Most of today's 64-bit platforms are capable of running both 32-bit and 64-bit applications. This asks for some clarifications on the advantages and drawbacks of 64-bit addressing for (SWI-)Prolog.
SWI-Prolog can be compiled for
a 32- or 64-bit address space on any system with a suitable C compiler.
Pointer arithmetic is based on the type (u)intptr_t from stdint.h,
with suitable emulation on MS-Windows.
Most of Prolog's memory usage consists of pointers. This indicates the primary drawback: Prolog memory usage almost doubles when using the 64-bit addressing model. Using more memory means copying more data between CPU and main memory, slowing down the system.
What then are the advantages? First of all, SWI-Prolog's addressing of the Prolog stacks does not cover the whole address space due to the use of type tag bits and garbage collection flags. On 32-bit hardware the stacks are limited to 128 MB each. This tends to be too low for demanding applications on modern hardware. On 64-bit hardware the limit is 2^32 times higher, exceeding the addressing capabilities of today's CPUs and operating systems. This implies Prolog can be started with stack sizes that use the full capabilities of your hardware.
Multi-threaded applications profit much more because every thread has its own set of stacks. The Prolog stacks start small and are dynamically expanded (see section 2.19.1). The C stack is also dynamically expanded, but the maximum size is reserved when a thread is started. Using 100 threads at the maximum default C stack of 8Mb (Linux) costs 800Mb virtual memory!24C-recursion over Prolog data structures is removed from most of SWI-Prolog. When removed from all predicates it will often be possible to use lower limits in threads. See http://www.swi-prolog.org/Devel/CStack.html
The implications of theoretical performance loss due to increased memory bandwidth implied by exchanging wider pointers depend on the design of the hardware. We only have data for the popular IA32 vs. AMD64 architectures. Here, it appears that the loss is compensated for by an instruction set that has been optimized for modern programming. In particular, the AMD64 has more registers and the relative addressing capabilities have been improved. Where we see a 10% performance degradation when placing the SWI-Prolog kernel in a Unix shared object, we cannot find a measurable difference on AMD64.
For those cases where we can choose between 32 and 64 bits, either because the hardware and OS support both or because we can still choose the hardware and OS, we give guidelines for this decision.
First of all, if SWI-Prolog needs to be linked against 32- or 64-bit native libraries, there is no choice as it is not possible to link 32- and 64-bit code into a single executable. Only if all required libraries are available in both sizes and there is no clear reason to use either do the different characteristics of Prolog become important.
Prolog applications that require more than the 128 MB stack limit provided in 32-bit addressing mode must use the 64-bit edition. Note however that the limits must be doubled to accommodate the same Prolog application.
If the system is tight on physical memory, 32-bit Prolog has the clear advantage of using only slightly more than half of the memory of 64-bit Prolog. This argument applies as long as the application fits in the virtual address space of the machine. The virtual address space of 32-bit hardware is 4GB, but in many cases the operating system provides less to user applications.
The only standard SWI-Prolog library adding significantly to this calculation is the RDF database provided by the semweb package. It uses approximately 80 bytes per triple on 32-bit hardware and 150 bytes on 64-bit hardware. Details depend on how many different resources and literals appear in the dataset as well as desired additional literal indexes.
Summarizing, if applications are small enough to fit comfortably in virtual and physical memory, simply take the model used by most of the applications on the OS. If applications require more than 128 MB per stack, use the 64-bit edition. If applications approach the size of physical memory, fit in the 128 MB stack limit and fit in virtual memory, the 32-bit version has clear advantages. For demanding applications on 64-bit hardware with more than about 6GB physical memory the 64-bit model is the model of choice.