On Wed, Sep 27, 2017 at 03:09:48PM +0200, Joerg Sonnenberger wrote:
> On Wed, Sep 27, 2017 at 08:40:26AM +0200, Alexandre Ratchov wrote:
> > Even on modern amd64s integer arithmetics and bitwise operations are
> > faster (and more precise in many cases) than floating point
> > equivalents.
>
> Can you actually substanciate this claim?
[OT]
I'm working on typical signal processing code doing mostly
multiplications, additions and table lookups. For instance, when we do
the following with floats:
float a, b, x, y;
y = a * x + b;
we get 24 bits of precision as floats have 24-bit mantissa. If, with
proper typedefs and #ifdefery, we replace floats by ints representing
the same numbers as 1:4:27 fixed-points:
int a, b, x, y;
y = ((long long)a * x >> 27) + b;
we get 28 bits of precision, i.e. 4 extra bits of precision compared
to floats.
I just did a quick test on a i5, gcc 4.2. The fixed-point version
consumes 10% to 50% less CPU depending on the algorithm. To test, I
just run the algorithm during 10 seconds and measure the throughput.
In the past, I've tested on other CPUs, with other compilers and the
floating point version of this code was never faster than the
fixed-point.
