llvm-project/flang/docs/Extensions.md

Fortran Extensions supported by Flang

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As a general principle, this compiler will accept by default and without complaint many legacy features, extensions to the standard language, and features that have been deleted from the standard, so long as the recognition of those features would not cause a standard-conforming program to be rejected or misinterpreted.

Other non-standard features, which do conflict with the current standard specification of the Fortran programming language, are accepted if enabled by command-line options.

Intentional violations of the standard

• Scalar INTEGER actual argument expressions (not variables!) are converted to the kinds of scalar INTEGER dummy arguments when the interface is explicit and the kinds differ. This conversion allows the results of the intrinsics like SIZE that (as mentioned below) may return non-default INTEGER results by default to be passed. A warning is emitted when truncation is possible.
• We are not strict on the contents of BLOCK DATA subprograms so long as they contain no executable code, no internal subprograms, and allocate no storage outside a named COMMON block. (C1415)

Extensions, deletions, and legacy features supported by default

• Tabs in source
• <> as synonym for .NE. and /=
• $ and @ as legal characters in names
• Initialization in type declaration statements using /values/
• Kind specification with *, e.g. REAL*4
• DOUBLE COMPLEX
• Signed complex literal constants
• DEC STRUCTURE, RECORD, UNION, and MAP
• Structure field access with .field
• BYTE as synonym for INTEGER(KIND=1)
• Quad precision REAL literals with Q
• X prefix/suffix as synonym for Z on hexadecimal literals
• B, O, Z, and X accepted as suffixes as well as prefixes
• Triplets allowed in array constructors
• %LOC, %VAL, and %REF
• Leading comma allowed before I/O item list
• Empty parentheses allowed in PROGRAM P()
• Missing parentheses allowed in FUNCTION F
• Cray based POINTER(p,x) and LOC() intrinsic (with %LOC() as an alias)
• Arithmetic IF. (Which branch should NaN take? Fall through?)
• ASSIGN statement, assigned GO TO, and assigned format
• PAUSE statement
• Hollerith literals and edit descriptors
• NAMELIST allowed in the execution part
• Omitted colons on type declaration statements with attributes
• COMPLEX constructor expression, e.g. (x+y,z)
• + and - before all primary expressions, e.g. x*-y
• .NOT. .NOT. accepted
• NAME= as synonym for FILE=
• Data edit descriptors without width or other details
• D lines in fixed form as comments or debug code
• CARRIAGECONTROL= on the OPEN and INQUIRE statements
• CONVERT= on the OPEN and INQUIRE statements
• DISPOSE= on the OPEN and INQUIRE statements
• Leading semicolons are ignored before any statement that could have a label
• The character & in column 1 in fixed form source is a variant form of continuation line.
• Character literals as elements of an array constructor without an explicit type specifier need not have the same length; the longest literal determines the length parameter of the implicit type, not the first.
• Outside a character literal, a comment after a continuation marker (&) need not begin with a comment marker (!).
• Classic C-style /comments/ are skipped, so multi-language header files are easier to write and use.
• $ and \ edit descriptors are supported in FORMAT to suppress newline output on user prompts.
• REAL and DOUBLE PRECISION variable and bounds in DO loops
• Integer literals without explicit kind specifiers that are out of range for the default kind of INTEGER are assumed to have the least larger kind that can hold them, if one exists.
• BOZ literals can be used as INTEGER values in contexts where the type is unambiguous: the right hand sides of assigments and initializations of INTEGER entities, and as actual arguments to a few intrinsic functions (ACHAR, BTEST, CHAR). But they cannot be used if the type would not be known (e.g., IAND(X'1',X'2')).
• BOZ literals can also be used as REAL values in some contexts where the type is unambiguous, such as initializations of REAL parameters.
• EQUIVALENCE of numeric and character sequences (a ubiquitous extension)
• Values for whole anonymous parent components in structure constructors (e.g., EXTENDEDTYPE(PARENTTYPE(1,2,3)) rather than EXTENDEDTYPE(1,2,3) or EXTENDEDTYPE(PARENTTYPE=PARENTTYPE(1,2,3))).
• Some intrinsic functions are specified in the standard as requiring the same type and kind for their arguments (viz., ATAN with two arguments, ATAN2, DIM, HYPOT, MAX, MIN, MOD, and MODULO); we allow distinct types to be used, promoting the arguments as if they were operands to an intrinsic + operator, and defining the result type accordingly.
• DOUBLE COMPLEX intrinsics DREAL, DCMPLX, DCONJG, and DIMAG.
• INT_PTR_KIND intrinsic returns the kind of c_intptr_t.
• Restricted specific conversion intrinsics FLOAT, SNGL, IDINT, IFIX, DREAL, and DCMPLX accept arguments of any kind instead of only the default kind or double precision kind. Their result kinds remain as specified.
• Specific intrinsics AMAX0, AMAX1, AMIN0, AMIN1, DMAX1, DMIN1, MAX0, MAX1, MIN0, and MIN1 accept more argument types than specified. They are replaced by the related generics followed by conversions to the specified result types.
• When a scalar CHARACTER actual argument of the same kind is known to have a length shorter than the associated dummy argument, it is extended on the right with blanks, similar to assignment.
• When a dummy argument is POINTER or ALLOCATABLE and is INTENT(IN), we relax enforcement of some requirements on actual arguments that must otherwise hold true for definable arguments.
• Assignment of LOGICAL to INTEGER and vice versa (but not other types) is allowed. The values are normalized.
• An effectively empty source file (no program unit) is accepted and produces an empty relocatable output file.
• A RETURN statement may appear in a main program.
• DATA statement initialization is allowed for procedure pointers outside structure constructors.
• Nonstandard intrinsic functions: ISNAN, SIZEOF
• A forward reference to a default INTEGER scalar dummy argument is permitted to appear in a specification expression, such as an array bound, in a scope with IMPLICIT NONE(TYPE) if the name of the dummy argument would have caused it to be implicitly typed as default INTEGER if IMPLICIT NONE(TYPE) were absent.
• OPEN(ACCESS='APPEND') is interpreted as OPEN(POSITION='APPEND') to ease porting from Sun Fortran.

