Re: [linux-audio-dev] Denormalisation considered harmful?

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Subject: Re: [linux-audio-dev] Denormalisation considered harmful?
From: Paul Barton-Davis (pbd_AT_Op.Net)
Date: Wed Nov 29 2000 - 15:31:23 EET

>PS For those who don't know, there is a problem when you try to do maths
> with floats that are very small on some chips. The FPU generates an
> interrupt, and normalises the bits of one float so that that maths
> won't loose precision. It's the best way to meet IEEE precision
> requirements aparentnly. Anyway, the interrupt is very expensive, so to
> aviod it people do nasty things like adding a DC offset (erk) or noise
> (Erk!) to the signal, or disabling interrups. There is a better hack
> that fixes it:

I have an even better way that involves basically no CPU cycles. This
is from snd:

-----------------------------------------------------------------------
#ifdef HAVE_FPU_CONTROL_H
  #include <fpu_control.h>
#endif

#ifdef HAVE_FPU_CONTROL_H
  #if __GLIBC_MINOR__ < 1
    /* in linux there's <fpu_control.h> with __setfpucw which Clisp calls as __setfpucw(_FPU_IEEE); */
    /* this appears to be useful in getting rid of idiotic NaN's */
    __setfpucw(_FPU_IEEE);
  #else
    #ifndef __alpha__
      int __fpu_ieee = _FPU_IEEE;
      _FPU_SETCW(__fpu_ieee);
      /* this bugfix thanks to Paul Barton-Davis */
    #else
      int __fpu_ieee = 0;
    #endif
  #endif
#endif
-----------------------------------------------------------------------

this turns off the interrupt for denormals.

--p
--p

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