|different results with different f77 optimizers email@example.com (Tom Keyes) (1995-09-24)|
|Re: different results with different f77 optimizers ah739@cleveland.Freenet.Edu (1995-09-29)|
|Re: different results with different f77 opti firstname.lastname@example.org (1995-10-05)|
|From:||email@example.com (Brian Bliss)|
|Keywords:||Fortran, optimize, arithmetic|
|Organization:||Engineering, Convex Computer Corporation, Richardson, Tx USA|
|Date:||Thu, 5 Oct 1995 22:21:25 GMT|
>>'Challenger' with IRIX 5.2. I compile the Fortran source with f77
>>-O3 -non_shared;if I try -O3 alone I get a message that the system does
>>quantities was radically different. I have checked enough now to verify
>>that these two different optimizers really do lead to different results.
>I would trust the unoptimized version more.
I would also, at first. However, if the difference is due to a mathematical
instability in the algorithm itself, the optmized version will typically
give a more accurate answer. Reduction functions can be performed on
specialized hardware which typically has extended precision, and if the
compiler used associative transformations to perform tree-height reduction
of expression trees, the resulting calculation have a better mathematical
stability, on the "average".
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