Code Gen Options (The GNU Fortran Compiler)

### 2.10 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 listed—the one that 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.

  Local variables or arrays having an explicit `SAVE` attribute are silently ignored unless the `-pedantic` option is added.

`-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 C—default `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 that were compiled with different calling conventions will break at execution time.

  Caution: This breaks code that 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. This is done to ensure compatibility with code produced by many UNIX Fortran compilers. Note this does not apply to names declared with C binding, or within a module.

  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-à-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 Fortran statement like

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

  is implemented as something akin to the C code:

      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 solution—getting 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 conflicts with a name in a system library, which could make finding unresolved-reference bugs quite difficult in some cases—they might occur at program run time, and show up only as buggy behavior at run time.

  See Naming and argument-passing conventions, for more information. Also note that declaring symbols as `bind(C)` is a more robust way to interface with code written in other languages or compiled with different Fortran compilers than the command-line options documented in this section.

`-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 names with no underscores.

  For example, 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.

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

`-fcoarray=<keyword>` ¶
  ‘`none`’

    Disable coarray support; using coarray declarations and image-control statements produces 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 such as http://opencoarrays.org needs to be linked. Alternatively, GCC’s `libcaf_single` library can be linked, albeit it only supports a single image.

`-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.

  ‘`bits`’

    Enable generation of run-time checks for invalid arguments to the bit manipulation intrinsics.

  ‘`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 are 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 that 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

  compiles the file with all checks enabled as specified above except warnings for generated array temporaries.

`-fbounds-check` ¶

  Deprecated alias for `-fcheck=bounds`.

`-ftail-call-workaround` ¶
`-ftail-call-workaround=n`

  Some C interfaces to Fortran codes violate the gfortran ABI by omitting the hidden character length arguments as described in See Argument passing conventions. This can lead to crashes because pushing arguments for tail calls can overflow the stack.

  To provide a workaround for existing binary packages, this option disables tail call optimization for gfortran procedures with character arguments. With `-ftail-call-workaround=2` tail call optimization is disabled in all gfortran procedures with character arguments, with `-ftail-call-workaround=1` or equivalent `-ftail-call-workaround` only in gfortran procedures with character arguments that call implicitly prototyped procedures.

  Using this option can lead to problems including crashes due to insufficient stack space.

  It is very strongly recommended to fix the code in question. The `-fc-prototypes-external` option can be used to generate prototypes that conform to gfortran’s ABI, for inclusion in the source code.

  Support for this option will likely be withdrawn in a future release of gfortran.

  The negative form, `-fno-tail-call-workaround` or equivalent `-ftail-call-workaround=0`, can be used to disable this option.

  Default is currently `-ftail-call-workaround`, this will change in future releases.

`-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 is 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 that 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 65536.

`-fstack-arrays` ¶

  Adding this option makes the Fortran compiler put all arrays of unknown size and array temporaries onto stack memory. If your program uses very large local arrays it is possible that you have to extend your runtime limits for stack memory on some operating systems. This flag is enabled by default at optimization level `-Ofast` unless `-fmax-stack-var-size` is specified.

`-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 makes GNU Fortran choose the smallest `INTEGER` kind a given enumerator set fits in, and give all its enumerators this kind.

`-finline-arg-packing` ¶

  When passing an assumed-shape argument of a procedure as actual argument to an assumed-size or explicit size or as argument to a procedure that does not have an explicit interface, the argument may have to be packed; that is, put into contiguous memory. An example is the call to `foo` in

        subroutine foo(a)
           real, dimension(*) :: a
        end subroutine foo
        subroutine bar(b)
           real, dimension(:) :: b
           call foo(b)
        end subroutine bar

  When `-finline-arg-packing` is in effect, this packing is performed by inline code. This allows for more optimization while increasing code size.

