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Fortran
2.9 Options for code generation conventions

These machine-independent options control the interface conventions used in code
generation.

Most of them have both positive and negative forms; the negative form of -ffoo
would be -fno-foo. In the table below, only one of the forms is listedthe one
which is not the default. You can figure out the other form by either removing
no- or adding it.

-fno-automatic
    Treat each program unit (except those marked as RECURSIVE) as if the SAVE
statement were specified for every local variable and array referenced in it.
Does not affect common blocks. (Some Fortran compilers provide this option under
the name -static or -save.) The default, which is -fautomatic, uses the stack
for local variables smaller than the value given by -fmax-stack-var-size. Use
the option -frecursive to use no static memory.
-ff2c
    Generate code designed to be compatible with code generated by g77 and f2c.

    The calling conventions used by g77 (originally implemented in f2c) require
functions that return type default REAL to actually return the C type double,
and functions that return type COMPLEX to return the values via an extra
argument in the calling sequence that points to where to store the return value.
Under the default GNU calling conventions, such functions simply return their
results as they would in GNU Cdefault REAL functions return the C type float,
and COMPLEX functions return the GNU C type complex. Additionally, this option
implies the -fsecond-underscore option, unless -fno-second-underscore is
explicitly requested.

    This does not affect the generation of code that interfaces with the
libgfortran library.

    Caution: It is not a good idea to mix Fortran code compiled with -ff2c with
code compiled with the default -fno-f2c calling conventions as, calling COMPLEX
or default REAL functions between program parts which were compiled with
different calling conventions will break at execution time.

    Caution: This will break code which passes intrinsic functions of type
default REAL or COMPLEX as actual arguments, as the library implementations use
the -fno-f2c calling conventions.
-fno-underscoring
    Do not transform names of entities specified in the Fortran source file by
appending underscores to them.

    With -funderscoring in effect, GNU Fortran appends one underscore to
external names with no underscores. This is done to ensure compatibility with
code produced by many UNIX Fortran compilers.

    Caution: The default behavior of GNU Fortran is incompatible with f2c and
g77, please use the -ff2c option if you want object files compiled with GNU
Fortran to be compatible with object code created with these tools.

    Use of -fno-underscoring is not recommended unless you are experimenting
with issues such as integration of GNU Fortran into existing system environments
(vis--vis existing libraries, tools, and so on).

    For example, with -funderscoring, and assuming that j() and max_count() are
external functions while my_var and lvar are local variables, a statement like

              I = J() + MAX_COUNT (MY_VAR, LVAR)

    is implemented as something akin to:

              i = j_() + max_count__(&my_var__, &lvar);

    With -fno-underscoring, the same statement is implemented as:

              i = j() + max_count(&my_var, &lvar);

    Use of -fno-underscoring allows direct specification of user-defined names
while debugging and when interfacing GNU Fortran code with other languages.

    Note that just because the names match does not mean that the interface
implemented by GNU Fortran for an external name matches the interface
implemented by some other language for that same name. That is, getting code
produced by GNU Fortran to link to code produced by some other compiler using
this or any other method can be only a small part of the overall
solutiongetting the code generated by both compilers to agree on issues other
than naming can require significant effort, and, unlike naming disagreements,
linkers normally cannot detect disagreements in these other areas.

    Also, note that with -fno-underscoring, the lack of appended underscores
introduces the very real possibility that a user-defined external name will
conflict with a name in a system library, which could make finding
unresolved-reference bugs quite difficult in some casesthey might occur at
program run time, and show up only as buggy behavior at run time.

    In future versions of GNU Fortran we hope to improve naming and linking
issues so that debugging always involves using the names as they appear in the
source, even if the names as seen by the linker are mangled to prevent
accidental linking between procedures with incompatible interfaces.
-fsecond-underscore
    By default, GNU Fortran appends an underscore to external names. If this
option is used GNU Fortran appends two underscores to names with underscores and
one underscore to external names with no underscores. GNU Fortran also appends
two underscores to internal names with underscores to avoid naming collisions
with external names.

    This option has no effect if -fno-underscoring is in effect. It is implied
by the -ff2c option.

