*evr, *stevr: use fast CSTEMR path for partial eigenvalue ranges on I…#1295
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…EEE machines The *evr and *stevr routines now call CSTEMR/DSTEMR/SSTEMR with the user-specified RANGE argument on IEEE-754 compliant machines, instead of only using the fast path when the full spectrum is requested (ALLEIG or IL=1, IU=N). Partial eigenvalue ranges with eigenvectors now also benefit from the faster algorithm. Fix a bug where WANTZ=.FALSE. with a partial range on IEEE machines would incorrectly set M=N and skip the DSTEBZ fallback, because INFO remained 0 from initialization when SSTERF was skipped (it can only compute all eigenvalues). The IF(INFO.EQ.0) success check is now moved inside each computational branch so it only triggers when SSTERF or CSTEMR was actually called and succeeded. Closes Reference-LAPACK#1277
langou
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Jul 13, 2026
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Thanks @jschueller. Long overdue. This is not a criticism of this PR since this PR is going in the right direction, but, personally, I do not like the fact that HEEVR calls STERF when IF( ALLEIG .OR. ( INDEIG .AND. IL.EQ.1 .AND. IU.EQ.N ) )I think HEEVR should always call STEMR, no matter what. If users want to use STERF in the case IF( .NOT.WANTZ ) THEN
IF( ALLEIG .OR. ( INDEIG .AND. IL.EQ.1 .AND. IU.EQ.N ) ) THENthen they can call HEEV directly. The way we do it now makes it hard for user to use STERF with IF( ALLEIG .OR. ( INDEIG .AND. IL.EQ.1 .AND. IU.EQ.N ) )This can useful for example if a user wants bit-by-bit eigenvalue reproducibility whether |
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…EEE machines
The *evr and *stevr routines now call CSTEMR/DSTEMR/SSTEMR with the user-specified RANGE argument on IEEE-754 compliant machines, instead of only using the fast path when the full spectrum is requested (ALLEIG or IL=1, IU=N). Partial eigenvalue ranges with eigenvectors now also benefit from the faster algorithm.
Fix a bug where WANTZ=.FALSE. with a partial range on IEEE machines would incorrectly set M=N and skip the DSTEBZ fallback, because INFO remained 0 from initialization when SSTERF was skipped (it can only compute all eigenvalues). The IF(INFO.EQ.0) success check is now moved inside each computational branch so it only triggers when SSTERF or CSTEMR was actually called and succeeded.
Closes #1277