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SUBROUTINE PSLATRD( UPLO, N, NB, A, IA, JA, DESCA, D, E, TAU, W,
$ IW, JW, DESCW, 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 UPLO
INTEGER IA, IW, JA, JW, N, NB
* ..
* .. Array Arguments ..
INTEGER DESCA( * ), DESCW( * )
REAL A( * ), D( * ), E( * ), TAU( * ), W( * ),
$ WORK( * )
* ..
*
* Purpose
* =======
*
* PSLATRD reduces NB rows and columns of a real symmetric distributed
* matrix sub( A ) = A(IA:IA+N-1,JA:JA+N-1) to symmetric tridiagonal
* form by an orthogonal similarity transformation Q' * sub( A ) * Q,
* and returns the matrices V and W which are needed to apply the
* transformation to the unreduced part of sub( A ).
*
* If UPLO = 'U', PSLATRD reduces the last NB rows and columns of a
* matrix, of which the upper triangle is supplied;
* if UPLO = 'L', PSLATRD reduces the first NB rows and columns of a
* matrix, of which the lower triangle is supplied.
*
* This is an auxiliary routine called by PSSYTRD.
*
* 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
* =========
*
* UPLO (global input) CHARACTER
* Specifies whether the upper or lower triangular part of the
* symmetric matrix sub( A ) is stored:
* = 'U': Upper triangular
* = 'L': Lower triangular
*
* N (global input) INTEGER
* The number of rows and columns to be operated on, i.e. the
* order of the distributed submatrix sub( A ). N >= 0.
*
* NB (global input) INTEGER
* The number of rows and columns to be reduced.
*
* A (local input/local output) REAL pointer into the
* local memory to an array of dimension (LLD_A,LOCc(JA+N-1)).
* On entry, this array contains the local pieces of the
* symmetric distributed matrix sub( A ). If UPLO = 'U', the
* leading N-by-N upper triangular part of sub( A ) contains
* the upper triangular part of the matrix, and its strictly
* lower triangular part is not referenced. If UPLO = 'L', the
* leading N-by-N lower triangular part of sub( A ) contains the
* lower triangular part of the matrix, and its strictly upper
* triangular part is not referenced.
* On exit, if UPLO = 'U', the last NB columns have been reduced
* to tridiagonal form, with the diagonal elements overwriting
* the diagonal elements of sub( A ); the elements above the
* diagonal with the array TAU, represent the orthogonal matrix
* Q as a product of elementary reflectors. If UPLO = 'L', the
* first NB columns have been reduced to tridiagonal form, with
* the diagonal elements overwriting the diagonal elements of
* sub( A ); the elements below the diagonal with the array TAU,
* represent the orthogonal matrix Q as a product of elementary
* reflectors; See Further Details.
*
* IA (global input) INTEGER
* The row index in the global array A indicating the first
* row of sub( A ).
*
* JA (global input) INTEGER
* The column index in the global array A indicating the
* first column of sub( A ).
*
* DESCA (global and local input) INTEGER array of dimension DLEN_.
* The array descriptor for the distributed matrix A.
*
* D (local output) REAL array, dimension LOCc(JA+N-1)
* The diagonal elements of the tridiagonal matrix T:
* D(i) = A(i,i). D is tied to the distributed matrix A.
*
* E (local output) REAL array, dimension LOCc(JA+N-1)
* if UPLO = 'U', LOCc(JA+N-2) otherwise. The off-diagonal
* elements of the tridiagonal matrix T: E(i) = A(i,i+1) if
* UPLO = 'U', E(i) = A(i+1,i) if UPLO = 'L'. E is tied to the
* distributed matrix A.
*
* TAU (local output) REAL, array, dimension
* LOCc(JA+N-1). This array contains the scalar factors TAU of
* the elementary reflectors. TAU is tied to the distributed
* matrix A.
*
* W (local output) REAL pointer into the local memory
* to an array of dimension (LLD_W,NB_W), This array contains
* the local pieces of the N-by-NB_W matrix W required to
* update the unreduced part of sub( A ).