Extensions supported when enabled by options

• C-style backslash escape sequences in quoted CHARACTER literals (but not Hollerith) [-fbackslash]
• Logical abbreviations .T., .F., .N., .A., .O., and .X. [-flogical-abbreviations]
• .XOR. as a synonym for .NEQV. [-fxor-operator]
• The default INTEGER type is required by the standard to occupy the same amount of storage as the default REAL type. Default REAL is of course 32-bit IEEE-754 floating-point today. This legacy rule imposes an artificially small constraint in some cases where Fortran mandates that something have the default INTEGER type: specifically, the results of references to the intrinsic functions SIZE, STORAGE_SIZE,LBOUND, UBOUND, SHAPE, and the location reductions FINDLOC, MAXLOC, and MINLOC in the absence of an explicit KIND= actual argument. We return INTEGER(KIND=8) by default in these cases when the -flarge-sizes option is enabled. SIZEOF and C_SIZEOF always return INTEGER(KIND=8).
• Treat each specification-part like is has IMPLICIT NONE [-fimplicit-none-type-always]
• Ignore occurrences of IMPLICIT NONE and IMPLICIT NONE(TYPE) [-fimplicit-none-type-never]
• Old-style PARAMETER pi=3.14 statement without parentheses [-falternative-parameter-statement]

Extensions and legacy features deliberately not supported

• .LG. as synonym for .NE.
• REDIMENSION
• Allocatable COMMON
• Expressions in formats
• ACCEPT as synonym for READ *
• TYPE as synonym for PRINT
• ARRAY as synonym for DIMENSION
• VIRTUAL as synonym for DIMENSION
• ENCODE and DECODE as synonyms for internal I/O
• IMPLICIT AUTOMATIC, IMPLICIT STATIC
• Default exponent of zero, e.g. 3.14159E
• Characters in defined operators that are neither letters nor digits
• B suffix on unquoted octal constants
• Z prefix on unquoted hexadecimal constants (dangerous)
• T and F as abbreviations for .TRUE. and .FALSE. in DATA (PGI/XLF)
• Use of host FORMAT labels in internal subprograms (PGI-only feature)
• ALLOCATE(TYPE(derived)::...) as variant of correct ALLOCATE(derived::...) (PGI only)
• Defining an explicit interface for a subprogram within itself (PGI only)
• USE association of a procedure interface within that same procedure's definition
• NULL() as a structure constructor expression for an ALLOCATABLE component (PGI).
• Conversion of LOGICAL to INTEGER in expressions.
• IF (integer expression) THEN ... END IF (PGI/Intel)
• Comparsion of LOGICAL with ==/.EQ. rather than .EQV. (also .NEQV.) (PGI/Intel)
• Procedure pointers in COMMON blocks (PGI/Intel)
• Underindexing multi-dimensional arrays (e.g., A(1) rather than A(1,1)) (PGI only)
• Legacy PGI NCHARACTER type and NC Kanji character literals
• Using non-integer expressions for array bounds (e.g., REAL A(3.14159)) (PGI/Intel)
• Mixing INTEGER types as operands to bit intrinsics (e.g., IAND); only two compilers support it, and they disagree on sign extension.
• Module & program names that conflict with an object inside the unit (PGI only).
• When the same name is brought into scope via USE association from multiple modules, the name must refer to a generic interface; PGI allows a name to be a procedure from one module and a generic interface from another.
• Type parameter declarations must come first in a derived type definition; some compilers allow them to follow PRIVATE, or be intermixed with the component declarations.
• Wrong argument types in calls to specific intrinsics that have different names than the related generics. Some accepted exceptions are listed above in the allowed extensions. PGI, Intel, and XLF support this in ways that are not numerically equivalent. PGI converts the arguments while Intel and XLF replace the specific by the related generic.

Preprocessing behavior

• The preprocessor is always run, whatever the filename extension may be.
• We respect Fortran comments in macro actual arguments (like GNU, Intel, NAG; unlike PGI and XLF) on the principle that macro calls should be treated like function references. Fortran's line continuation methods also work.

Standard features not silently accepted

• Fortran explicitly ignores type declaration statements when they attempt to type the name of a generic intrinsic function (8.2 p3). One can declare CHARACTER::COS and still get a real result from COS(3.14159), for example. f18 will complain when a generic intrinsic function's inferred result type does not match an explicit declaration. This message is a warning.