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SUBROUTINE PDLARZT( DIRECT, STOREV, N, K, V, IV, JV, DESCV, TAU,
$ T, WORK )
*
* -- ScaLAPACK auxiliary routine (version 1.7) --
* University of Tennessee, Knoxville, Oak Ridge National Laboratory,
* and University of California, Berkeley.
* May 1, 1997
*
* .. Scalar Arguments ..
CHARACTER DIRECT, STOREV
INTEGER IV, JV, K, N
* ..
* .. Array Arguments ..
INTEGER DESCV( * )
DOUBLE PRECISION TAU( * ), T( * ), V( * ), WORK( * )
* ..
*
* Purpose
* =======
*
* PDLARZT forms the triangular factor T of a real block reflector
* H of order > n, which is defined as a product of k elementary
* reflectors as returned by PDTZRZF.
*
* If DIRECT = 'F', H = H(1) H(2) . . . H(k) and T is upper triangular;
*
* If DIRECT = 'B', H = H(k) . . . H(2) H(1) and T is lower triangular.
*
* If STOREV = 'C', the vector which defines the elementary reflector
* H(i) is stored in the i-th column of the array V, and
*
* H = I - V * T * V'
*
* If STOREV = 'R', the vector which defines the elementary reflector
* H(i) is stored in the i-th row of the array V, and
*
* H = I - V' * T * V
*
* Currently, only STOREV = 'R' and DIRECT = 'B' are supported.
*
* Notes
* =====
*
* Each global data object is described by an associated description
* vector. This vector stores the information required to establish
* the mapping between an object element and its corresponding process
* and memory location.
*
* Let A be a generic term for any 2D block cyclicly distributed array.
* Such a global array has an associated description vector DESCA.
* In the following comments, the character _ should be read as
* "of the global array".
*
* NOTATION STORED IN EXPLANATION
* --------------- -------------- --------------------------------------
* DTYPE_A(global) DESCA( DTYPE_ )The descriptor type. In this case,
* DTYPE_A = 1.
* CTXT_A (global) DESCA( CTXT_ ) The BLACS context handle, indicating
* the BLACS process grid A is distribu-
* ted over. The context itself is glo-
* bal, but the handle (the integer
* value) may vary.
* M_A (global) DESCA( M_ ) The number of rows in the global
* array A.
* N_A (global) DESCA( N_ ) The number of columns in the global
* array A.
* MB_A (global) DESCA( MB_ ) The blocking factor used to distribute
* the rows of the array.
* NB_A (global) DESCA( NB_ ) The blocking factor used to distribute
* the columns of the array.
* RSRC_A (global) DESCA( RSRC_ ) The process row over which the first
* row of the array A is distributed.
* CSRC_A (global) DESCA( CSRC_ ) The process column over which the
* first column of the array A is
* distributed.
* LLD_A (local) DESCA( LLD_ ) The leading dimension of the local
* array. LLD_A >= MAX(1,LOCr(M_A)).
*
* Let K be the number of rows or columns of a distributed matrix,
* and assume that its process grid has dimension p x q.
* LOCr( K ) denotes the number of elements of K that a process
* would receive if K were distributed over the p processes of its
* process column.
* Similarly, LOCc( K ) denotes the number of elements of K that a
* process would receive if K were distributed over the q processes of
* its process row.
* The values of LOCr() and LOCc() may be determined via a call to the
* ScaLAPACK tool function, NUMROC:
* LOCr( M ) = NUMROC( M, MB_A, MYROW, RSRC_A, NPROW ),
* LOCc( N ) = NUMROC( N, NB_A, MYCOL, CSRC_A, NPCOL ).
