ilaenv(3S)

NAME
     ILAENV - i called from the LAPACK routines to choose problem-dependent
     parameters for the local environment

SYNOPSIS
     INTEGER FUNCTION ILAENV( ISPEC, NAME, OPTS, N1, N2, N3, N4 )

         CHARACTER*(  * ) NAME, OPTS

         INTEGER      ISPEC, N1, N2, N3, N4

IMPLEMENTATION
     These routines are part of the SCSL Scientific Library and can be loaded
     using either the -lscs or the -lscs_mp option.  The -lscs_mp option
     directs the linker to use the multi-processor version of the library.

     When linking to SCSL with -lscs or -lscs_mp, the default integer size is
     4 bytes (32 bits). Another version of SCSL is available in which integers
     are 8 bytes (64 bits).  This version allows the user access to larger
     memory sizes and helps when porting legacy Cray codes.  It can be loaded
     by using the -lscs_i8 option or the -lscs_i8_mp option. A program may use
     only one of the two versions; 4-byte integer and 8-byte integer library
     calls cannot be mixed.

PURPOSE
     ILAENV is called from the LAPACK routines to choose problem-dependent
     parameters for the local environment. See ISPEC for a description of the
     parameters.

     This version provides a set of parameters which should give good, but not
     optimal, performance on many of the currently available computers.  Users
     are encouraged to modify this subroutine to set the tuning parameters for
     their particular machine using the option and problem size information in
     the arguments.

     This routine will not function correctly if it is converted to all lower
     case.  Converting it to all upper case is allowed.


ARGUMENTS
     ISPEC   (input) INTEGER
             Specifies the parameter to be returned as the value of ILAENV.  =
             1: the optimal blocksize; if this value is 1, an unblocked
             algorithm will give the best performance.  = 2: the minimum block
             size for which the block routine should be used; if the usable
             block size is less than this value, an unblocked routine should
             be used.  = 3: the crossover point (in a block routine, for N
             less than this value, an unblocked routine should be used) = 4:
             the number of shifts, used in the nonsymmetric eigenvalue
             routines = 5: the minimum column dimension for blocking to be
             used; rectangular blocks must have dimension at least k by m,
             where k is given by ILAENV(2,...) and m by ILAENV(5,...)  = 6:
             the crossover point for the SVD (when reducing an m by n matrix
             to bidiagonal form, if max(m,n)/min(m,n) exceeds this value, a QR
             factorization is used first to reduce the matrix to a triangular
             form.)  = 7: the number of processors
             = 8: the crossover point for the multishift QR and QZ methods for
             nonsymmetric eigenvalue problems.  = 9: maximum size of the
             subproblems at the bottom of the computation tree in the divide-
             and-conquer algorithm (used by xGELSD and xGESDD) =10: ieee NaN
             arithmetic can be trusted not to trap
             =11: infinity arithmetic can be trusted not to trap

     NAME    (input) CHARACTER*(*)
             The name of the calling subroutine, in either upper case or lower
             case.

     OPTS    (input) CHARACTER*(*)
             The character options to the subroutine NAME, concatenated into a
             single character string.  For example, UPLO = 'U', TRANS = 'T',
             and DIAG = 'N' for a triangular routine would be specified as
             OPTS = 'UTN'.

     N1      (input) INTEGER
             N2      (input) INTEGER N3      (input) INTEGER N4      (input)
             INTEGER Problem dimensions for the subroutine NAME; these may not
             all be required.

             >= 0: the value of the parameter specified by ISPEC < 0:  if
             ILAENV = -k, the k-th argument had an illegal value.

FURTHER DETAILS
     The following conventions have been used when calling ILAENV from the
     LAPACK routines:
     1)  OPTS is a concatenation of all of the character options to
         subroutine NAME, in the same order that they appear in the
         argument list for NAME, even if they are not used in determining
         the value of the parameter specified by ISPEC.
     2)  The problem dimensions N1, N2, N3, N4 are specified in the order
         that they appear in the argument list for NAME.  N1 is used
         first, N2 second, and so on, and unused problem dimensions are
         passed a value of -1.
     3)  The parameter value returned by ILAENV is checked for validity in
         the calling subroutine.  For example, ILAENV is used to retrieve
         the optimal blocksize for STRTRI as follows:

         NB = ILAENV( 1, 'STRTRI', UPLO // DIAG, N, -1, -1, -1 )
         IF( NB.LE.1 ) NB = MAX( 1, N )


SEE ALSO
     INTRO_LAPACK(3S), INTRO_SCSL(3S)

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