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stedc scales only if norm is tiny or huge (and not unconditionally) #1053

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stedc scales only if norm is tiny or huge (and not unconditionally) #1053

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2 changes: 1 addition & 1 deletion SRC/dlaed0.f
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Original file line number Diff line number Diff line change
Expand Up @@ -74,7 +74,7 @@
*> On exit, its eigenvalues.
*> \endverbatim
*>
*> \param[in] E
*> \param[in,out] E
*> \verbatim
*> E is DOUBLE PRECISION array, dimension (N-1)
*> The off-diagonal elements of the tridiagonal matrix.
Expand Down
44 changes: 32 additions & 12 deletions SRC/dstedc.f
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Expand Up @@ -201,9 +201,10 @@ SUBROUTINE DSTEDC( COMPZ, N, D, E, Z, LDZ, WORK, LWORK, IWORK,
* ..
* .. Local Scalars ..
LOGICAL LQUERY
INTEGER FINISH, I, ICOMPZ, II, J, K, LGN, LIWMIN,
$ LWMIN, M, SMLSIZ, START, STOREZ, STRTRW
DOUBLE PRECISION EPS, ORGNRM, P, TINY
INTEGER FINISH, I, ICOMPZ, II, ISCALE, J, K, LGN,
$ LIWMIN, LWMIN, M, SMLSIZ, START, STOREZ, STRTRW
DOUBLE PRECISION EPS, EPS2, ORGNRM, P, SAFMAX, SAFMIN,
$ SSFMAX, SSFMIN, TINY
* ..
* .. External Functions ..
LOGICAL LSAME
Expand Down Expand Up @@ -339,8 +340,15 @@ SUBROUTINE DSTEDC( COMPZ, N, D, E, Z, LDZ, WORK, LWORK, IWORK,
ORGNRM = DLANST( 'M', N, D, E )
IF( ORGNRM.EQ.ZERO )
$ GO TO 50
*
* Determine the unit roundoff and over/underflow thresholds.
*
EPS = DLAMCH( 'Epsilon' )
EPS2 = EPS**2
SAFMIN = DLAMCH( 'S' )
SAFMAX = ONE / SAFMIN
SSFMAX = SQRT( SAFMAX ) / ( ONE + TWO )
SSFMIN = SQRT( SAFMIN ) / EPS2
*
START = 1
*
Expand Down Expand Up @@ -378,12 +386,20 @@ SUBROUTINE DSTEDC( COMPZ, N, D, E, Z, LDZ, WORK, LWORK, IWORK,
* Scale.
*
ORGNRM = DLANST( 'M', M, D( START ), E( START ) )
CALL DLASCL( 'G', 0, 0, ORGNRM, ONE, M, 1, D( START ),
$ M,
$ INFO )
CALL DLASCL( 'G', 0, 0, ORGNRM, ONE, M-1, 1,
$ E( START ),
$ M-1, INFO )
ISCALE = 0
IF( ORGNRM.GT.SSFMAX ) THEN
ISCALE = 1
CALL DLASCL( 'G', 0, 0, ORGNRM, SSFMAX, M, 1,
$ D( START ), M, INFO )
CALL DLASCL( 'G', 0, 0, ORGNRM, SSFMAX, M-1, 1,
$ E( START ), M-1, INFO )
ELSE IF( ORGNRM.LT.SSFMIN ) THEN
ISCALE = 2
CALL DLASCL( 'G', 0, 0, ORGNRM, SSFMIN, M, 1,
$ D( START ), M, INFO )
CALL DLASCL( 'G', 0, 0, ORGNRM, SSFMIN, M-1, 1,
$ E( START ), M-1, INFO )
END IF
*
IF( ICOMPZ.EQ.1 ) THEN
STRTRW = 1
Expand All @@ -401,9 +417,13 @@ SUBROUTINE DSTEDC( COMPZ, N, D, E, Z, LDZ, WORK, LWORK, IWORK,
*
* Scale back.
*
CALL DLASCL( 'G', 0, 0, ONE, ORGNRM, M, 1, D( START ),
$ M,
$ INFO )
IF( ISCALE.EQ.1 ) THEN
CALL DLASCL( 'G', 0, 0, SSFMAX, ORGNRM, M, 1,
$ D( START ), M, INFO )
ELSE IF( ISCALE.EQ.2 ) THEN
CALL DLASCL( 'G', 0, 0, SSFMIN, ORGNRM, M, 1,
$ D( START ), M, INFO )
END IF
*
ELSE
IF( ICOMPZ.EQ.1 ) THEN
Expand Down
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