arranged in memory (something not supported by the PE format), any addresses reference that is both relative to the current instruction pointer and refers to a location within the confines of the current image will continue to refer to the correct location, no matter where the image is placed at load time.
As a result, many x64 images have a greatly reduced number of fixups, due to the fact that most operations can be performed in an RIP-relative fashion. For example, the base relocation information (not including alignment padding) on the 64-bit ntdll.dll (for Windows Vista) is a mere 560 bytes total, compared to 18092 bytes in the Wow64 (x86) version.
Fewer fixups also means better memory usage when a binary is relocated, as there is a higher probability that a particular page will not need to be modified by the base relocation process, and thus can still remain shared even if a particular process needs to relocate a particular DLL.