negative form of -ffoo would be -fno-foo. This manual
documents only one of these two forms, whichever one is not
the default.
OPTIONS
Here is a summary of all the options specific to GNU
Fortran, grouped by type. Explanations are in the following
sections.
Overall Options
.
UNIX users typically use the file.f and file.F nomenclature.
Users of other operating systems, especially those that
cannot distinguish upper-case letters from lower-case
letters in their file names, typically use the file.for and
file.fpp nomenclature.
Use of the preprocessor cpp allows use of C-like constructs
such as "#define" and "#include", but can lead to
unexpected, even mistaken, results due to Fortran's source
file format. It is recommended that use of the C
preprocessor be limited to "#include" and, in conjunction
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GNU G77(1)
with "#define", only "#if" and related directives, thus
avoiding in-line macro expansion entirely. This
recommendation applies especially when using the traditional
fixed source form. With free source form, fewer unexpected
transformations are likely to happen, but use of constructs
such as Hollerith and character constants can nevertheless
present problems, especially when these are continued across
multiple source lines. These problems result, primarily,
from differences between the way such constants are
interpreted by the C preprocessor and by a Fortran compiler.
Another example of a problem that results from using the C
preprocessor is that a Fortran comment line that happens to
contain any characters ``interesting'' to the C
preprocessor, such as a backslash at the end of the line, is
not recognized by the preprocessor as a comment line, so
instead of being passed through ``raw'', the line is edited
according to the rules for the preprocessor. For example,
the backslash at the end of the line is removed, along with
the subsequent newline, resulting in the next line being
effectively commented out---unfortunate if that line is a
non-comment line of important code!
Note: The -traditional and -undef flags are supplied to cpp
by default, to help avoid unpleasant surprises.
This means that ANSI C preprocessor features (such as the #
operator) aren't available, and only variables in the C
reserved namespace (generally, names with a leading
underscore) are liable to substitution by C predefines.
Thus, if you want to do system-specific tests, use, for
example, #ifdef __linux__ rather than #ifdef linux. Use the
-v option to see exactly how the preprocessor is invoked.
Unfortunately, the -traditional flag will not avoid an error
from anything that cpp sees as an unterminated C comment,
such as:
C Some Fortran compilers accept /* as starting
C an inline comment.
The following options that affect overall processing are
recognized by the g77 and gcc commands in a GNU Fortran
installation:
-fversion
Ensure that the g77 version of the compiler phase is
reported, if run, and, starting in "egcs" version 1.1,
that internal consistency checks in the f771 program are
run.
This option is supplied automatically when -v or
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--verbose is specified as a command-line option for g77
or gcc and when the resulting commands compile Fortran
source files.
In GCC 3.1, this is changed back to the behavior gcc
displays for .c files.
-fset-g77-defaults
Version info: This option was obsolete as of "egcs"
version 1.1. The effect is instead achieved by the
"lang_init_options" routine in gcc/gcc/f/com.c.
Set up whatever gcc options are to apply to Fortran
compilations, and avoid running internal consistency
checks that might take some time.
This option is supplied automatically when compiling
Fortran code via the g77 or gcc command. The
description of this option is provided so that users
seeing it in the output of, say, g77 -v understand why
it is there.
Also, developers who run "f771" directly might want to
specify it by hand to get the same defaults as they
would running "f771" via g77 or gcc However, such
developers should, after linking a new "f771"
executable, invoke it without this option once, e.g. via
"./f771 -quiet < /dev/null", to ensure that they have
not introduced any internal inconsistencies (such as in
the table of intrinsics) before proceeding---g77 will
crash with a diagnostic if it detects an inconsistency.
-fno-silent
Print (to "stderr") the names of the program units as
they are compiled, in a form similar to that used by
popular UNIX f77 implementations and f2c
Shorthand Options
The following options serve as ``shorthand'' for other
options accepted by the compiler:
-fugly
Note: This option is no longer supported. The
information, below, is provided to aid in the conversion
of old scripts.
Specify that certain ``ugly'' constructs are to be
quietly accepted. Same as:
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-fugly-args -fugly-assign -fugly-assumed
-fugly-comma -fugly-complex -fugly-init
-fugly-logint
These constructs are considered inappropriate to use in
new or well-maintained portable Fortran code, but widely
used in old code.
-fno-ugly
Specify that all ``ugly'' constructs are to be noisily
rejected. Same as:
-fno-ugly-args -fno-ugly-assign -fno-ugly-assumed
-fno-ugly-comma -fno-ugly-complex -fno-ugly-init
-fno-ugly-logint
-ff66
Specify that the program is written in idiomatic FORTRAN
66. Same as -fonetrip -fugly-assumed.
The -fno-f66 option is the inverse of -ff66. As such,
it is the same as -fno-onetrip -fno-ugly-assumed.
The meaning of this option is likely to be refined as
future versions of g77 provide more compatibility with
other existing and obsolete Fortran implementations.
-ff77
Specify that the program is written in idiomatic UNIX
FORTRAN 77 and/or the dialect accepted by the f2c
product. Same as -fbackslash -fno-typeless-boz.
