A Friday 22 May 2009 11:55:31 Gregor Thalhammer escrigué: > dmitrey schrieb: > > Hi all, > > I expected to have some speedup via using ldexp or multiplying an > > array by a power of 2 (doesn't it have to perform a simple shift of > > mantissa?), but I don't see the one. > > > > # Let me also note - > > # 1) using b = 2 * ones(N) or b = zeros(N) doesn't yield any speedup > > vs b = rand() > > # 2) using A * 2.0 (or mere 2) instead of 2.1 doesn't yield any > > speedup, despite it is exact integer power of 2. > > On recent processors multiplication is very fast and takes 1.5 clock > cycles (float, double precision), independent of the values. There is > very little gain by using bit shift operators.
...unless you use the vectorization capabilities of modern Intel-compatible processors and shift data in bunches of up to 4 elements (i.e. the number of floats that fits on a 128-bit SSE2 register), in which case you can perform operations up to a speed of 0.25 cycles/element. Indeed, that requires dealing with SSE2 instructions in your code, but using latest GCC, ICC or MSVC implementations, this is not that difficult. Cheers, -- Francesc Alted _______________________________________________ Numpy-discussion mailing list Numpy-discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