    Otherwise, with this option, an external name such as MAX_COUNT is
implemented as a reference to the link-time external symbol max_count__, instead
of max_count_. This is required for compatibility with g77 and f2c, and is
implied by use of the -ff2c option.
-fcoarray=<keyword>

    none
        Disable coarray support; using coarray declarations and image-control
statements will produce a compile-time error. (Default)
    single
        Single-image mode, i.e. num_images() is always one.
    lib
        Library-based coarray parallelization; a suitable GNU Fortran coarray
library needs to be linked. 


-fcheck=<keyword>
    Enable the generation of run-time checks; the argument shall be a
comma-delimited list of the following keywords. Prefixing a check with no-
disables it if it was activated by a previous specification.

    all
        Enable all run-time test of -fcheck.
    array-temps
        Warns at run time when for passing an actual argument a temporary array
had to be generated. The information generated by this warning is sometimes
useful in optimization, in order to avoid such temporaries.

        Note: The warning is only printed once per location.
    bounds
        Enable generation of run-time checks for array subscripts and against
the declared minimum and maximum values. It also checks array indices for
assumed and deferred shape arrays against the actual allocated bounds and
ensures that all string lengths are equal for character array constructors
without an explicit typespec.

        Some checks require that -fcheck=bounds is set for the compilation of
the main program.

        Note: In the future this may also include other forms of checking, e.g.,
checking substring references.
    do
        Enable generation of run-time checks for invalid modification of loop
iteration variables.
    mem
        Enable generation of run-time checks for memory allocation. Note: This
option does not affect explicit allocations using the ALLOCATE statement, which
will be always checked.
    pointer
        Enable generation of run-time checks for pointers and allocatables.
    recursion
        Enable generation of run-time checks for recursively called subroutines
and functions which are not marked as recursive. See also -frecursive. Note:
This check does not work for OpenMP programs and is disabled if used together
with -frecursive and -fopenmp. 

    Example: Assuming you have a file foo.f90, the command

                gfortran -fcheck=all,no-array-temps foo.f90

    will compile the file with all checks enabled as specified above except
warnings for generated array temporaries.
-fbounds-check
    Deprecated alias for -fcheck=bounds.
-fcheck-array-temporaries
    Deprecated alias for -fcheck=array-temps.
-fmax-array-constructor=n
    This option can be used to increase the upper limit permitted in array
constructors. The code below requires this option to expand the array at compile
time.

              program test
              implicit none
              integer j
              integer, parameter :: n = 100000
              integer, parameter :: i(n) = (/ (2*j, j = 1, n) /)
              print '(10(I0,1X))', i
              end program test

    Caution: This option can lead to long compile times and excessively large
object files.

    The default value for n is 65535.
-fmax-stack-var-size=n
    This option specifies the size in bytes of the largest array that will be
put on the stack; if the size is exceeded static memory is used (except in
procedures marked as RECURSIVE). Use the option -frecursive to allow for
recursive procedures which do not have a RECURSIVE attribute or for parallel
programs. Use -fno-automatic to never use the stack.

    This option currently only affects local arrays declared with constant
bounds, and may not apply to all character variables. Future versions of GNU
Fortran may improve this behavior.

    The default value for n is 32768.
-fstack-arrays
    Adding this option will make the Fortran compiler put all local arrays, even
those of unknown size onto stack memory. If your program uses very large local
arrays it is possible that you will have to extend your runtime limits for stack
memory on some operating systems. This flag is enabled by default at
optimization level -Ofast.
-fpack-derived
    This option tells GNU Fortran to pack derived type members as closely as
possible. Code compiled with this option is likely to be incompatible with code
compiled without this option, and may execute slower.
-frepack-arrays
    In some circumstances GNU Fortran may pass assumed shape array sections via
a descriptor describing a noncontiguous area of memory. This option adds code to
the function prologue to repack the data into a contiguous block at runtime.

    This should result in faster accesses to the array. However it can introduce
significant overhead to the function call, especially when the passed data is
noncontiguous.
-fshort-enums
    This option is provided for interoperability with C code that was compiled
with the -fshort-enums option. It will make GNU Fortran choose the smallest
INTEGER kind a given enumerator set will fit in, and give all its enumerators
this kind.
-fexternal-blas
    This option will make gfortran generate calls to BLAS functions for some
matrix operations like MATMUL, instead of using our own algorithms, if the size
of the matrices involved is larger than a given limit (see -fblas-matmul-limit).
This may be profitable if an optimized vendor BLAS library is available. The
BLAS library will have to be specified at link time.
-fblas-matmul-limit=n
    Only significant when -fexternal-blas is in effect. Matrix multiplication of
matrices with size larger than (or equal to) n will be performed by calls to
BLAS functions, while others will be handled by gfortran internal algorithms. If
the matrices involved are not square, the size comparison is performed using the
geometric mean of the dimensions of the argument and result matrices.