The meaning of this option is likely to be refined as
future versions of g77 provide more compatibility with
other existing and obsolete Fortran implementations.
-fno-f77
The -fno-f77 option is not the inverse of -ff77. It
specifies that the program is not written in idiomatic
UNIX FORTRAN 77 or f2c but in a more widely portable
dialect. -fno-f77 is the same as -fno-backslash.
The meaning of this option is likely to be refined as
future versions of g77 provide more compatibility with
other existing and obsolete Fortran implementations.
Options Controlling Fortran Dialect
The following options control the dialect of Fortran that
the compiler accepts:
-ffree-form
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-fno-fixed-form
Specify that the source file is written in free form
(introduced in Fortran 90) instead of the more-
traditional fixed form.
-ff90
Allow certain Fortran-90 constructs.
This option controls whether certain Fortran 90
constructs are recognized. (Other Fortran 90 constructs
might or might not be recognized depending on other
options such as -fvxt, -ff90-intrinsics-enable, and the
current level of support for Fortran 90.)
-fvxt
Specify the treatment of certain constructs that have
different meanings depending on whether the code is
written in GNU Fortran (based on FORTRAN 77 and akin to
Fortran 90) or VXT Fortran (more like VAX FORTRAN).
The default is -fno-vxt. -fvxt specifies that the VXT
Fortran interpretations for those constructs are to be
chosen.
-fdollar-ok
Allow $ as a valid character in a symbol name.
-fno-backslash
Specify that \ is not to be specially interpreted in
character and Hollerith constants a la C and many UNIX
Fortran compilers.
For example, with -fbackslash in effect, A\nB specifies
three characters, with the second one being newline.
With -fno-backslash, it specifies four characters, A, \,
n, and B.
Note that g77 implements a fairly general form of
backslash processing that is incompatible with the
narrower forms supported by some other compilers. For
example, 'A\003B' is a three-character string in g77
whereas other compilers that support backslash might not
support the three-octal-digit form, and thus treat that
string as longer than three characters.
-fno-ugly-args
Disallow passing Hollerith and typeless constants as
actual arguments (for example, CALL FOO(4HABCD)).
-fugly-assign
Use the same storage for a given variable regardless of
whether it is used to hold an assigned-statement label
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GNU G77(1)
(as in ASSIGN 10 TO I) or used to hold numeric data (as
in I = 3).
-fugly-assumed
Assume any dummy array with a final dimension specified
as 1 is really an assumed-size array, as if * had been
specified for the final dimension instead of 1.
For example, DIMENSION X(1) is treated as if it had read
DIMENSION X(*).
-fugly-comma
In an external-procedure invocation, treat a trailing
comma in the argument list as specification of a
trailing null argument, and treat an empty argument list
as specification of a single null argument.
For example, CALL FOO(,) is treated as CALL FOO(%VVVVAAAALLLL(0),
%VVVVAAAALLLL(0)). That is, two null arguments are specified by
the procedure call when -fugly-comma is in force. And F
= FFFFUUUUNNNNCCCC(((()))) is treated as F = FUNC(%VVVVAAAALLLL(0)).
The default behavior, -fno-ugly-comma, is to ignore a
single trailing comma in an argument list. So, by
default, CALL FOO(X,) is treated exactly the same as
CALL FOO(X).
-fugly-complex
Do not complain about REAL(expr) or AIMAG(expr) when
expr is a "COMPLEX" type other than
"COMPLEX(KIND=1)"---usually this is used to permit
"COMPLEX(KIND=2)" ("DOUBLE COMPLEX") operands.
The -ff90 option controls the interpretation of this
construct.
-fno-ugly-init
Disallow use of Hollerith and typeless constants as
initial values (in "PARAMETER" and "DATA" statements),
and use of character constants to initialize numeric
types and vice versa.
For example, DATA I/'F'/, CHRVAR/65/, J/4HABCD/ is
disallowed by -fno-ugly-init.
-fugly-logint
Treat "INTEGER" and "LOGICAL" variables and expressions
as potential stand-ins for each other.
For example, automatic conversion between "INTEGER" and
"LOGICAL" is enabled, for many contexts, via this
option.
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-fonetrip
Executable iterative "DO" loops are to be executed at
least once each time they are reached.
ANSI FORTRAN 77 and more recent versions of the Fortran
standard specify that the body of an iterative "DO" loop
is not executed if the number of iterations calculated
from the parameters of the loop is less than 1. (For
example, DO 10 I = 1, 0.) Such a loop is called a
zero-trip loop.
Prior to ANSI FORTRAN 77, many compilers implemented
"DO" loops such that the body of a loop would be
executed at least once, even if the iteration count was
zero. Fortran code written assuming this behavior is
said to require one-trip loops. For example, some code
written to the FORTRAN 66 standard expects this behavior
from its "DO" loops, although that standard did not
specify this behavior.
The -fonetrip option specifies that the source file(s)
being compiled require one-trip loops.
