ssytrd_2stage.f

Go to the documentation of this file.

*> \brief \b SSYTRD_2STAGE
*
*  @generated from zhetrd_2stage.f, fortran z -> s, Sun Nov  6 19:34:06 2016
*
*  =========== DOCUMENTATION ===========
*
* Online html documentation available at
*            http://www.netlib.org/lapack/explore-html/
*
*> Download SSYTRD_2STAGE + dependencies
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/ssytrd_2stage.f">
*> [TGZ]</a>
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/ssytrd_2stage.f">
*> [ZIP]</a>
*> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/ssytrd_2stage.f">
*> [TXT]</a>
*
*  Definition:
*  ===========
*
*       SUBROUTINE SSYTRD_2STAGE( VECT, UPLO, N, A, LDA, D, E, TAU,
*                                 HOUS2, LHOUS2, WORK, LWORK, INFO )
*
*       IMPLICIT NONE
*
*      .. Scalar Arguments ..
*       CHARACTER          VECT, UPLO
*       INTEGER            N, LDA, LWORK, LHOUS2, INFO
*      ..
*      .. Array Arguments ..
*       REAL               D( * ), E( * )
*       REAL               A( LDA, * ), TAU( * ),
*                          HOUS2( * ), WORK( * )
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*> SSYTRD_2STAGE reduces a real symmetric matrix A to real symmetric
*> tridiagonal form T by a orthogonal similarity transformation:
*> Q1**T Q2**T* A * Q2 * Q1 = T.
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] VECT
*> \verbatim
*>          VECT is CHARACTER*1
*>          = 'N':  No need for the Housholder representation,
*>                  in particular for the second stage (Band to
*>                  tridiagonal) and thus LHOUS2 is of size max(1, 4*N);
*>          = 'V':  the Householder representation is needed to
*>                  either generate Q1 Q2 or to apply Q1 Q2,
*>                  then LHOUS2 is to be queried and computed.
*>                  (NOT AVAILABLE IN THIS RELEASE).
*> \endverbatim
*>
*> \param[in] UPLO
*> \verbatim
*>          UPLO is CHARACTER*1
*>          = 'U':  Upper triangle of A is stored;
*>          = 'L':  Lower triangle of A is stored.
*> \endverbatim
*>
*> \param[in] N
*> \verbatim
*>          N is INTEGER
*>          The order of the matrix A.  N >= 0.
*> \endverbatim
*>
*> \param[in,out] A
*> \verbatim
*>          A is REAL array, dimension (LDA,N)
*>          On entry, the symmetric matrix A.  If UPLO = 'U', the leading
*>          N-by-N upper triangular part of A contains the upper
*>          triangular part of the matrix A, and the strictly lower
*>          triangular part of A is not referenced.  If UPLO = 'L', the
*>          leading N-by-N lower triangular part of A contains the lower
*>          triangular part of the matrix A, and the strictly upper
*>          triangular part of A is not referenced.
*>          On exit, if UPLO = 'U', the band superdiagonal
*>          of A are overwritten by the corresponding elements of the
*>          internal band-diagonal matrix AB, and the elements above
*>          the KD superdiagonal, with the array TAU, represent the orthogonal
*>          matrix Q1 as a product of elementary reflectors; if UPLO
*>          = 'L', the diagonal and band subdiagonal of A are over-
*>          written by the corresponding elements of the internal band-diagonal
*>          matrix AB, and the elements below the KD subdiagonal, with
*>          the array TAU, represent the orthogonal matrix Q1 as a product
*>          of elementary reflectors. See Further Details.
*> \endverbatim
*>
*> \param[in] LDA
*> \verbatim
*>          LDA is INTEGER
*>          The leading dimension of the array A.  LDA >= max(1,N).
*> \endverbatim
*>
*> \param[out] D
*> \verbatim
*>          D is REAL array, dimension (N)
*>          The diagonal elements of the tridiagonal matrix T.
*> \endverbatim
*>
*> \param[out] E
*> \verbatim
*>          E is REAL array, dimension (N-1)
*>          The off-diagonal elements of the tridiagonal matrix T.
