From:	"Vyacheslav O. Myskin" <V DOT O DOT Myskin AT inp DOT nsk DOT su>
Organization:	BINP RAS, Novosibirsk, Russia
To:	kagel AT quasar DOT bloomberg DOT com
Date:	Fri, 7 Feb 1997 00:06:30 +0600
Subject:	Re: double-->int: What's wrong here?
Reply-To:	V DOT O DOT Myskin AT inp DOT nsk DOT su
Cc:	djgpp AT delorie DOT com
In-Reply-To:	<9702061427.AA26252@quasar.bloomberg.com >
References:	<Pine DOT SUN DOT 3 DOT 91 DOT 970206083831 DOT 4379F-100000 AT is> (message from Eli Zaretskii on Thu, 6 Feb 1997 08:44:11 +0200 (IST))
Message-Id:	<6CF0313A6D@csd.inp.nsk.su>

Hello!


>    > The output is:
>    > n1=214  n2=215  d2=215.000000
>    >
>    > So why  n1 is not equal to n2? It's not fun because I used n1 as an
>    > argument to malloc() and kept getting SIGSEGVs later :(
>
>    Never assume floating-point computations are exact: they aren't.  The
>    exact reason for what your example printed are immaterial (it's a long
>    and quite dull story; you might consider adding -S to gcc command line
>    and looking at the assembly it generates to see what's going on).  The
>    real lesson is that you should always round the FP numbers explicitly
>    before using them as counters of anything.  In your case, say n1 =
>    .05/d1 + 0.5 and live happily ever after.
>
> But Eli, 1.0/4300.0 ==> 0.000232558139535
> and n1 = .05/d1 ==> 215.000000000000000
> to the limit of a double's precision (the next 5 digits would 86000 in the
> 80bit internal x87 representation.  There is no imprecision in this
> calculation due to binary floating point effects and adding 0.5 will not
> change the results. Vyacheslav may have indeed found something. 
> Vyacheslav are you using the emulator or an x87?  Which?

I ran it on 486DX, so it's x87. The assembler output ( gcc -S ...) shows
that n1 is stored as an integer (fistp) immediately after calculation; n2 is
converted after saving the value of d2 and reloading it (as it should be for
unoptimized output). The ONLY difference is this fstp/fld combination. Is it
OK that the data saved in memory is not identical to that in the FPU
registers (can't assume anything else) ? Shame on me, my competence in FPU
topics is equal to zero, hope haven't misspelled instruction names ;)

Vyacheslav

>
> --
> Art S. Kagel, kagel AT quasar DOT bloomberg DOT com
>
> A proverb is no proverb to you 'till life has illustrated it.  -- John
> Keats

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