This option affects only those loops specified by the
(iterative) "DO" statement and by implied-"DO" lists in
I/O statements. Loops specified by implied-"DO" lists
in "DATA" and specification (non-executable) statements
are not affected.
-ftypeless-boz
Specifies that prefix-radix non-decimal constants, such
as Z'ABCD', are typeless instead of "INTEGER(KIND=1)".
You can test for yourself whether a particular compiler
treats the prefix form as "INTEGER(KIND=1)" or typeless
by running the following program:
EQUIVALENCE (I, R)
R = Z'ABCD1234'
J = Z'ABCD1234'
IF (J .EQ. I) PRINT *, 'Prefix form is TYPELESS'
IF (J .NE. I) PRINT *, 'Prefix form is INTEGER'
END
Reports indicate that many compilers process this form
as "INTEGER(KIND=1)", though a few as typeless, and at
least one based on a command-line option specifying some
kind of compatibility.
-fintrin-case-initcap
-fintrin-case-upper
-fintrin-case-lower
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-fintrin-case-any
Specify expected case for intrinsic names.
-fintrin-case-lower is the default.
-fmatch-case-initcap
-fmatch-case-upper
-fmatch-case-lower
-fmatch-case-any
Specify expected case for keywords. -fmatch-case-lower
is the default.
-fsource-case-upper
-fsource-case-lower
-fsource-case-preserve
Specify whether source text other than character and
Hollerith constants is to be translated to uppercase, to
lowercase, or preserved as is. -fsource-case-lower is
the default.
-fsymbol-case-initcap
-fsymbol-case-upper
-fsymbol-case-lower
-fsymbol-case-any
Specify valid cases for user-defined symbol names.
-fsymbol-case-any is the default.
-fcase-strict-upper
Same as -fintrin-case-upper -fmatch-case-upper
-fsource-case-preserve -fsymbol-case-upper. (Requires
all pertinent source to be in uppercase.)
-fcase-strict-lower
Same as -fintrin-case-lower -fmatch-case-lower
-fsource-case-preserve -fsymbol-case-lower. (Requires
all pertinent source to be in lowercase.)
-fcase-initcap
Same as -fintrin-case-initcap -fmatch-case-initcap
-fsource-case-preserve -fsymbol-case-initcap. (Requires
all pertinent source to be in initial capitals, as in
Print *,SqRt(Value).)
-fcase-upper
Same as -fintrin-case-any -fmatch-case-any
-fsource-case-upper -fsymbol-case-any. (Maps all
pertinent source to uppercase.)
-fcase-lower
Same as -fintrin-case-any -fmatch-case-any
-fsource-case-lower -fsymbol-case-any. (Maps all
pertinent source to lowercase.)
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GNU G77(1)
-fcase-preserve
Same as -fintrin-case-any -fmatch-case-any
-fsource-case-preserve -fsymbol-case-any. (Preserves
all case in user-defined symbols, while allowing any-
case matching of intrinsics and keywords. For example,
call Foo(i,I) would pass two different variables named i
and I to a procedure named Foo.)
-fbadu77-intrinsics-delete
-fbadu77-intrinsics-hide
-fbadu77-intrinsics-disable
-fbadu77-intrinsics-enable
Specify status of UNIX intrinsics having inappropriate
forms. -fbadu77-intrinsics-enable is the default.
-ff2c-intrinsics-delete
-ff2c-intrinsics-hide
-ff2c-intrinsics-disable
-ff2c-intrinsics-enable
Specify status of f2c-specific intrinsics.
-ff2c-intrinsics-enable is the default.
-ff90-intrinsics-delete
-ff90-intrinsics-hide
-ff90-intrinsics-disable
-ff90-intrinsics-enable
Specify status of F90-specific intrinsics.
-ff90-intrinsics-enable is the default.
-fgnu-intrinsics-delete
-fgnu-intrinsics-hide
-fgnu-intrinsics-disable
-fgnu-intrinsics-enable
Specify status of Digital's COMPLEX-related intrinsics.
-fgnu-intrinsics-enable is the default.
-fmil-intrinsics-delete
-fmil-intrinsics-hide
-fmil-intrinsics-disable
-fmil-intrinsics-enable
Specify status of MIL-STD-1753-specific intrinsics.
-fmil-intrinsics-enable is the default.
-funix-intrinsics-delete
-funix-intrinsics-hide
-funix-intrinsics-disable
-funix-intrinsics-enable
Specify status of UNIX intrinsics.
-funix-intrinsics-enable is the default.
-fvxt-intrinsics-delete
-fvxt-intrinsics-hide
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GNU G77(1)
-fvxt-intrinsics-disable
-fvxt-intrinsics-enable
Specify status of VXT intrinsics.
-fvxt-intrinsics-enable is the default.
-ffixed-line-length-n
Set column after which characters are ignored in typical
fixed-form lines in the source file, and through which
spaces are assumed (as if padded to that length) after
the ends of short fixed-form lines.