*> \endverbatim
*>
*> \param[out] TAU
*> \verbatim
*>          TAU is REAL array, dimension (N-KD)
*>          The scalar factors of the elementary reflectors of
*>          the first stage (see Further Details).
*> \endverbatim
*>
*> \param[out] HOUS2
*> \verbatim
*>          HOUS2 is REAL array, dimension (MAX(1,LHOUS2))
*>          Stores the Householder representation of the stage2
*>          band to tridiagonal.
*> \endverbatim
*>
*> \param[in] LHOUS2
*> \verbatim
*>          LHOUS2 is INTEGER
*>          The dimension of the array HOUS2.
*>          LHOUS2 >= 1.
*>
*>          If LWORK = -1, or LHOUS2 = -1,
*>          then a query is assumed; the routine
*>          only calculates the optimal size of the HOUS2 array, returns
*>          this value as the first entry of the HOUS2 array, and no error
*>          message related to LHOUS2 is issued by XERBLA.
*>          If VECT='N', LHOUS2 = max(1, 4*n);
*>          if VECT='V', option not yet available.
*> \endverbatim
*>
*> \param[out] WORK
*> \verbatim
*>          WORK is REAL array, dimension (LWORK)
*> \endverbatim
*>
*> \param[in] LWORK
*> \verbatim
*>          LWORK is INTEGER
*>          The dimension of the array WORK.
*>          If N = 0, LWORK >= 1, else LWORK = MAX(1, dimension).
*>
*>          If LWORK = -1, or LHOUS2 = -1,
*>          then a workspace query is assumed; the routine
*>          only calculates the optimal size of the WORK array, returns
*>          this value as the first entry of the WORK array, and no error
*>          message related to LWORK is issued by XERBLA.
*>          LWORK = MAX(1, dimension) where
*>          dimension   = max(stage1,stage2) + (KD+1)*N
*>                      = N*KD + N*max(KD+1,FACTOPTNB)
*>                        + max(2*KD*KD, KD*NTHREADS)
*>                        + (KD+1)*N
*>          where KD is the blocking size of the reduction,
*>          FACTOPTNB is the blocking used by the QR or LQ
*>          algorithm, usually FACTOPTNB=128 is a good choice
*>          NTHREADS is the number of threads used when
*>          openMP compilation is enabled, otherwise =1.
*> \endverbatim
*>
*> \param[out] INFO
*> \verbatim
*>          INFO is INTEGER
*>          = 0:  successful exit
*>          < 0:  if INFO = -i, the i-th argument had an illegal value
*> \endverbatim
*
*  Authors:
*  ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \ingroup hetrd_2stage
*
*> \par Further Details:
*  =====================
*>
*> \verbatim
*>
*>  Implemented by Azzam Haidar.
*>
*>  All details are available on technical report, SC11, SC13 papers.
*>
*>  Azzam Haidar, Hatem Ltaief, and Jack Dongarra.
*>  Parallel reduction to condensed forms for symmetric eigenvalue problems
*>  using aggregated fine-grained and memory-aware kernels. In Proceedings
*>  of 2011 International Conference for High Performance Computing,
*>  Networking, Storage and Analysis (SC '11), New York, NY, USA,
*>  Article 8 , 11 pages.
*>  http://doi.acm.org/10.1145/2063384.2063394
*>
*>  A. Haidar, J. Kurzak, P. Luszczek, 2013.
*>  An improved parallel singular value algorithm and its implementation
*>  for multicore hardware, In Proceedings of 2013 International Conference
*>  for High Performance Computing, Networking, Storage and Analysis (SC '13).
*>  Denver, Colorado, USA, 2013.
*>  Article 90, 12 pages.
*>  http://doi.acm.org/10.1145/2503210.2503292
*>
*>  A. Haidar, R. Solca, S. Tomov, T. Schulthess and J. Dongarra.
*>  A novel hybrid CPU-GPU generalized eigensolver for electronic structure
*>  calculations based on fine-grained memory aware tasks.
*>  International Journal of High Performance Computing Applications.
*>  Volume 28 Issue 2, Pages 196-209, May 2014.
