I did some timing tests to compare the new PowerPC IEEE 128-bit results to the current implementation of long double using the IBM extended format.
The test consisted a short loop doing the operation over arrays of 1,024 elements, reading in two values, doing the operation, and then storing it back. This loop in turn was done multiple times, with the idea that most of the values would be in the cache, and we didn't have to worry about pre-fetching, etc. The float, double tests were done with vectorization disabled, while the vector float and vector double tests, the compiler was allowed to do the normal auto vectorization. The number reported was how much longer the second column took over the first: Generally, the __float128 is 2x slower than the current IBM extended double format, except for divide, where it is 5x slower. I must say, the software floating point emulation routines worked well, and once the proper macros were setup, I only needed to override the type used for IEEE 128-bit. Add loop ======== float vs double: 2.00x float vs vector float: 4.97x double vs vector double: 2.63x long double vs double: 16.85x __float128 vs double: 23.34x __float128 vs long double: 1.39x Subtract loop ============= float vs double: 1.99x float vs vector float: 4.66x double vs vector double: 2.63x long double vs double: 14.47x __float128 vs double: 27.65x __float128 vs long double: 1.91x Multiply loop ============= float vs double: 2.05x float vs vector float: 5.18x double vs vector double: 2.59x long double vs double: 11.58x __float128 vs double: 27.44x __float128 vs long double: 2.37x Divide loop =========== float vs double: 0.82x float vs vector float: 2.11x double vs vector double: 2.00x long double vs double: 5.90x __float128 vs double: 34.57x __float128 vs long double: 5.86x Maximum via comparison and ?: ============================= float vs double: 1.74x float vs vector float: 4.62x double vs vector double: 2.62x long double vs double: 5.07x __float128 vs double: 18.02x __float128 vs long double: 3.55x Minimum via comparison and ?: ============================= float vs double: 1.74x float vs vector float: 4.52x double vs vector double: 2.62x long double vs double: 5.38x __float128 vs double: 15.14x __float128 vs long double: 2.82x -- Michael Meissner, IBM IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA email: meiss...@linux.vnet.ibm.com, phone: +1 (978) 899-4797