Popular values for n include 72 (the standard and the
default), 80 (card image), and 132 (corresponds to
``extended-source'' options in some popular compilers).
n may be none, meaning that the entire line is
meaningful and that continued character constants never
have implicit spaces appended to them to fill out the
line. -ffixed-line-length-0 means the same thing as
-ffixed-line-length-none.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions
which are not inherently erroneous but which are risky or
suggest there might have been an error.
You can request many specific warnings with options
beginning -W, for example -Wimplicit to request warnings on
implicit declarations. Each of these specific warning
options also has a negative form beginning -Wno- to turn off
warnings; for example, -Wno-implicit. This manual lists
only one of the two forms, whichever is not the default.
These options control the amount and kinds of warnings
produced by GNU Fortran:
-fsyntax-only
Check the code for syntax errors, but don't do anything
beyond that.
-pedantic
Issue warnings for uses of extensions to ANSI FORTRAN
77. -pedantic also applies to C-language constructs
where they occur in GNU Fortran source files, such as
use of \e in a character constant within a directive
like #include.
Valid ANSI FORTRAN 77 programs should compile properly
with or without this option. However, without this
option, certain GNU extensions and traditional Fortran
features are supported as well. With this option, many
of them are rejected.
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GNU G77(1)
Some users try to use -pedantic to check programs for
strict ANSI conformance. They soon find that it does
not do quite what they want---it finds some non-ANSI
practices, but not all. However, improvements to g77 in
this area are welcome.
-pedantic-errors
Like -pedantic, except that errors are produced rather
than warnings.
-fpedantic
Like -pedantic, but applies only to Fortran constructs.
-w Inhibit all warning messages.
-Wno-globals
Inhibit warnings about use of a name as both a global
name (a subroutine, function, or block data program
unit, or a common block) and implicitly as the name of
an intrinsic in a source file.
Also inhibit warnings about inconsistent invocations
and/or definitions of global procedures (function and
subroutines). Such inconsistencies include different
numbers of arguments and different types of arguments.
-Wimplicit
Warn whenever a variable, array, or function is
implicitly declared. Has an effect similar to using the
"IMPLICIT NONE" statement in every program unit. (Some
Fortran compilers provide this feature by an option
named -u or /WARNINGS=DECLARATIONS.)
-Wunused
Warn whenever a variable is unused aside from its
declaration.
-Wuninitialized
Warn whenever an automatic variable is used without
first being initialized.
These warnings are possible only in optimizing
compilation, because they require data-flow information
that is computed only when optimizing. If you don't
specify -O, you simply won't get these warnings.
These warnings occur only for variables that are
candidates for register allocation. Therefore, they do
not occur for a variable whose address is taken, or
whose size is other than 1, 2, 4 or 8 bytes. Also, they
do not occur for arrays, even when they are in
registers.
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GNU G77(1)
Note that there might be no warning about a variable
that is used only to compute a value that itself is
never used, because such computations may be deleted by
data-flow analysis before the warnings are printed.
These warnings are made optional because GNU Fortran is
not smart enough to see all the reasons why the code
might be correct despite appearing to have an error.
Here is one example of how this can happen:
SUBROUTINE DISPAT(J)
IF (J.EQ.1) I=1
IF (J.EQ.2) I=4
IF (J.EQ.3) I=5
CALL FOO(I)
END
If the value of "J" is always 1, 2 or 3, then "I" is
always initialized, but GNU Fortran doesn't know this.
Here is another common case:
SUBROUTINE MAYBE(FLAG)
LOGICAL FLAG
IF (FLAG) VALUE = 9.4
...
IF (FLAG) PRINT *, VALUE
END
This has no bug because "VALUE" is used only if it is
set.
-Wall
The -Wunused and -Wuninitialized options combined.
These are all the options which pertain to usage that we
recommend avoiding and that we believe is easy to avoid.
(As more warnings are added to g77 some might be added
to the list enabled by -Wall.)
The remaining -W... options are not implied by -Wall because
they warn about constructions that we consider reasonable to
use, on occasion, in clean programs.
-Wsurprising
Warn about ``suspicious'' constructs that are
interpreted by the compiler in a way that might well be
surprising to someone reading the code. These
differences can result in subtle, compiler-dependent
(even machine-dependent) behavioral differences. The
constructs warned about include:
+o Expressions having two arithmetic operators in a
row, such as X*-Y. Such a construct is nonstandard,
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GNU G77(1)
and can produce unexpected results in more
complicated situations such as X**-Y*Z. g77 along
with many other compilers, interprets this example
differently than many programmers, and a few other
compilers. Specifically, g77 interprets X**-Y*Z as
(X**(-Y))*Z, while others might think it should be
interpreted as X**(-(Y*Z)).
A revealing example is the constant expression
2**-2*1., which g77 evaluates to .25, while others
might evaluate it to 0., the difference resulting
from the way precedence affects type promotion.
(The -fpedantic option also warns about expressions
having two arithmetic operators in a row.)