    The default value for n is 30.
-finline-matmul-limit=n
    When front-end optimiztion is active, some calls to the MATMUL intrinsic
function will be inlined. This may result in code size increase if the size of
the matrix cannot be determined at compile time, as code for both cases is
generated. Setting -finline-matmul-limit=0 will disable inlining in all cases.
Setting this option with a value of n will produce inline code for matrices with
size up to n. If the matrices involved are not square, the size comparison is
performed using the geometric mean of the dimensions of the argument and result
matrices.

    The default value for n is the value specified for -fblas-matmul-limit if
this option is specified, or unlimitited otherwise.
-frecursive
    Allow indirect recursion by forcing all local arrays to be allocated on the
stack. This flag cannot be used together with -fmax-stack-var-size= or
-fno-automatic.
-finit-local-zero
-finit-derived
-finit-integer=n
-finit-real=<zero|inf|-inf|nan|snan>
-finit-logical=<true|false>
-finit-character=n
    The -finit-local-zero option instructs the compiler to initialize local
INTEGER, REAL, and COMPLEX variables to zero, LOGICAL variables to false, and
CHARACTER variables to a string of null bytes. Finer-grained initialization
options are provided by the -finit-integer=n,
-finit-real=<zero|inf|-inf|nan|snan> (which also initializes the real and
imaginary parts of local COMPLEX variables), -finit-logical=<true|false>, and
-finit-character=n (where n is an ASCII character value) options. Components of
derived type variables will be initialized according to these flags only with
-finit-derived. These options do not initialize

        allocatable arrays
        variables that appear in an EQUIVALENCE statement. 

    (These limitations may be removed in future releases).

    Note that the -finit-real=nan option initializes REAL and COMPLEX variables
with a quiet NaN. For a signalling NaN use -finit-real=snan; note, however, that
compile-time optimizations may convert them into quiet NaN and that trapping
needs to be enabled (e.g. via -ffpe-trap).

    Finally, note that enabling any of the -finit-* options will silence
warnings that would have been emitted by -Wuninitialized for the affected local
variables.
-falign-commons
    By default, gfortran enforces proper alignment of all variables in a COMMON
block by padding them as needed. On certain platforms this is mandatory, on
others it increases performance. If a COMMON block is not declared with
consistent data types everywhere, this padding can cause trouble, and
-fno-align-commons can be used to disable automatic alignment. The same form of
this option should be used for all files that share a COMMON block. To avoid
potential alignment issues in COMMON blocks, it is recommended to order objects
from largest to smallest.
-fno-protect-parens
    By default the parentheses in expression are honored for all optimization
levels such that the compiler does not do any re-association. Using
-fno-protect-parens allows the compiler to reorder REAL and COMPLEX expressions
to produce faster code. Note that for the re-association optimization
-fno-signed-zeros and -fno-trapping-math need to be in effect. The parentheses
protection is enabled by default, unless -Ofast is given.
-frealloc-lhs
    An allocatable left-hand side of an intrinsic assignment is automatically
(re)allocated if it is either unallocated or has a different shape. The option
is enabled by default except when -std=f95 is given. See also -Wrealloc-lhs.
-faggressive-function-elimination
    Functions with identical argument lists are eliminated within statements,
regardless of whether these functions are marked PURE or not. For example, in

                a = f(b,c) + f(b,c)

    there will only be a single call to f. This option only works if
-ffrontend-optimize is in effect.
-ffrontend-optimize
    This option performs front-end optimization, based on manipulating parts the
Fortran parse tree. Enabled by default by any -O option. Optimizations enabled
by this option include inlining calls to MATMUL, elimination of identical
function calls within expressions, removing unnecessary calls to TRIM in
comparisons and assignments and replacing TRIM(a) with a(1:LEN_TRIM(a)). It can
be deselected by specifying -fno-frontend-optimize. 

See Options for Code Generation Conventions, fo