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Another possibility for an Opteron compiler is the new Pathscale compiler. We are currently collecting addresses of people who'd like to be in our January beta. General availability is estimated to be March. For details, see http://pathscale.com/products1.html. On Mon, Dec 15, 2003 at 03:11:00PM -0500, Peter Shenkin wrote: > I suspect the only thing a 64-bit compilation will buy > you on Opteron is extra address space, which is good if > you need it. On the Opteron, 64-bit mode also gets twice as many registers, and a better calling sequence. > But we have found on other platforms, such > as IRIX64, that 32-bit executables actually run faster > than 64-bit executables on the 64-bit OS, because of > better cache warmth. On Irix, the equivalent of the Opteron 32-bit mode is "o32", which nobody uses anymore. o32 has fewer registers. n32 got the best of both worlds: the extra registers, but small datatypes. There is a negative effect from datatypes which are widened from 32 to 64 bits, such as C longs and C and Fortran pointers. If your program doesn't use many pointers, then the only cache effect is that the executable code can be a bit bigger, but sometimes it's actually smaller, too... more registers can mean fewer spills, and many fewer instructions. Anyway, to sum up: Fortran programs should almost always see a benefit from the Opteron's 64-bit mode. -- greg