+o Expressions with a unary minus followed by an
operand and then a binary operator other than plus
or minus. For example, -2**2 produces a warning,
because the precedence is -(2**2), yielding -4, not
(-2)**2, which yields 4, and which might represent
what a programmer expects.
An example of an expression producing different
results in a surprising way is -I*S, where I holds
the value -2147483648 and S holds 0.5. On many
systems, negating I results in the same value, not a
positive number, because it is already the lower
bound of what an "INTEGER(KIND=1)" variable can
hold. So, the expression evaluates to a positive
number, while the ``expected'' interpretation,
(-I)*S, would evaluate to a negative number.
Even cases such as -I*J produce warnings, even
though, in most configurations and situations, there
is no computational difference between the results
of the two interpretations---the purpose of this
warning is to warn about differing interpretations
and encourage a better style of coding, not to
identify only those places where bugs might exist in
the user's code.
+o "DO" loops with "DO" variables that are not of
integral type---that is, using "REAL" variables as
loop control variables. Although such loops can be
written to work in the ``obvious'' way, the way g77
is required by the Fortran standard to interpret
such code is likely to be quite different from the
way many programmers expect. (This is true of all
"DO" loops, but the differences are pronounced for
non-integral loop control variables.)
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GNU G77(1)
-Werror
Make all warnings into errors.
-W Turns on ``extra warnings'' and, if optimization is
specified via -O, the -Wuninitialized option. (This
might change in future versions of g77
``Extra warnings'' are issued for:
+o Unused parameters to a procedure (when -Wunused also
is specified).
+o Overflows involving floating-point constants (not
available for certain configurations).
Some of these have no effect when compiling programs written
in Fortran:
-Wcomment
-Wformat
-Wparentheses
-Wswitch
-Wswitch-default
-Wswitch-enum
-Wtraditional
-Wshadow
-Wid-clash-len
-Wlarger-than-len
-Wconversion
-Waggregate-return
-Wredundant-decls
These options all could have some relevant meaning for
GNU Fortran programs, but are not yet supported.
Options for Debugging Your Program or GNU Fortran
GNU Fortran has various special options that are used for
debugging either your program or g77
-g Produce debugging information in the operating system's
native format (stabs, COFF, XCOFF, or DWARF). GDB can
work with this debugging information.
A sample debugging session looks like this (note the use
of the breakpoint):
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GNU G77(1)
$ cat gdb.f
PROGRAM PROG
DIMENSION A(10)
DATA A /1.,2.,3.,4.,5.,6.,7.,8.,9.,10./
A(5) = 4.
PRINT*,A
END
$ g77 -g -O gdb.f
$ gdb a.out
...
(gdb) break MAIN__
Breakpoint 1 at 0x8048e96: file gdb.f, line 4.
(gdb) run
Starting program: /home/toon/g77-bugs/./a.out
Breakpoint 1, MAIN__ () at gdb.f:4
4 A(5) = 4.
Current language: auto; currently fortran
(gdb) print a(5)
$1 = 5
(gdb) step
5 PRINT*,A
(gdb) print a(5)
$2 = 4
...
One could also add the setting of the breakpoint and the
first run command to the file .gdbinit in the current
directory, to simplify the debugging session.
Options That Control Optimization
Most Fortran users will want to use no optimization when
developing and testing programs, and use -O or -O2 when
compiling programs for late-cycle testing and for production
use. However, note that certain diagnostics---such as for
uninitialized variables---depend on the flow analysis done
by -O, i.e. you must use -O or -O2 to get such diagnostics.
The following flags have particular applicability when
compiling Fortran programs:
-malign-double
(Intel x86 architecture only.)
Noticeably improves performance of g77 programs making
heavy use of "REAL(KIND=2)" ("DOUBLE PRECISION") data on
some systems. In particular, systems using Pentium,
Pentium Pro, 586, and 686 implementations of the i386
architecture execute programs faster when "REAL(KIND=2)"
("DOUBLE PRECISION") data are aligned on 64-bit
boundaries in memory.
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GNU G77(1)
This option can, at least, make benchmark results more
consistent across various system configurations,
versions of the program, and data sets.
Note: The warning in the gcc documentation about this
option does not apply, generally speaking, to Fortran
code compiled by g77
Also also note: The negative form of -malign-double is
-mno-align-double, not -benign-double.
-ffloat-store
Might help a Fortran program that depends on exact IEEE
conformance on some machines, but might slow down a
program that doesn't.
This option is effective when the floating-point unit is
set to work in IEEE 854 `extended precision'---as it
typically is on x86 and m68k GNU systems---rather than
IEEE 754 double precision. -ffloat-store tries to
remove the extra precision by spilling data from
floating-point registers into memory and this typically
involves a big performance hit. However, it doesn't
affect intermediate results, so that it is only
partially effective. `Excess precision' is avoided in
code like:
a = b + c
d = a * e
but not in code like:
d = (b + c) * e
For another, potentially better, way of controlling the
precision, see Floating-point precision.
-fforce-mem
-fforce-addr
Might improve optimization of loops.
