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:
Changing pointer bitwidth (64 or 32 bits)
and in order to force 64 bit pointers, pass:
(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.
R. Clint Whaley