double precision calculations in single precision code

Branko Drevenšek
Sat Oct 27 19:40:00 GMT 2012


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);
     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

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