-fno-inline
Don't compile statement functions inline. Might reduce
the size of a program unit---which might be at expense
of some speed (though it should compile faster). Note
that if you are not optimizing, no functions can be
expanded inline.
-ffast-math
Might allow some programs designed to not be too
dependent on IEEE behavior for floating-point to run
faster, or die trying. Sets
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GNU G77(1)
-funsafe-math-optimizations, -ffinite-math-only, and
-fno-trapping-math.
-funsafe-math-optimizations
Allow optimizations that may be give incorrect results
for certain IEEE inputs.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that
assume that arguments and results are not NaNs or
+-Infs.
This option should never be turned on by any -O option
since it can result in incorrect output for programs
which depend on an exact implementation of IEEE or ISO
rules/specifications.
The default is -fno-finite-math-only.
-fno-trapping-math
Allow the compiler to assume that floating-point
arithmetic will not generate traps on any inputs. This
is useful, for example, when running a program using
IEEE "non-stop" floating-point arithmetic.
-fstrength-reduce
Might make some loops run faster.
-frerun-cse-after-loop
-fexpensive-optimizations
-fdelayed-branch
-fschedule-insns
-fschedule-insns2
-fcaller-saves
Might improve performance on some code.
-funroll-loops
Typically improves performance on code using iterative
"DO" loops by unrolling them and is probably generally
appropriate for Fortran, though it is not turned on at
any optimization level. Note that outer loop unrolling
isn't done specifically; decisions about whether to
unroll a loop are made on the basis of its instruction
count.
Also, no `loop discovery'[1] is done, so only loops
written with "DO" benefit from loop optimizations,
including---but not limited to---unrolling. Loops
written with "IF" and "GOTO" are not currently
recognized as such. This option unrolls only iterative
"DO" loops, not "DO WHILE" loops.
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GNU G77(1)
-funroll-all-loops
Probably improves performance on code using "DO WHILE"
loops by unrolling them in addition to iterative "DO"
loops. In the absence of "DO WHILE", this option is
equivalent to -funroll-loops but possibly slower.
-fno-move-all-movables
-fno-reduce-all-givs
-fno-rerun-loop-opt
In general, the optimizations enabled with these options
will lead to faster code being generated by GNU Fortran;
hence they are enabled by default when issuing the g77
command.
-fmove-all-movables and -freduce-all-givs will enable
loop optimization to move all loop-invariant index
computations in nested loops over multi-rank array dummy
arguments out of these loops.
-frerun-loop-opt will move offset calculations resulting
from the fact that Fortran arrays by default have a
lower bound of 1 out of the loops.
These three options are intended to be removed someday,
once loop optimization is sufficiently advanced to
perform all those transformations without help from
these options.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on
each C source file before actual compilation.
Some of these options also affect how g77 processes the
"INCLUDE" directive. Since this directive is processed even
when preprocessing is not requested, it is not described in
this section.
However, the "INCLUDE" directive does not apply
preprocessing to the contents of the included file itself.
Therefore, any file that contains preprocessor directives
(such as "#include", "#define", and "#if") must be included
via the "#include" directive, not via the "INCLUDE"
directive. Therefore, any file containing preprocessor
directives, if included, is necessarily included by a file
that itself contains preprocessor directives.
Options for Directory Search
These options affect how the cpp preprocessor searches for
files specified via the "#include" directive. Therefore,
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GNU G77(1)
when compiling Fortran programs, they are meaningful when
the preprocessor is used.
Some of these options also affect how g77 searches for files
specified via the "INCLUDE" directive, although files
included by that directive are not, themselves,
preprocessed. These options are:
-I-
-Idir
These affect interpretation of the "INCLUDE" directive
(as well as of the "#include" directive of the cpp
preprocessor).
Note that -Idir must be specified without any spaces
between -I and the directory name---that is, -Ifoo/bar
is valid, but -I foo/bar is rejected by the g77 compiler
(though the preprocessor supports the latter form).
Also note that the general behavior of -I and "INCLUDE"
is pretty much the same as of -I with "#include" in the
cpp preprocessor, with regard to looking for header.gcc
files and other such things.
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 as if the "SAVE" statement was
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.)
-finit-local-zero
Specify that variables and arrays that are local to a
program unit (not in a common block and not passed as an
argument) are to be initialized to binary zeros.
Since there is a run-time penalty for initialization of
variables that are not given the "SAVE" attribute, it
might be a good idea to also use -fno-automatic with
-finit-local-zero.
-fno-f2c
Do not generate code designed to be compatible with code
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GNU G77(1)
generated by f2c use the GNU calling conventions
instead.
The f2c calling conventions require functions that
return type "REAL(KIND=1)" 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 GNU calling conventions, such functions simply
return their results as they would in GNU
C---"REAL(KIND=1)" functions return the C type "float",
and "COMPLEX" functions return the GNU C type "complex"
(or its "struct" equivalent).
This does not affect the generation of code that
interfaces with the "libg2c" library.
