PGI User's Guide

Chapter 13. Programming Considerations for 64-Bit Environmentsstack spaceStack space can be a problem for data that is stack-based. In Win64, stack spacecan be increased by using this link-time switch, where N is the desired stack size:–Wl,-stack:NNoteIn linux86-64, stack size is increased in the environment. Settingstacksize to unlimited often is not large enough.page swappingconfigured spacesupport for largeaddress offsets inobject file formatlimit stacksize new_size ! in cshulimit –s new_size ! in bashIf your executable is much larger than the physical size of memory, page swappingcan cause it to run dramatically slower; it may even fail. This is not a compilerproblem. Try smaller data sets to determine whether or not a problem is due topage thrashing.Be sure your linux86-64 system is configured with swap space sufficiently large tosupport the data sets used in your application(s). If your memory+swap space isnot sufficiently large, your application will likely encounter a segmentation fault atruntime.Arrays that are not dynamically allocated are limited by how the compiler canexpress the ‘distance’ between them when generating code. A field in the objectfile stores this ‘distance’ value, which is limited to 32-bits on Win32, Win64,linux86, and linux86-64 with –mcmodel=small. It is 64-bits on linux86-64 with –mcmodel=medium.NoteWithout the 64-bit offset support in the object file format, large arrayscannot be declared statically or locally stack-based.Medium Memory Model and Large Array in CConsider the following example, where the aggregate size of the arrays exceeds 2GB.% cat bigadd.#include #define SIZE 600000000 /* > 2GB/4 */static float a[SIZE], b[SIZE];intmain(){long long i, n, m;float c[SIZE]; /* goes on stack */n = SIZE;m = 0;for (i = 0; i < n; i += 10000) {a[i] = i + 1;b[i] = 2.0 * (i + 1);c[i] = a[i] + b[i];Example 13.1. Medium Memory Model and Large Array in C171