* An upper bound for these quantities may be computed by:
* LOCr( M ) <= ceil( ceil(M/MB_A)/NPROW )*MB_A
* LOCc( N ) <= ceil( ceil(N/NB_A)/NPCOL )*NB_A
*
* Arguments
* =========
*
* DIRECT (global input) CHARACTER
* Specifies the order in which the elementary reflectors are
* multiplied to form the block reflector:
* = 'F': H = H(1) H(2) . . . H(k) (Forward, not supported yet)
* = 'B': H = H(k) . . . H(2) H(1) (Backward)
*
* STOREV (global input) CHARACTER
* Specifies how the vectors which define the elementary
* reflectors are stored (see also Further Details):
* = 'C': columnwise (not supported yet)
* = 'R': rowwise
*
* N (global input) INTEGER
* The number of meaningful entries of the block reflector H.
* N >= 0.
*
* K (global input) INTEGER
* The order of the triangular factor T (= the number of
* elementary reflectors). 1 <= K <= MB_V (= NB_V).
*
* V (input/output) DOUBLE PRECISION pointer into the local memory
* to an array of local dimension (LOCr(IV+K-1),LOCc(JV+N-1)).
* The distributed matrix V contains the Householder vectors.
* See further details.
*
* IV (global input) INTEGER
* The row index in the global array V indicating the first
* row of sub( V ).
*
* JV (global input) INTEGER
* The column index in the global array V indicating the
* first column of sub( V ).
*
* DESCV (global and local input) INTEGER array of dimension DLEN_.
* The array descriptor for the distributed matrix V.
*
* TAU (local input) DOUBLE PRECISION array, dimension LOCr(IV+K-1)
* if INCV = M_V, and LOCc(JV+K-1) otherwise. This array
* contains the Householder scalars related to the Householder
* vectors. TAU is tied to the distributed matrix V.
*
* T (local output) DOUBLE PRECISION array, dimension (MB_V,MB_V)
* It contains the k-by-k triangular factor of the block
* reflector associated with V. T is lower triangular.
*
* WORK (local workspace) DOUBLE PRECISION array,
* dimension (K*(K-1)/2)
*
* Further Details
* ===============
*
* The shape of the matrix V and the storage of the vectors which define
* the H(i) is best illustrated by the following example with n = 5 and
* k = 3. The elements equal to 1 are not stored; the corresponding
* array elements are modified but restored on exit. The rest of the
* array is not used.
*
* DIRECT = 'F' and STOREV = 'C': DIRECT = 'F' and STOREV = 'R':
*
* ______V_____
* ( v1 v2 v3 ) / \
* ( v1 v2 v3 ) ( v1 v1 v1 v1 v1 . . . . 1 )
* V = ( v1 v2 v3 ) ( v2 v2 v2 v2 v2 . . . 1 )
* ( v1 v2 v3 ) ( v3 v3 v3 v3 v3 . . 1 )
* ( v1 v2 v3 )
* . . .
* . . .
* 1 . .
* 1 .
* 1
*
* DIRECT = 'B' and STOREV = 'C': DIRECT = 'B' and STOREV = 'R':
*
* ______V_____
* 1 / \
* . 1 ( 1 . . . . v1 v1 v1 v1 v1 )
* . . 1 ( . 1 . . . v2 v2 v2 v2 v2 )
* . . . ( . . 1 . . v3 v3 v3 v3 v3 )
* . . .
* ( v1 v2 v3 )
* ( v1 v2 v3 )
* V = ( v1 v2 v3 )
* ( v1 v2 v3 )
* ( v1 v2 v3 )
*
* =====================================================================
*
* .. Parameters ..
INTEGER BLOCK_CYCLIC_2D, CSRC_, CTXT_, DLEN_, DTYPE_,
$ LLD_, MB_, M_, NB_, N_, RSRC_
PARAMETER ( BLOCK_CYCLIC_2D = 1, DLEN_ = 9, DTYPE_ = 1,
$ CTXT_ = 2, M_ = 3, N_ = 4, MB_ = 5, NB_ = 6,
$ RSRC_ = 7, CSRC_ = 8, LLD_ = 9 )
DOUBLE PRECISION ZERO
PARAMETER ( ZERO = 0.0D+0 )
* ..
* .. Local Scalars ..
