By Darryl Gove-Oracle on Sep 28, 2009
Just updated the Selecting The Best Compiler Options article for the developer portal. Minor changes, mainly a bit more clarification on floating point optimisations.
Documentation on the AMD64 architecture.
One of the questions people ask is when to develop 64-bit apps vs 32-bit apps. The answer is not totally clear cut, it depends on the application and the platform. So here's my take on it.
First, let's discuss SPARC. The SPARC V8 architecture defines the 32-bit SPARC ISA (Instruction Set Architecture). This was found on a selection of SPARC processors that appeared before the UltraSPARC I, ie quite a long time ago. Later the SPARC V9 architecture appeared. (As an aside, these are open standards, so anyone can download the specs and make one, of course not everyone has the time and resources to do that, but it's nice in theory
The SPARC V9 architecture added a few instructions, but mainly added the capability to use 64-bit addressing. The ABI (Application Binary Interface) was also improved (floating point values passed in FP registers rather than the odd V8 choice of using the integer registers). The UltraSPARC I and onwards have implemented the V9 architecture, which means that they can execute both V8 and V9 binaries.
One of the things that it's easy to get taken in by is the animal farm idea that if 32-bits is good 64-bits must be better. The trouble with 64-bit address space is that it takes more instructions to set up an address, pointers go from 4 bytes to 8 bytes, and the memory footprint of the application increases.
A hybrid mode was also defined, which took the 32-bit address space, together with a selection of the new instructions. This was called v8plus, or more recently sparcvis. This has been the default architecture for SPARC binaries for quite a while now, and it combines the smaller memory footprint of SPARC V8 with the more recent instructions from SPARC V9. For applications that don't require 64-bit address space, v8plus or sparcvis is the way to go.
Moving to the x86 side things are slightly more complex. The high level view is similar. You have the x86 ISA, or IA32 as its been called. Then you have the 64-bit ISA, called AMD64 or EMT64. EMT64 gives you both 64-bit addressing, a new ABI, a number of new instructions, and perhaps most importantly a bundle of new registers. The x86 has impressively few registers, EMT64 fixes that quite nicely.
In the same way as SPARC, moving to a 64-bit address space does cost some performance due to the increased memory footprint. However, the x64 gains a number of additional registers, which usually more than make up for this loss in performance. So the general rule is that 64-bits is better, unless the application makes extensive use of pointers.
Unlike SPARC, EMT64 does not currently provide a mode which gives the instruction set extensions and registers with a 32-bit address space.