*>  http://hpc.sagepub.com/content/28/2/196
*>
*> \endverbatim
*>
*  =====================================================================
      SUBROUTINE ssytrd_2stage( VECT, UPLO, N, A, LDA, D, E, TAU,
     $                          HOUS2, LHOUS2, WORK, LWORK, INFO )
*
      IMPLICIT NONE
*
*  -- LAPACK computational routine --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      CHARACTER          VECT, UPLO
      INTEGER            N, LDA, LWORK, LHOUS2, INFO
*     ..
*     .. Array Arguments ..
      REAL               D( * ), E( * )
      REAL               A( LDA, * ), TAU( * ),
     $                   hous2( * ), work( * )
*     ..
*
*  =====================================================================
*     ..
*     .. Local Scalars ..
      LOGICAL            LQUERY, UPPER, WANTQ
      INTEGER            KD, IB, LWMIN, LHMIN, LWRK, LDAB, WPOS, ABPOS
*     ..
*     .. External Subroutines ..
      EXTERNAL           xerbla, ssytrd_sy2sb, ssytrd_sb2st
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      INTEGER            ILAENV2STAGE
      EXTERNAL           lsame, ilaenv2stage
*     ..
*     .. Executable Statements ..
*
*     Test the input parameters
*
      info   = 0
      wantq  = lsame( vect, 'V' )
      upper  = lsame( uplo, 'U' )
      lquery = ( lwork.EQ.-1 ) .OR. ( lhous2.EQ.-1 )
*
*     Determine the block size, the workspace size and the hous size.
*
      kd     = ilaenv2stage( 1, 'SSYTRD_2STAGE', vect, n, -1, -1,
     $                      -1 )
      ib     = ilaenv2stage( 2, 'SSYTRD_2STAGE', vect, n, kd, -1,
     $                      -1 )
      IF( n.EQ.0 ) THEN
         lhmin = 1
         lwmin = 1
      ELSE
         lhmin = ilaenv2stage( 3, 'SSYTRD_2STAGE', vect, n, kd, ib,
     $                        -1 )
         lwmin = ilaenv2stage( 4, 'SSYTRD_2STAGE', vect, n, kd, ib,
     $                        -1 )
      END IF
*
      IF( .NOT.lsame( vect, 'N' ) ) THEN
         info = -1
      ELSE IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
         info = -2
      ELSE IF( n.LT.0 ) THEN
         info = -3
      ELSE IF( lda.LT.max( 1, n ) ) THEN
         info = -5
      ELSE IF( lhous2.LT.lhmin .AND. .NOT.lquery ) THEN
         info = -10
      ELSE IF( lwork.LT.lwmin .AND. .NOT.lquery ) THEN
         info = -12
      END IF
*
      IF( info.EQ.0 ) THEN
         hous2( 1 ) = real( lhmin )
         work( 1 )  = real( lwmin )
      END IF
*
      IF( info.NE.0 ) THEN
         CALL xerbla( 'SSYTRD_2STAGE', -info )
         RETURN
      ELSE IF( lquery ) THEN
         RETURN
      END IF
*
*     Quick return if possible
*
      IF( n.EQ.0 ) THEN
         work( 1 ) = 1
         RETURN
      END IF
*
*     Determine pointer position
*
      ldab  = kd+1
      lwrk  = lwork-ldab*n
      abpos = 1
      wpos  = abpos + ldab*n
      CALL ssytrd_sy2sb( uplo, n, kd, a, lda, work( abpos ), ldab,
     $                   tau, work( wpos ), lwrk, info )
      IF( info.NE.0 ) THEN
         CALL xerbla( 'SSYTRD_SY2SB', -info )
         RETURN
      END IF
      CALL ssytrd_sb2st( 'Y', vect, uplo, n, kd,
     $                   work( abpos ), ldab, d, e,
     $                   hous2, lhous2, work( wpos ), lwrk, info )
      IF( info.NE.0 ) THEN
         CALL xerbla( 'SSYTRD_SB2ST', -info )
         RETURN
      END IF
*
*
      work( 1 ) = real( lwmin )
      RETURN
*
*     End of SSYTRD_2STAGE
*
      END