*
* IW (global input) INTEGER
* The row index in the global array W indicating the first
* row of sub( W ).
*
* JW (global input) INTEGER
* The column index in the global array W indicating the
* first column of sub( W ).
*
* DESCW (global and local input) INTEGER array of dimension DLEN_.
* The array descriptor for the distributed matrix W.
*
* WORK (local workspace) REAL array, dimension (NB_A)
*
* Further Details
* ===============
*
* If UPLO = 'U', the matrix Q is represented as a product of elementary
* reflectors
*
* Q = H(n) H(n-1) . . . H(n-nb+1).
*
* Each H(i) has the form
*
* H(i) = I - tau * v * v'
*
* where tau is a real scalar, and v is a real vector with
* v(i:n) = 0 and v(i-1) = 1; v(1:i-1) is stored on exit in
* A(ia:ia+i-2,ja+i), and tau in TAU(ja+i-1).
*
* If UPLO = 'L', the matrix Q is represented as a product of elementary
* reflectors
*
* Q = H(1) H(2) . . . H(nb).
*
* Each H(i) has the form
*
* H(i) = I - tau * v * v'
*
* where tau is a real scalar, and v is a real vector with
* v(1:i) = 0 and v(i+1) = 1; v(i+2:n) is stored on exit in
* A(ia+i+1:ia+n-1,ja+i-1), and tau in TAU(ja+i-1).
*
* The elements of the vectors v together form the N-by-NB matrix V
* which is needed, with W, to apply the transformation to the unreduced
* part of the matrix, using a symmetric rank-2k update of the form:
* sub( A ) := sub( A ) - V*W' - W*V'.
*
* The contents of A on exit are illustrated by the following examples
* with n = 5 and nb = 2:
*
* if UPLO = 'U': if UPLO = 'L':
*
* ( a a a v4 v5 ) ( d )
* ( a a v4 v5 ) ( 1 d )
* ( a 1 v5 ) ( v1 1 a )
* ( d 1 ) ( v1 v2 a a )
* ( d ) ( v1 v2 a a a )
*
* where d denotes a diagonal element of the reduced matrix, a denotes
* an element of the original matrix that is unchanged, and vi denotes
* an element of the vector defining H(i).
*
* =====================================================================
*
* .. 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 )
REAL HALF, ONE, ZERO
PARAMETER ( HALF = 0.5E+0, ONE = 1.0E+0, ZERO = 0.0E+0 )
* ..
* .. Local Scalars ..
INTEGER I, IACOL, IAROW, ICTXT, II, J, JJ, JP, JWK, K,
$ KW, MYCOL, MYROW, NPCOL, NPROW, NQ
REAL ALPHA
* ..
* .. Local Arrays ..
INTEGER DESCD( DLEN_ ), DESCE( DLEN_ ), DESCWK( DLEN_ )
* ..
* .. External Subroutines ..
EXTERNAL BLACS_GRIDINFO, DESCSET, INFOG2L, PSAXPY,
$ PSDOT, PSELGET, PSELSET, PSGEMV,
$ PSLARFG, PSSCAL, PSSYMV, SGEBR2D,
$ SGEBS2D
* ..
* .. External Functions ..
LOGICAL LSAME
INTEGER NUMROC
EXTERNAL LSAME, NUMROC
* ..
* .. Intrinsic Functions ..
INTRINSIC MIN
* ..
* .. Executable Statements ..
