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Re: [avr-libc-dev] Bug in math library
From: |
Marek Michalkiewicz |
Subject: |
Re: [avr-libc-dev] Bug in math library |
Date: |
Sat, 8 Jan 2005 22:17:43 +0100 |
User-agent: |
Mutt/1.5.6+20040907i |
Hi,
On Sat, Jan 08, 2005 at 03:47:34PM -0400, Colin O'Flynn wrote:
> At any rate the problem lies in the fp_split.S file. The file says this:
>
> CLR rBE ; clear fraction extension
> cp rBE,rB3 ; set C if nonzero exponent. -MM 2000-11-25
> brcs 1f
> ldi rB3,1 ; zero exponent really means 1 with hidden bit = 0
>
> If the exponent is nonzero, it sets it to 1.
If the exponent is _zero_, it sets it to 1.
Note the order of "cp" operands: if (0 < rB3) then skip the "ldi".
> This breaks the test completely -
> as far as I can tell this is a pretty big problem. I don't know enough about
> avr-libc's "special" floating-point format to understand why there is this
> 'hidden bit' thought, which is why I just didn't make a patch.
This is the standard IEEE float format, "hidden bit" is explained here:
http://en.wikipedia.org/wiki/Floating_point
> At any rate it is my guess that the math library has been broken since
> 2000-11-15, which is when it looks like that fragment was added. To me this
> seems fairly large, as it also breaks a number of other routines with "zero
> tests" in them.
If I remember correctly (it was long time ago...), this was a fix
for some bugs with very small (close to zero) numbers. But, you may
be right - it looks like one bugfix introduced another bug :(
I suspect these "zero tests" should test the fraction (rA2:rA1:rA0),
not the exponent (rA3). The "hidden bit" is un-hidden by fp_split
and becomes rA2 bit 7. For approximate testing (very small "denormal"
numbers considered equal to zero), test only rA2. For testing if
a number is exactly equal to 0, test all of rA2:rA1:rA0 (not sure
about rAE where it is used - seems to be zero almost all the time).
Disclaimer: please don't trust me too much, and double check to
be sure whatever I said above makes any sense ;)
Hope this helps,
Marek
- [avr-libc-dev] Bug in math library, Colin O'Flynn, 2005/01/08
- Re: [avr-libc-dev] Bug in math library,
Marek Michalkiewicz <=
- Message not available
- Message not available
- Re: [avr-libc-dev] Bug in math library, Colin O'Flynn, 2005/01/08
- Re: [avr-libc-dev] Bug in math library, Colin O'Flynn, 2005/01/08
- Re: [avr-libc-dev] Bug in math library, Marek Michalkiewicz, 2005/01/08
- Re: [avr-libc-dev] Bug in math library, Colin O'Flynn, 2005/01/09
- Re: [avr-libc-dev] Bug in math library, Paul Schlie, 2005/01/09
- Re: [avr-libc-dev] Bug in math library, Paul Schlie, 2005/01/09
- Re: [avr-libc-dev] Bug in math library, Bob Paddock, 2005/01/09
- Re: [avr-libc-dev] Bug in math library, Colin O'Flynn, 2005/01/09
- Re: [avr-libc-dev] Bug in math library, Paul Schlie, 2005/01/09
- Re: [avr-libc-dev] Bug in math library, Colin O'Flynn, 2005/01/09