LAPACK 3.3.1 Linear Algebra PACKage

# VARIANTS/lu/CR/sgetrf.f

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```00001       SUBROUTINE SGETRF ( M, N, A, LDA, IPIV, INFO)
00002 *
00003 *  -- LAPACK routine (version 3.1) --
00004 *     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
00005 *     March 2008
00006 *
00007 *     .. Scalar Arguments ..
00008       INTEGER            INFO, LDA, M, N
00009 *     ..
00010 *     .. Array Arguments ..
00011       INTEGER            IPIV( * )
00012       REAL               A( LDA, * )
00013 *     ..
00014 *
00015 *  Purpose
00016 *  =======
00017 *
00018 *  SGETRF computes an LU factorization of a general M-by-N matrix A
00019 *  using partial pivoting with row interchanges.
00020 *
00021 *  The factorization has the form
00022 *     A = P * L * U
00023 *  where P is a permutation matrix, L is lower triangular with unit
00024 *  diagonal elements (lower trapezoidal if m > n), and U is upper
00025 *  triangular (upper trapezoidal if m < n).
00026 *
00027 *  This is the Crout Level 3 BLAS version of the algorithm.
00028 *
00029 *  Arguments
00030 *  =========
00031 *
00032 *  M       (input) INTEGER
00033 *          The number of rows of the matrix A.  M >= 0.
00034 *
00035 *  N       (input) INTEGER
00036 *          The number of columns of the matrix A.  N >= 0.
00037 *
00038 *  A       (input/output) REAL array, dimension (LDA,N)
00039 *          On entry, the M-by-N matrix to be factored.
00040 *          On exit, the factors L and U from the factorization
00041 *          A = P*L*U; the unit diagonal elements of L are not stored.
00042 *
00043 *  LDA     (input) INTEGER
00044 *          The leading dimension of the array A.  LDA >= max(1,M).
00045 *
00046 *  IPIV    (output) INTEGER array, dimension (min(M,N))
00047 *          The pivot indices; for 1 <= i <= min(M,N), row i of the
00048 *          matrix was interchanged with row IPIV(i).
00049 *
00050 *  INFO    (output) INTEGER
00051 *          = 0:  successful exit
00052 *          < 0:  if INFO = -i, the i-th argument had an illegal value
00053 *          > 0:  if INFO = i, U(i,i) is exactly zero. The factorization
00054 *                has been completed, but the factor U is exactly
00055 *                singular, and division by zero will occur if it is used
00056 *                to solve a system of equations.
00057 *
00058 *  =====================================================================
00059 *
00060 *     .. Parameters ..
00061       REAL               ONE
00062       PARAMETER          ( ONE = 1.0E+0 )
00063 *     ..
00064 *     .. Local Scalars ..
00065       INTEGER            I, IINFO, J, JB, NB
00066 *     ..
00067 *     .. External Subroutines ..
00068       EXTERNAL           SGEMM, SGETF2, SLASWP, STRSM, XERBLA
00069 *     ..
00070 *     .. External Functions ..
00071       INTEGER            ILAENV
00072       EXTERNAL           ILAENV
00073 *     ..
00074 *     .. Intrinsic Functions ..
00075       INTRINSIC          MAX, MIN, MOD
00076 *     ..
00077 *     .. Executable Statements ..
00078 *
00079 *     Test the input parameters.
00080 *
00081       INFO = 0
00082       IF( M.LT.0 ) THEN
00083          INFO = -1
00084       ELSE IF( N.LT.0 ) THEN
00085          INFO = -2
00086       ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
00087          INFO = -4
00088       END IF
00089       IF( INFO.NE.0 ) THEN
00090          CALL XERBLA( 'SGETRF', -INFO )
00091          RETURN
00092       END IF
00093 *
00094 *     Quick return if possible
00095 *
00096       IF( M.EQ.0 .OR. N.EQ.0 )
00097      \$   RETURN
00098 *
00099 *     Determine the block size for this environment.
00100 *
00101       NB = ILAENV( 1, 'SGETRF', ' ', M, N, -1, -1 )
00102       IF( NB.LE.1 .OR. NB.GE.MIN( M, N ) ) THEN
00103 *
00104 *        Use unblocked code.
00105 *
00106          CALL SGETF2( M, N, A, LDA, IPIV, INFO )
00107       ELSE
00108 *
00109 *        Use blocked code.
00110 *
00111          DO 20 J = 1, MIN( M, N ), NB
00112             JB = MIN( MIN( M, N )-J+1, NB )
00113 *
00114 *           Update current block.
00115 *
00116             CALL SGEMM( 'No transpose', 'No transpose',
00117      \$                 M-J+1, JB, J-1, -ONE,
00118      \$                 A( J, 1 ), LDA, A( 1, J ), LDA, ONE,
00119      \$                 A( J, J ), LDA )
00120
00121 *
00122 *           Factor diagonal and subdiagonal blocks and test for exact
00123 *           singularity.
00124 *
00125             CALL SGETF2( M-J+1, JB, A( J, J ), LDA, IPIV( J ), IINFO )
00126 *
00127 *           Adjust INFO and the pivot indices.
00128 *
00129             IF( INFO.EQ.0 .AND. IINFO.GT.0 )
00130      \$         INFO = IINFO + J - 1
00131             DO 10 I = J, MIN( M, J+JB-1 )
00132                IPIV( I ) = J - 1 + IPIV( I )
00133    10       CONTINUE
00134 *
00135 *           Apply interchanges to column 1:J-1
00136 *
00137             CALL SLASWP( J-1, A, LDA, J, J+JB-1, IPIV, 1 )
00138 *
00139             IF ( J+JB.LE.N ) THEN
00140 *
00141 *              Apply interchanges to column J+JB:N
00142 *
00143                CALL SLASWP( N-J-JB+1, A( 1, J+JB ), LDA, J, J+JB-1,
00144      \$                     IPIV, 1 )
00145 *
00146                CALL SGEMM( 'No transpose', 'No transpose',
00147      \$                    JB, N-J-JB+1, J-1, -ONE,
00148      \$                    A( J, 1 ), LDA, A( 1, J+JB ), LDA, ONE,
00149      \$                    A( J, J+JB ), LDA )
00150 *
00151 *              Compute block row of U.
00152 *
00153                CALL STRSM( 'Left', 'Lower', 'No transpose', 'Unit',
00154      \$                    JB, N-J-JB+1, ONE, A( J, J ), LDA,
00155      \$                    A( J, J+JB ), LDA )
00156             END IF
00157
00158    20    CONTINUE
00159
00160       END IF
00161       RETURN
00162 *
00163 *     End of SGETRF
00164 *
00165       END
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