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Postings from June 2002
On Sat, Jun 15, 2002 at 06:56:12PM -0500, Craig A. Berry wrote: > A for loop in pp_pow is constructed in such a way that it squares the > base again even after it's iterated through all the bits of the > exponent. This could be a problem on any system where floating > overflow causes an exception; I noticed it with the D_FLOAT format on > OpenVMS Alpha, where it was causing Perl to blow up when running > p/op/pow.t. The patch below fixes it by exiting the loop as soon as > we know we can. > > If someone strongly objects to having this test inside the loop, I > can ifdef it. Not really. I wasn't too worried about tweaking the performance of the integer bits of the pp_pow loop once it was working (in log(n) time). Having looked at how GNU bc does the calculation, I realised that there was at least one way of rewriting it (sorry, I forget how offhand) that would use slightly less calculation each iteration. For information (which I don't think has reached p5p) the reason for having messy loops in pp_pow rather than using pow() or log(), exp() and a multiply is that on Irix with longdoubles, 2**31 was giving a floating point value slightly *less* than 1<<31, which was really screwing up some tests. (and by implication (nearly) every perl programmer's assumption that 2**n is an integer) Nicholas Clark -- Even better than the real thing: http://nms-cgi.sourceforge.net/Thread Previous | Thread Next