[Numpy-discussion] where are the benefits of ldexp and/or "array times 2"?

[Francesc Alted]

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