Handling of floating point numbers changed from GMP 4.1.4 to GMP 4.2.1

[Vincent Lefevre]

On 2007-09-12 17:14:21 +0200, Roberto Bagnara wrote:
> Vincent Lefevre wrote:
>> On 2007-09-12 10:42:48 +0200, Roberto Bagnara wrote:
>>> The results are different depending on whether it is linked to GMP 4.1.4
>>> or with GMP 4.2.1.  For example, on my Fedora 7, x86_64 machine where I
>>> have GMP 4.1.4 in /usr/lib64 and GMP 4.2.1 in /usr/local/lib, I use the
>>> script
>>>
>>> g++ -I/usr/local/include -o bug.o -c bug.cc
>>> g++ -static -o bug-gmp-4.2.1 bug.o -L/usr/local/lib -lm -lgmpxx -lgmp
>>> g++ -static -o bug-gmp-4.1.4 bug.o -L/usr/lib64 -lm -lgmpxx -lgmp
>>> ./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
>>>
>>> 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.
>>
>> Were they compiled with the same compiler and the same compile switches?
>> If not, perhaps one uses SSE2 while the other one uses the traditional
>> FPU.
>
> Same compiler and same switches as per the script above.

I meant the GMP libraries (not the bug-gmp-* binaries). Were these
libraries compiled *exactly* in the same way?

Moreover, you compiled bug.cc with the GMP headers from
/usr/local/include. This isn't necessarily correct if you
use the GMP library from /usr/lib64.

-- 
Vincent Lefèvre <vincent at vinc17.org> - Web: <http://www.vinc17.org/>
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