However, because the "libg2c" library uses f2c calling
conventions, g77 rejects attempts to pass intrinsics
implemented by routines in this library as actual
arguments when -fno-f2c is used, to avoid bugs when they
are actually called by code expecting the GNU calling
conventions to work.
For example, INTRINSIC ABS;CALL FOO(ABS) is rejected
when -fno-f2c is in force. (Future versions of the g77
run-time library might offer routines that provide GNU-
callable versions of the routines that implement the f2c
intrinsics that may be passed as actual arguments, so
that valid programs need not be rejected when -fno-f2c
is used.)
Caution: If -fno-f2c is used when compiling any source
file used in a program, it must be used when compiling
all Fortran source files used in that program.
-ff2c-library
Specify that use of "libg2c" (or the original "libf2c")
is required. This is the default for the current
version of g77
Currently it is not valid to specify -fno-f2c-library.
This option is provided so users can specify it in shell
scripts that build programs and libraries that require
the "libf2c" library, even when being compiled by future
versions of g77 that might otherwise default to
generating code for an incompatible library.
-fno-underscoring
Do not transform names of entities specified in the
Fortran source file by appending underscores to them.
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GNU G77(1)
With -funderscoring in effect, g77 appends two
underscores to names with underscores and one underscore
to external names with no underscores. (g77 also
appends two underscores to internal names with
underscores to avoid naming collisions with external
names. The -fno-second-underscore option disables
appending of the second underscore in all cases.)
This is done to ensure compatibility with code produced
by many UNIX Fortran compilers, including f2c which
perform the same transformations.
Use of -fno-underscoring is not recommended unless you
are experimenting with issues such as integration of
(GNU) Fortran into existing system environments
(vis-a-vis existing libraries, tools, and so on).
For example, with -funderscoring, and assuming other
defaults like -fcase-lower and that j() and mmmmaaaaxxxx_ccccoooouuuunnnntttt(((())))
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
g77 code with other languages.
Note that just because the names match does not mean
that the interface implemented by g77 for an external
name matches the interface implemented by some other
language for that same name. That is, getting code
produced by g77 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
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GNU G77(1)
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 g77 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.
-fno-second-underscore
Do not append a second underscore to names of entities
specified in the Fortran source file.
This option has no effect if -fno-underscoring is in
effect.
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__.
-fno-ident
Ignore the #ident directive.
-fzeros
Treat initial values of zero as if they were any other
value.
As of version 0.5.18, g77 normally treats "DATA" and
other statements that are used to specify initial values
of zero for variables and arrays as if no values were
actually specified, in the sense that no diagnostics
regarding multiple initializations are produced.
This is done to speed up compiling of programs that
initialize large arrays to zeros.
Use -fzeros to revert to the simpler, slower behavior
that can catch multiple initializations by keeping track
of all initializations, zero or otherwise.
Caution: Future versions of g77 might disregard this
option (and its negative form, the default) or interpret
it somewhat differently. The interpretation changes
will affect only non-standard programs; standard-
conforming programs should not be affected.
-femulate-complex
Implement "COMPLEX" arithmetic via emulation, instead of
using the facilities of the gcc back end that provide
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GNU G77(1)
direct support of "complex" arithmetic.
(gcc had some bugs in its back-end support for "complex"
arithmetic, due primarily to the support not being
completed as of version 2.8.1 and "egcs" 1.1.2.)
Use -femulate-complex if you suspect code-generation
bugs, or experience compiler crashes, that might result
from g77 using the "COMPLEX" support in the gcc back
end. If using that option fixes the bugs or crashes you
are seeing, that indicates a likely g77 bugs (though,
all compiler crashes are considered bugs), so, please
report it. (Note that the known bugs, now believed
fixed, produced compiler crashes rather than causing the
generation of incorrect code.)
Use of this option should not affect how Fortran code
compiled by g77 works in terms of its interfaces to
other code, e.g. that compiled by f2c
As of GCC version 3.0, this option is not necessary
anymore.
Caution: Future versions of g77 might ignore both forms
of this option.
-falias-check
-fargument-alias
-fargument-noalias
-fno-argument-noalias-global
Version info: These options are not supported by
versions of g77 based on gcc version 2.8.
These options specify to what degree aliasing (overlap)
is permitted between arguments (passed as pointers) and
"COMMON" (external, or public) storage.
The default for Fortran code, as mandated by the FORTRAN
77 and Fortran 90 standards, is
-fargument-noalias-global. The default for code written
in the C language family is -fargument-alias.
Note that, on some systems, compiling with -fforce-addr
in effect can produce more optimal code when the default
aliasing options are in effect (and when optimization is
enabled).
-fno-globals
Disable diagnostics about inter-procedural analysis
problems, such as disagreements about the type of a
function or a procedure's argument, that might cause a
compiler crash when attempting to inline a reference to
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GNU G77(1)
a procedure within a program unit. (The diagnostics
themselves are still produced, but as warnings, unless
-Wno-globals is specified, in which case no relevant
diagnostics are produced.)
Further, this option disables such inlining, to avoid
compiler crashes resulting from incorrect code that
would otherwise be diagnosed.