*
* Quick return if possible
*
IF( N.LE.0 )
$ RETURN
*
ICTXT = DESCA( CTXT_ )
CALL BLACS_GRIDINFO( ICTXT, NPROW, NPCOL, MYROW, MYCOL )
NQ = MAX( 1, NUMROC( JA+N-1, DESCA( NB_ ), MYCOL, DESCA( CSRC_ ),
$ NPCOL ) )
CALL DESCSET( DESCD, 1, JA+N-1, 1, DESCA( NB_ ), MYROW,
$ DESCA( CSRC_ ), DESCA( CTXT_ ), 1 )
*
IF( LSAME( UPLO, 'U' ) ) THEN
*
CALL INFOG2L( N+IA-NB, N+JA-NB, DESCA, NPROW, NPCOL, MYROW,
$ MYCOL, II, JJ, IAROW, IACOL )
CALL DESCSET( DESCWK, 1, DESCW( NB_ ), 1, DESCW( NB_ ), IAROW,
$ IACOL, ICTXT, 1 )
CALL DESCSET( DESCE, 1, JA+N-1, 1, DESCA( NB_ ), MYROW,
$ DESCA( CSRC_ ), DESCA( CTXT_ ), 1 )
*
* Reduce last NB columns of upper triangle
*
DO 10 J = JA+N-1, JA+N-NB, -1
I = IA + J - JA
K = J - JA + 1
KW = MOD( K-1, DESCA( MB_ ) ) + 1
*
* Update A(IA:I,I)
*
CALL PSGEMV( 'No transpose', K, N-K, -ONE, A, IA, J+1,
$ DESCA, W, IW+K-1, JW+KW, DESCW, DESCW( M_ ),
$ ONE, A, IA, J, DESCA, 1 )
CALL PSGEMV( 'No transpose', K, N-K, -ONE, W, IW, JW+KW,
$ DESCW, A, I, J+1, DESCA, DESCA( M_ ), ONE, A,
$ IA, J, DESCA, 1 )
IF( N-K.GT.0 )
$ CALL PSELSET( A, I, J+1, DESCA, E( JP ) )
*
* Generate elementary reflector H(i) to annihilate
* A(IA:I-2,I)
*
JP = MIN( JJ+KW-1, NQ )
CALL PSLARFG( K-1, E( JP ), I-1, J, A, IA, J, DESCA, 1,
$ TAU )
CALL PSELSET( A, I-1, J, DESCA, ONE )
*
* Compute W(IW:IW+K-2,JW+KW-1)
*
CALL PSSYMV( 'Upper', K-1, ONE, A, IA, JA, DESCA, A, IA, J,
$ DESCA, 1, ZERO, W, IW, JW+KW-1, DESCW, 1 )
*
JWK = MOD( K-1, DESCWK( NB_ ) ) + 2
CALL PSGEMV( 'Transpose', K-1, N-K, ONE, W, IW, JW+KW,
$ DESCW, A, IA, J, DESCA, 1, ZERO, WORK, 1, JWK,
$ DESCWK, DESCWK( M_ ) )
CALL PSGEMV( 'No transpose', K-1, N-K, -ONE, A, IA, J+1,
$ DESCA, WORK, 1, JWK, DESCWK, DESCWK( M_ ), ONE,
$ W, IW, JW+KW-1, DESCW, 1 )
CALL PSGEMV( 'Transpose', K-1, N-K, ONE, A, IA, J+1, DESCA,
$ A, IA, J, DESCA, 1, ZERO, WORK, 1, JWK, DESCWK,
$ DESCWK( M_ ) )
CALL PSGEMV( 'No transpose', K-1, N-K, -ONE, W, IW, JW+KW,
$ DESCW, WORK, 1, JWK, DESCWK, DESCWK( M_ ), ONE,
$ W, IW, JW+KW-1, DESCW, 1 )
CALL PSSCAL( K-1, TAU( JP ), W, IW, JW+KW-1, DESCW, 1 )
*
CALL PSDOT( K-1, ALPHA, W, IW, JW+KW-1, DESCW, 1, A, IA, J,
$ DESCA, 1 )
IF( MYCOL.EQ.IACOL )
$ ALPHA = -HALF*TAU( JP )*ALPHA
CALL PSAXPY( K-1, ALPHA, A, IA, J, DESCA, 1, W, IW, JW+KW-1,
$ DESCW, 1 )
IF( MYCOL.EQ.IACOL ) THEN
CALL PSELGET( 'E', ' ', D( JP ), A, I, J, DESCA )
