ILAENV - i called from the LAPACK routines to choose prob
lem-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
PURPOSE
ILAENV is called from the LAPACK routines to choose prob
lem-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 con
verted 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 non
symmetric eigenvalue routines = 5: the minimum
column dimension for blocking to be used; rectan
gular 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
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, con
catenated 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 subrou
tine 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 dimen
sions 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 )
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