INTEGER ICOFF, ICTXT, II, IIV, INFO, IVCOL, IVROW,
$ ITMP0, ITMP1, IW, JJV, LDV, MYCOL, MYROW,
$ NPCOL, NPROW, NQ
* ..
* .. External Subroutines ..
EXTERNAL BLACS_ABORT, BLACS_GRIDINFO, DCOPY, DGEMV,
$ DGSUM2D, DLASET, DTRMV, INFOG2L,
$ PXERBLA
* ..
* .. External Functions ..
LOGICAL LSAME
INTEGER NUMROC
EXTERNAL LSAME, NUMROC
* ..
* .. Intrinsic Functions ..
INTRINSIC MOD
* ..
* .. Executable Statements ..
*
* Get grid parameters
*
ICTXT = DESCV( CTXT_ )
CALL BLACS_GRIDINFO( ICTXT, NPROW, NPCOL, MYROW, MYCOL )
*
* Check for currently supported options
*
INFO = 0
IF( .NOT.LSAME( DIRECT, 'B' ) ) THEN
INFO = -1
ELSE IF( .NOT.LSAME( STOREV, 'R' ) ) THEN
INFO = -2
END IF
IF( INFO.NE.0 ) THEN
CALL PXERBLA( ICTXT, 'PDLARZT', -INFO )
CALL BLACS_ABORT( ICTXT, 1 )
RETURN
END IF
*
CALL INFOG2L( IV, JV, DESCV, NPROW, NPCOL, MYROW, MYCOL,
$ IIV, JJV, IVROW, IVCOL )
*
IF( MYROW.EQ.IVROW ) THEN
IW = 1
ITMP0 = 0
LDV = DESCV( LLD_ )
ICOFF = MOD( JV-1, DESCV( NB_ ) )
NQ = NUMROC( N+ICOFF, DESCV( NB_ ), MYCOL, IVCOL, NPCOL )
IF( MYCOL.EQ.IVCOL )
$ NQ = NQ - ICOFF
*
DO 10 II = IIV+K-2, IIV, -1
*
* T(i+1:k,i) = -tau( iv+i-1 ) *
* V(iv+i:iv+k-1,jv:jv+n-1) * V(iv+i-1,jv:jv+n-1)'
*
ITMP0 = ITMP0 + 1
IF( NQ.GT.0 ) THEN
CALL DGEMV( 'No transpose', ITMP0, NQ, -TAU( II ),
$ V( II+1+(JJV-1)*LDV ), LDV,
$ V( II+(JJV-1)*LDV ), LDV, ZERO, WORK( IW ),
$ 1 )
ELSE
CALL DLASET( 'All', ITMP0, 1, ZERO, ZERO, WORK( IW ),
$ ITMP0 )
END IF
IW = IW + ITMP0
*
10 CONTINUE
*
CALL DGSUM2D( ICTXT, 'Rowwise', ' ', IW-1, 1, WORK, IW-1,
$ MYROW, IVCOL )
*
IF( MYCOL.EQ.IVCOL ) THEN
*
IW = 1
ITMP0 = 0
ITMP1 = K + 1 + (K-1) * DESCV( MB_ )
*
T( ITMP1-1 ) = TAU( IIV+K-1 )
*
DO 20 II = IIV+K-2, IIV, -1
*
* T(i+1:k,i) = T(i+1:k,i+1:k) * T(i+1:k,i)
*
ITMP0 = ITMP0 + 1
ITMP1 = ITMP1 - DESCV( MB_ ) - 1
CALL DCOPY( ITMP0, WORK( IW ), 1, T( ITMP1 ), 1 )
IW = IW + ITMP0
*
CALL DTRMV( 'Lower', 'No transpose', 'Non-unit', ITMP0,
$ T( ITMP1+DESCV( MB_ ) ), DESCV( MB_ ),
$ T( ITMP1 ), 1 )
T( ITMP1-1 ) = TAU( II )
*
20 CONTINUE
*
END IF
*
END IF
*
RETURN
*
* End of PDLARZT
*
END
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