END IF
*
10 CONTINUE
*
ELSE
*
CALL INFOG2L( IA, JA, DESCA, NPROW, NPCOL, MYROW, MYCOL, II,
$ JJ, IAROW, IACOL )
CALL DESCSET( DESCWK, 1, DESCW( NB_ ), 1, DESCW( NB_ ), IAROW,
$ IACOL, ICTXT, 1 )
CALL DESCSET( DESCE, 1, JA+N-2, 1, DESCA( NB_ ), MYROW,
$ DESCA( CSRC_ ), DESCA( CTXT_ ), 1 )
*
* Reduce first NB columns of lower triangle
*
DO 20 J = JA, JA+NB-1
I = IA + J - JA
K = J - JA + 1
*
* Update A(J:JA+N-1,J)
*
CALL PSGEMV( 'No transpose', N-K+1, K-1, -ONE, A, I, JA,
$ DESCA, W, IW+K-1, JW, DESCW, DESCW( M_ ), ONE,
$ A, I, J, DESCA, 1 )
CALL PSGEMV( 'No transpose', N-K+1, K-1, -ONE, W, IW+K-1,
$ JW, DESCW, A, I, JA, DESCA, DESCA( M_ ), ONE,
$ A, I, J, DESCA, 1 )
IF( K.GT.1 )
$ CALL PSELSET( A, I, J-1, DESCA, E( JP ) )
*
*
* Generate elementary reflector H(i) to annihilate
* A(I+2:IA+N-1,I)
*
JP = MIN( JJ+K-1, NQ )
CALL PSLARFG( N-K, E( JP ), I+1, J, A, I+2, J, DESCA, 1,
$ TAU )
CALL PSELSET( A, I+1, J, DESCA, ONE )
*
* Compute W(IW+K:IW+N-1,JW+K-1)
*
CALL PSSYMV( 'Lower', N-K, ONE, A, I+1, J+1, DESCA, A, I+1,
$ J, DESCA, 1, ZERO, W, IW+K, JW+K-1, DESCW, 1 )
*
CALL PSGEMV( 'Transpose', N-K, K-1, ONE, W, IW+K, JW, DESCW,
$ A, I+1, J, DESCA, 1, ZERO, WORK, 1, 1, DESCWK,
$ DESCWK( M_ ) )
CALL PSGEMV( 'No transpose', N-K, K-1, -ONE, A, I+1, JA,
$ DESCA, WORK, 1, 1, DESCWK, DESCWK( M_ ), ONE, W,
$ IW+K, JW+K-1, DESCW, 1 )
CALL PSGEMV( 'Transpose', N-K, K-1, ONE, A, I+1, JA, DESCA,
$ A, I+1, J, DESCA, 1, ZERO, WORK, 1, 1, DESCWK,
$ DESCWK( M_ ) )
CALL PSGEMV( 'No transpose', N-K, K-1, -ONE, W, IW+K, JW,
$ DESCW, WORK, 1, 1, DESCWK, DESCWK( M_ ), ONE, W,
$ IW+K, JW+K-1, DESCW, 1 )
CALL PSSCAL( N-K, TAU( JP ), W, IW+K, JW+K-1, DESCW, 1 )
CALL PSDOT( N-K, ALPHA, W, IW+K, JW+K-1, DESCW, 1, A, I+1,
$ J, DESCA, 1 )
IF( MYCOL.EQ.IACOL )
$ ALPHA = -HALF*TAU( JP )*ALPHA
CALL PSAXPY( N-K, ALPHA, A, I+1, J, DESCA, 1, W, IW+K,
$ JW+K-1, DESCW, 1 )
IF( MYCOL.EQ.IACOL ) THEN
CALL PSELGET( 'E', ' ', D( JP ), A, I, J, DESCA )
END IF
*
20 CONTINUE
*
END IF
*
* Broadcast columnwise the diagonal elements into D.
*
IF( MYCOL.EQ.IACOL ) THEN
IF( MYROW.EQ.IAROW ) THEN
CALL SGEBS2D( ICTXT, 'Columnwise', ' ', 1, NB, D( JJ ), 1 )
ELSE
CALL SGEBR2D( ICTXT, 'Columnwise', ' ', 1, NB, D( JJ ), 1,
$ IAROW, MYCOL )
END IF
END IF
*
RETURN
*
* End of PSLATRD
*
END
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