LAPACK  3.6.1 LAPACK: Linear Algebra PACKage
sgetrf.f
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1 C> \brief \b SGETRF VARIANT: iterative version of Sivan Toledo's recursive LU algorithm
2 *
3 * =========== DOCUMENTATION ===========
4 *
5 * Online html documentation available at
6 * http://www.netlib.org/lapack/explore-html/
7 *
8 * Definition:
9 * ===========
10 *
11 * SUBROUTINE SGETRF( M, N, A, LDA, IPIV, INFO )
12 *
13 * .. Scalar Arguments ..
14 * INTEGER INFO, LDA, M, N
15 * ..
16 * .. Array Arguments ..
17 * INTEGER IPIV( * )
18 * REAL A( LDA, * )
19 * ..
20 *
21 * Purpose
22 * =======
23 *
24 C>\details \b Purpose:
25 C>\verbatim
26 C>
27 C> SGETRF computes an LU factorization of a general M-by-N matrix A
28 C> using partial pivoting with row interchanges.
29 C>
30 C> The factorization has the form
31 C> A = P * L * U
32 C> where P is a permutation matrix, L is lower triangular with unit
33 C> diagonal elements (lower trapezoidal if m > n), and U is upper
34 C> triangular (upper trapezoidal if m < n).
35 C>
36 C> This code implements an iterative version of Sivan Toledo's recursive
37 C> LU algorithm[1]. For square matrices, this iterative versions should
38 C> be within a factor of two of the optimum number of memory transfers.
39 C>
40 C> The pattern is as follows, with the large blocks of U being updated
41 C> in one call to STRSM, and the dotted lines denoting sections that
42 C> have had all pending permutations applied:
43 C>
44 C> 1 2 3 4 5 6 7 8
45 C> +-+-+---+-------+------
46 C> | |1| | |
47 C> |.+-+ 2 | |
48 C> | | | | |
49 C> |.|.+-+-+ 4 |
50 C> | | | |1| |
51 C> | | |.+-+ |
52 C> | | | | | |
53 C> |.|.|.|.+-+-+---+ 8
54 C> | | | | | |1| |
55 C> | | | | |.+-+ 2 |
56 C> | | | | | | | |
57 C> | | | | |.|.+-+-+
58 C> | | | | | | | |1|
59 C> | | | | | | |.+-+
60 C> | | | | | | | | |
61 C> |.|.|.|.|.|.|.|.+-----
62 C> | | | | | | | | |
63 C>
64 C> The 1-2-1-4-1-2-1-8-... pattern is the position of the last 1 bit in
65 C> the binary expansion of the current column. Each Schur update is
66 C> applied as soon as the necessary portion of U is available.
67 C>
68 C> [1] Toledo, S. 1997. Locality of Reference in LU Decomposition with
69 C> Partial Pivoting. SIAM J. Matrix Anal. Appl. 18, 4 (Oct. 1997),
70 C> 1065-1081. http://dx.doi.org/10.1137/S0895479896297744
71 C>
72 C>\endverbatim
73 *
74 * Arguments:
75 * ==========
76 *
77 C> \param[in] M
78 C> \verbatim
79 C> M is INTEGER
80 C> The number of rows of the matrix A. M >= 0.
81 C> \endverbatim
82 C>
83 C> \param[in] N
84 C> \verbatim
85 C> N is INTEGER
86 C> The number of columns of the matrix A. N >= 0.
87 C> \endverbatim
88 C>
89 C> \param[in,out] A
90 C> \verbatim
91 C> A is REAL array, dimension (LDA,N)
92 C> On entry, the M-by-N matrix to be factored.
93 C> On exit, the factors L and U from the factorization
94 C> A = P*L*U; the unit diagonal elements of L are not stored.
95 C> \endverbatim
96 C>
97 C> \param[in] LDA
98 C> \verbatim
99 C> LDA is INTEGER
100 C> The leading dimension of the array A. LDA >= max(1,M).
101 C> \endverbatim
102 C>
103 C> \param[out] IPIV
104 C> \verbatim
105 C> IPIV is INTEGER array, dimension (min(M,N))
106 C> The pivot indices; for 1 <= i <= min(M,N), row i of the
107 C> matrix was interchanged with row IPIV(i).
108 C> \endverbatim
109 C>
110 C> \param[out] INFO
111 C> \verbatim
112 C> INFO is INTEGER
113 C> = 0: successful exit
114 C> < 0: if INFO = -i, the i-th argument had an illegal value
115 C> > 0: if INFO = i, U(i,i) is exactly zero. The factorization
116 C> has been completed, but the factor U is exactly
117 C> singular, and division by zero will occur if it is used
118 C> to solve a system of equations.
119 C> \endverbatim
120 C>
121 *
122 * Authors:
123 * ========
124 *
125 C> \author Univ. of Tennessee
126 C> \author Univ. of California Berkeley
127 C> \author Univ. of Colorado Denver
128 C> \author NAG Ltd.
129 *
130 C> \date November 2011
131 *
132 C> \ingroup variantsGEcomputational
133 *
134 * =====================================================================
135  SUBROUTINE sgetrf( M, N, A, LDA, IPIV, INFO )
136 *
137 * -- LAPACK computational routine (version 3.X) --
138 * -- LAPACK is a software package provided by Univ. of Tennessee, --
139 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
140 * November 2011
141 *
142 * .. Scalar Arguments ..
143  INTEGER INFO, LDA, M, N
144 * ..
145 * .. Array Arguments ..
146  INTEGER IPIV( * )
147  REAL A( lda, * )
148 * ..
149 *
150 * =====================================================================
151 *
152 * .. Parameters ..
