On 02/22/2011 02:52 AM, Richard B. Kreckel wrote:

On 02/21/2011 07:05 AM, Michael Miller wrote:

...

The following code (to calculate x^100000):

cl_R x="1.0L100000", y=1; for (int i=1; i<=100000; i++) y=x*y; cout<< y<< "\n"; cout<< exp(100000*ln(x))<< "\n";;

results in the output

1.00000000000000302104L10000000000 terminate called after throwing an instance of 'cln::floating_point_overflow_exception' what(): floating point overflow. Aborted

I know that these are very large numbers, but the two calculations are computing the same value. If the first doesn't overflow, then why does the second?

It is not only the final result that matters. When an intermediate result overflows an exception is thrown, too. You may wish to have a look at the code in src/float/transcendental/.

I have looked at the code, and it seems that the problem is in scale_float, which throws a floating point exception if one tries to scale the exponent to more than 2^32-1. This appears to be unnecessary. For example, in the code below: cl_I m="1073741824"; // 2^30 cl_LF x="1L0"; cout << 3*m << " " << 6*m << "\n"; cout << scale_float(scale_float(x, 3*m), 3*m) << "\n"; cout << scale_float(x, 6*m) << "\n"; the results of the last two output lines should be identical, but I get 3221225472 6442450944 5.4667795091064729912L1939370979 terminate called after throwing an instance of 'cln::floating_point_overflow_exception' what(): floating point overflow. Aborted Now it's true that the exponent of the number being calculated (6*2^30) is larger than LF_exp_high (given as 2^32-1 in cl_LF.h), but that didn't interfere with the value computed by shifting twice by 3*2^30. Is the comparison with LF_exp_high actually necessary? MM