Fix division by zero in [z]rotg #4235
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The cases [ c s ] * [ 0 ] = [ |db_i| ] [-s c ] [ i*db_i ] [ 0 ] and [ c s ] * [ 0 ] = [ |db_r| ] [-s c ] [ db_r ] [ 0 ] computed s incorrectly. To flip the entries of vector, s should be conjg(db)/|db| and not conjg(db) / da, where da == 0.0.
* The check da != ZERO is no longer necessary since there is a special case ada == ZERO, where ada = |da|. * Add the missing check c != ZERO before the division. Note that with these two changes the long double code follows the float/double version of the code.
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@vladimir-ch You have commented on rotg before and may want to have a look |
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Thanks for the notification @angsch. I've checked and our Gonum BLAS already has this fix. |
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Fix division by zero.
With that
ifcondition, the division byda_rcorresponds to a division by zero.OpenBLAS/interface/zrotg.c
Lines 63 to 68 in bb2f1ec
Fixed to such that
S = conjg(db)/|db|Remove redundant branch.
With the algorithm change in reference LAPACK, the case
da == ZEROis a special branch.OpenBLAS/interface/rotg.c
Line 34 in bb2f1ec
OpenBLAS/interface/rotg.c
Lines 51 to 56 in bb2f1ec
Consequently, there is no longer the need to check.
OpenBLAS/interface/rotg.c
Line 69 in bb2f1ec
The commit removes the branch.
Add missing check for zero.
OpenBLAS/interface/rotg.c
Lines 72 to 73 in bb2f1ec
does not have a check
c != 0in the long double code. The float/double version does have this check. The new code is essentially identical to the float/double version