When AMD launched its now widely used 64-bit architecture in year 2000, it started marketing it as a significant step forward because of it having longer word and pointer size. One just has to check the advertising material and notice those big “64″ being touted as the main improvement.
But it is commonly accepted that most applications don’t need a 64-bit address space for anything. Building them in LP64 model is just a waste of memory due to increase of pointer size. Even though this made most applications lag behind, the new architecture still was an improvement in terms of speed because of the AMD revised ISA, featuring changes like:
- The program counter register (%rip) can be accessed directly. This means that PIC (position-independent code) can be implemented sanely without doing strange, inefficient gimmicks (on i386, one had to perform a dummy “call” and retrieve the top of the stack inmediately afterwards).
- Add 8 new general-purpose registers to the first set of 8 registers, most of which were either claimed as implicit argument in some instructions or claimed by the ABI, and as such not really “general purpose”.
To summarize, most of the merit from AMD64 architecture was in fixing some of the insanity of Intel 386 ISA (instruction set architecture), rather than the increased pointer size which was a source of inefficiency most of the time.
Perhaps AMD didn’t evaluate this correctly, or perhaps its marketing side won over technical merit. However, although the new hardware is biased towards LP64 data model, it’s not actually enforced. It was a matter of time until an independent project took over and attempted to fix this, combining the AMD64 ISA with ILP32 data model.
I’ve been reading with much interest in the binutils and gcc mailing lists that such project is beginning to take shape. A port of binutils, GCC, GDB and Linux is already available. Future plans include porting Glibc which will make it possible to build a standard GNU derivative out of this.
Sadly, I don’t have the time to devote to this project myself, but I’ll continue following its progress. I’m looking forward to bringing this speed boost to my machines.