As such, this option might be quite useful when
compiling existing, ``working'' code that happens to
have a few bugs that do not generally show themselves,
but which g77 diagnoses.
Use of this option therefore has the effect of
instructing g77 to behave more like it did up through
version 0.5.19.1, when it paid little or no attention to
disagreements between program units about a procedure's
type and argument information, and when it performed no
inlining of procedures (except statement functions).
Without this option, g77 defaults to performing the
potentially inlining procedures as it started doing in
version 0.5.20, but as of version 0.5.21, it also
diagnoses disagreements that might cause such inlining
to crash the compiler as (fatal) errors, and warns about
similar disagreements that are currently believed to not
likely to result in the compiler later crashing or
producing incorrect code.
-fflatten-arrays
Use back end's C-like constructs (pointer plus offset)
instead of its "ARRAY_REF" construct to handle all array
references.
Note: This option is not supported. It is intended for
use only by g77 developers, to evaluate code-generation
issues. It might be removed at any time.
-fbounds-check
-ffortran-bounds-check
Enable generation of run-time checks for array
subscripts and substring start and end points against
the (locally) declared minimum and maximum values.
The current implementation uses the "libf2c" library
routine "s_rnge" to print the diagnostic.
However, whereas f2c generates a single check per
reference for a multi-dimensional array, of the computed
offset against the valid offset range (0 through the
size of the array), g77 generates a single check per
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GNU G77(1)
subscript expression. This catches some cases of
potential bugs that f2c does not, such as references to
below the beginning of an assumed-size array.
g77 also generates checks for "CHARACTER" substring
references, something f2c currently does not do.
Use the new -ffortran-bounds-check option to specify
bounds-checking for only the Fortran code you are
compiling, not necessarily for code written in other
languages.
Note: To provide more detailed information on the
offending subscript, g77 provides the "libg2c" run-time
library routine "s_rnge" with somewhat differently-
formatted information. Here's a sample diagnostic:
Subscript out of range on file line 4, procedure rnge.f/bf.
Attempt to access the -6-th element of variable b[subscript-2-of-2].
Aborted
The above message indicates that the offending source
line is line 4 of the file rnge.f, within the program
unit (or statement function) named bf. The offended
array is named b. The offended array dimension is the
second for a two-dimensional array, and the offending,
computed subscript expression was -6.
For a "CHARACTER" substring reference, the second line
has this appearance:
Attempt to access the 11-th element of variable a[start-substring].
This indicates that the offended "CHARACTER" variable or
array is named a, the offended substring position is the
starting (leftmost) position, and the offending
substring expression is 11.
(Though the verbage of "s_rnge" is not ideal for the
purpose of the g77 compiler, the above information
should provide adequate diagnostic abilities to it
users.)
Some of these do not work when compiling programs written in
Fortran:
-fpcc-struct-return
-freg-struct-return
You should not use these except strictly the same way as
you used them to build the version of "libg2c" with
which you will be linking all code compiled by g77 with
the same option.
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GNU G77(1)
-fshort-double
This probably either has no effect on Fortran programs,
or makes them act loopy.
-fno-common
Do not use this when compiling Fortran programs, or
there will be Trouble.
-fpack-struct
This probably will break any calls to the "libg2c"
library, at the very least, even if it is built with the
same option.
ENVIRONMENT
GNU Fortran currently does not make use of any environment
variables to control its operation above and beyond those
that affect the operation of gcc.
BUGS
For instructions on reporting bugs, see
. Use of the gccbug script to
report bugs is recommended.
FOOTNOTES
1. loop discovery refers to the process by which a
compiler, or indeed any reader of a program, determines
which portions of the program are more likely to be
executed repeatedly as it is being run. Such discovery
typically is done early when compiling using
optimization techniques, so the ``discovered'' loops get
more attention---and more run-time resources, such as
registers---from the compiler. It is easy to
``discover'' loops that are constructed out of looping
constructs in the language (such as Fortran's "DO").
For some programs, ``discovering'' loops constructed out
of lower-level constructs (such as "IF" and "GOTO") can
lead to generation of more optimal code than otherwise.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), gcc(1),
as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1) and the Info
entries for gcc, cpp, g77, as, ld, binutils and gdb.
AUTHOR
See the Info entry for g77 for contributors to GCC and G77.
COPYRIGHT
Copyright (c) 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2004
Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation
gcc-3.4.3 Last change: 2004-11-05 28
GNU G77(1)
License, Version 1.2 or any later version published by the
Free Software Foundation; with the Invariant Sections being
``GNU General Public License'' and ``Funding Free
Software'', the Front-Cover texts being (a) (see below), and
with the Back-Cover Texts being (b) (see below). A copy of
the license is included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
_______________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE|
|____________________|__________________|_
| Availability | developer/gcc-3|
|____________________|__________________|_
| Interface Stability| External |
|____________________|_________________|
NOTES
Source for gcc is available on http://opensolaris.org.
gcc-3.4.3 Last change: 2004-11-05 29