The GNU Fortran Compiler: Code Gen Options

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### 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 listed—the 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 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 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-à-vis existing libraries, tools, and so on).

  For example, with `-funderscoring`, and assuming other defaults like `-fcase-lower` and 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 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 will conflict 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.

  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.

  ‘`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`.

`-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.

`-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-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. These options do not initialize

  - allocatable arrays
  - components of derived type variables
  - 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 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 in Using the GNU Compiler Collection (GCC), for information on more options offered by the GBE shared by `gfortran`, `gcc`, and other GNU compilers.

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