[PPL-devel] [Fwd: Handling of floating point numbers changed from GMP 4.1.4 to GMP 4.2.1]

[Roberto Bagnara]

-------- Original Message --------
Subject: Handling of floating point numbers changed from GMP 4.1.4 to GMP 4.2.1
Date: Wed, 12 Sep 2007 14:03:56 +0200
From: Torbjorn Granlund <tg at swox.com>
To: gmp-discuss at swox.com

   ./bug-gmp-4.2.1
   ./bug-gmp-4.1.4

   This prints

   real_coeff = 137171200400403985/1125899906842624
   float = 121.83249999999999602
   real_coeff = 137171200400403985/1125899906842624
   float = 121.83250000000001023

OK, so it seems GMP 4.1.4 did not comply to the documentation (or your
compiler uses the wrong fp->unsigned instruction, not uncommon!).  I
don't recall this specifically, but I do recall that we rewrote much
of this code, for speed and perhaps for correctness.

IIRC, C is defined to always do truncation for float -> int assignments, so
no matter the rounding mode,

    intvar = 1.999999;

should put 1 in intvar.

   The mismatch does not happen if one does not touch the rounding mode
   or if one sets it to FE_TONEAREST, FE_TOWARDZERO, FE_DOWNWARD.
   So, on this example, while GMP 4.1.4 results do not depend on the
   rounding mode, with GMP 4.2.1 the rounding mode matters.

I think you've got it backwards.

(I tested this with GMP 4.2.2 on my x86_64 system and see no problems.
I have not tried GMP 4.1.4, but if your output is not mixed up, it is
GMP 4.1.4 that misbehaves for you.)

   Perhaps the answer is simply that GMP provides no guarantee
   when invoked with the rounding mode set to anything different from
   FE_TONEAREST, but I have not found that in the documentation.
   Instead, the documentation says, e.g., that

      double mpq_get_d (mpq_t op)
      Convert op to a double, truncating if necessary (ie.: rounding towards zero).

Well, we don't provide a guarantee, but we do have very strong
feelings about complying to our own documentation.  :-)

-- 
Torbjörn
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-- 
Prof. Roberto Bagnara
Computer Science Group
Department of Mathematics, University of Parma, Italy
http://www.cs.unipr.it/~bagnara/
mailto:bagnara at cs.unipr.it