Re: double precision calculations in single precision code

[Craig Howland <howland at LGSInnovations dot com>]

Branko:

I think that the best and easiest thing for you to do is to rebuild to
avoid mathfp by not giving --enable-newlib-hw-fp to configure.  That
is, use libm/math, not libm/mathfp.

You should be OK using libm/math for the build in terms of not having
ay double operations "polluting" float functions.  If there are any,
they ought to be addressed.

Craig

On 10/24/2012 07:30 AM, Branko Drevenšek wrote:
> Hi.
>
> We have Newlib ported to our architecture. One of configurations that we support
> is hardware single precision and soft-float double precision.
>
> I have noticed unexpected double precision soft-float function calls in simple
> code like x = sinf(y), where both x and y are single precision.
>
> I have noticed that Newlib sinef function (libm/mathfp/sf_sine.c) contains lines
> like:
> /* Calculate the exponent. */
> if (y < 0.0)
> N = (int) (y * ONE_OVER_PI - 0.5);
> else
> N = (int) (y * ONE_OVER_PI + 0.5);
>
> While y * ONE_OVER_PI gets calculated in single precision, that +- 0.5 causes
> the result of multiplication being converted to double and then addition is done
> in double using soft-float. For our case this has big performance penalty and
> also requires double soft-float functions to be linked in, causing binary to grow.
>
> Is 0.5 (instead of 0.5f) used by purpose or is that just wrong (and needs
> fixing)? Where did those math. functions originate from?
>
> Best regards,
> Branko Drevensek