153  REAL ONE, ZERO, NEGONE
154  parameter ( one = 1.0e+0, zero = 0.0e+0 )
155  parameter ( negone = -1.0e+0 )
156 * ..
157 * .. Local Scalars ..
158  REAL SFMIN, TMP
159  INTEGER I, J, JP, NSTEP, NTOPIV, NPIVED, KAHEAD
160  INTEGER KSTART, IPIVSTART, JPIVSTART, KCOLS
161 * ..
162 * .. External Functions ..
163  REAL SLAMCH
164  INTEGER ISAMAX
165  LOGICAL SISNAN
166  EXTERNAL slamch, isamax, sisnan
167 * ..
168 * .. External Subroutines ..
169  EXTERNAL strsm, sscal, xerbla, slaswp
170 * ..
171 * .. Intrinsic Functions ..
172  INTRINSIC max, min, iand
173 * ..
174 * .. Executable Statements ..
175 *
176 * Test the input parameters.
177 *
178  info = 0
179  IF( m.LT.0 ) THEN
180  info = -1
181  ELSE IF( n.LT.0 ) THEN
182  info = -2
183  ELSE IF( lda.LT.max( 1, m ) ) THEN
184  info = -4
185  END IF
186  IF( info.NE.0 ) THEN
187  CALL xerbla( 'SGETRF', -info )
188  RETURN
189  END IF
190 *
191 * Quick return if possible
192 *
193  IF( m.EQ.0 .OR. n.EQ.0 )
194  \$ RETURN
195 *
196 * Compute machine safe minimum
197 *
198  sfmin = slamch( 'S' )
199 *
200  nstep = min( m, n )
201  DO j = 1, nstep
202  kahead = iand( j, -j )
203  kstart = j + 1 - kahead
204  kcols = min( kahead, m-j )
205 *
206 * Find pivot.
207 *
208  jp = j - 1 + isamax( m-j+1, a( j, j ), 1 )
209  ipiv( j ) = jp
210
211 ! Permute just this column.
212  IF (jp .NE. j) THEN
213  tmp = a( j, j )
214  a( j, j ) = a( jp, j )
215  a( jp, j ) = tmp
216  END IF
217
218 ! Apply pending permutations to L
219  ntopiv = 1
220  ipivstart = j
221  jpivstart = j - ntopiv
222  DO WHILE ( ntopiv .LT. kahead )
223  CALL slaswp( ntopiv, a( 1, jpivstart ), lda, ipivstart, j,
224  \$ ipiv, 1 )
225  ipivstart = ipivstart - ntopiv;
226  ntopiv = ntopiv * 2;
227  jpivstart = jpivstart - ntopiv;
228  END DO
229
230 ! Permute U block to match L
231  CALL slaswp( kcols, a( 1,j+1 ), lda, kstart, j, ipiv, 1 )
232
233 ! Factor the current column
234  IF( a( j, j ).NE.zero .AND. .NOT.sisnan( a( j, j ) ) ) THEN
235  IF( abs(a( j, j )) .GE. sfmin ) THEN
236  CALL sscal( m-j, one / a( j, j ), a( j+1, j ), 1 )
237  ELSE
238  DO i = 1, m-j
239  a( j+i, j ) = a( j+i, j ) / a( j, j )
240  END DO
241  END IF
242  ELSE IF( a( j,j ) .EQ. zero .AND. info .EQ. 0 ) THEN
243  info = j
244  END IF
245
246 ! Solve for U block.
247  CALL strsm( 'Left', 'Lower', 'No transpose', 'Unit', kahead,
248  \$ kcols, one, a( kstart, kstart ), lda,
249  \$ a( kstart, j+1 ), lda )
250 ! Schur complement.
251  CALL sgemm( 'No transpose', 'No transpose', m-j,
252  \$ kcols, kahead, negone, a( j+1, kstart ), lda,
253  \$ a( kstart, j+1 ), lda, one, a( j+1, j+1 ), lda )
254  END DO
255
256 ! Handle pivot permutations on the way out of the recursion
257  npived = iand( nstep, -nstep )
258  j = nstep - npived
259  DO WHILE ( j .GT. 0 )
260  ntopiv = iand( j, -j )
261  CALL slaswp( ntopiv, a( 1, j-ntopiv+1 ), lda, j+1, nstep,
262  \$ ipiv, 1 )
263  j = j - ntopiv
264  END DO
265
266 ! If short and wide, handle the rest of the columns.
267  IF ( m .LT. n ) THEN
268  CALL slaswp( n-m, a( 1, m+kcols+1 ), lda, 1, m, ipiv, 1 )
269  CALL strsm( 'Left', 'Lower', 'No transpose', 'Unit', m,
270  \$ n-m, one, a, lda, a( 1,m+kcols+1 ), lda )
271  END IF
272
273  RETURN
274 *
275 * End of SGETRF
276 *
277  END
subroutine strsm(SIDE, UPLO, TRANSA, DIAG, M, N, ALPHA, A, LDA, B, LDB)
STRSM
Definition: strsm.f:183
subroutine sgemm(TRANSA, TRANSB, M, N, K, ALPHA, A, LDA, B, LDB, BETA, C, LDC)
SGEMM
Definition: sgemm.f:189
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:62
subroutine slaswp(N, A, LDA, K1, K2, IPIV, INCX)
SLASWP performs a series of row interchanges on a general rectangular matrix.
Definition: slaswp.f:116
subroutine sgetrf(M, N, A, LDA, IPIV, INFO)
SGETRF
Definition: sgetrf.f:110
subroutine sscal(N, SA, SX, INCX)
SSCAL
Definition: sscal.f:55