  `-finline-arg-packing` is implied by any of the `-O` options except when optimizing for size via `-Os`. If the code contains a very large number of argument that have to be packed, code size and also compilation time may become excessive. If that is the case, it may be better to disable this option. Instances of packing can be found by using `-Warray-temporaries`.

`-fexternal-blas` ¶

  This option makes `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 has to be specified at link time. This option specifies a BLAS library with integer arguments of default kind (32 bits). It cannot be used together with `-fexternal-blas64`.

`-fexternal-blas64` ¶

  makes `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 has to be specified at link time. This option specifies a BLAS library with integer arguments of `KIND=8` (64 bits). It cannot be used together with `-fexternal-blas`, and requires a 64-bit system. This option also requires `-ffrontend-optimize`.

`-fblas-matmul-limit=n` ¶

  Only significant when `-fexternal-blas` or `-fexternal-blas64` are in effect. Matrix multiplication of matrices with size larger than or equal to n is performed by calls to BLAS functions, while others are 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-intrinsics` ¶
`-finline-intrinsics=intr1,intr2,...`

  Prefer generating inline code over calls to libgfortran functions to implement intrinsics.

  Usage of intrinsics can be implemented either by generating a call to the libgfortran library function or by directly generating inline code. For most intrinsics, only a single variant is available, and there is no choice of implementation. However, some intrinsics can use a library function or inline code, where inline code typically offers opportunities for additional optimization over a library function. With `-finline-intrinsics=...` or `-fno-inline-intrinsics=...`, the choice applies only to the intrinsics present in the comma-separated list provided as argument.

  For each intrinsic, if no choice of implementation was made through either of the flag variants, a default behavior is chosen depending on optimization: library calls are generated when not optimizing or when optimizing for size; otherwise inline code is preferred.

  The set of intrinsics allowed as argument to `-finline-intrinsics=` is currently limited to `MAXLOC` and `MINLOC`. The effect of the flag is moreover limited to calls of those intrinsics without `DIM` argument and with `ARRAY` of a non-`CHARACTER` type. The case of rank-1 argument and `DIM` argument present, i.e. `MAXLOC(A(:),DIM=1)` or `MINLOC(A(:),DIM=1)` is inlined unconditionally for numeric rank-1 array argument `A`.

`-finline-matmul-limit=n` ¶

  When front-end optimization is active, some calls to the `MATMUL` intrinsic function are 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` disables inlining in all cases. Setting this option with a value of n produces 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 30. The `-fblas-matmul-limit` can be used to change this value.

`-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.

  With `-finit-derived`, components of derived type variables are initialized according to these flags. Components whose type is not covered by an explicit `-finit-*` flag are treated as described above with `-finit-local-zero`.

  These options do not initialize

  - objects with the `POINTER` attribute
  - 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`).

  The `-finit-integer` option parses the value into an integer of type `INTEGER(kind=C_LONG)` on the host. Said value is then assigned to the integer variables in the Fortran code, which might result in wraparound if the value is too large for the kind.

  Finally, note that enabling any of the `-finit-*` options silences 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 reassociation. Using `-fno-protect-parens` allows the compiler to reorder `REAL` and `COMPLEX` expressions to produce faster code. Note that for the reassociation 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 is only 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 of the Fortran parse tree. Enabled by default by any `-O` option except `-O0` and `-Og`. 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,
  - replacing `TRIM(a)` with `a(1:LEN_TRIM(a))` and
  - short-circuiting of logical operators (`.AND.` and `.OR.`).

  It can be deselected by specifying `-fno-frontend-optimize`.

`-ffrontend-loop-interchange` ¶

  Attempt to interchange loops in the Fortran front end where profitable. Enabled by default by any `-O` option. At the moment, this option only affects `FORALL` and `DO CONCURRENT` statements with several forall triplets.

See Options for Code Generation Conventions in Using the GNU Compiler Collection (GCC), for information on more options offered by the back end shared by `gfortran`, `gcc`, and other GNU compilers.