Changing pointer bitwidth (64 or 32 bits)

Most modern platforms allow for compiling libraries to handle either 32 or 64 bit address spaces. On the x86, this selection strongly affects the ISA used (eg., whether to use IA32 or x86-64). The x86-64 ISA, with 16 rather than 8 registers, is more amenable to optimization than the IA32, so if the user has no preference, 64-bit pointers are recommended. If ATLAS's guess is not correct, you can tell configure what address space to build for. In order to force 32-bit pointer width, pass the flag:

   -b 32

and in order to force 64 bit pointers, pass:

   -b 64

(the b stands for bitwidth).

This tells ATLAS to throw the appropriate compiler flags for compilers it knows about, as well as effecting various configure probes. Therefore, if you override ATLAS's compiler choices, be sure that you give the correct flags to match this setting.