llvm-project/flang/docs/FortranStandardsSupport.md

Flang Fortran Standards Support

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This document summarizes Fortran standards support in Flang. The information is only provided as a guideline. The TODOs/Not Yet Implemented messages emitted by the compiler for unimplemented features should be treated as authoritative.

The standards support information is provided as a table with three columns that are self explanatory. The Status column uses the letters Y, P, N for the implementation status:

• Y : Yes. When the implementation is complete
• P : Partial. When the implementation is incomplete for a few cases
• N : No. When the implementation is absent

There is no distinction made between support in the Parser/Semantics and the MLIR or Lowering stages.

Note: The two major missing features in Flang at present are coarrays and parameterized derived types (PDTs) with length type parameters.

Fortran 2023

See document for a brief discussion about the new features in Fortran 2023. The following table summarizes the status of all important Fortran 2023 features. The table entries are based on the document The new features in Fortran 2023.

Feature	Status	Comments
Allow longer statement lines and overall statement length	Y
Automatic allocation of lengths of character variables	N
The specifiers typeof and classof	N
Conditional expressions and arguments	N
More use of boz constants	P	All usages other than enum are supported
Intrinsics for extracting tokens from a string	N
Intrinsics for Trig functions that work in degrees	Y
Intrinsics for Trig functions that work in half revolutions	Y
Changes to system_clock	N
Changes for conformance with the new IEEE standard	Y
Additional named constants to specify kinds	Y
Extensions for c_f_pointer intrinsic	Y
Procedures for converting between fortran and c strings	N
The at edit descriptor	N
Control over leading zeros in output of real values	N
Extensions for Namelist	N
Allow an object of a type with a coarray ultimate component to be an array or allocatable	N
Put with Notify	N
Error conditions in collectives	N
Simple procedures	N
Using integer arrays to specify subscripts	N
Using integer arrays to specify rank and bound of an array	N
Using an integer constant to specify rank	N
Reduction specifier for do concurrent	P	Syntax is accepted
Enumerations	N

Fortran 2018

All features except those listed in the following table are supported. Almost all of the unsupported features are related to the multi-image execution. The table entries are based on the document The new features in Fortran 2018.

Feature	Status	Comments
Asynchronous communication	P	Syntax is accepted
Teams	N	Multi-image/Coarray feature
Image failure	P	Multi-image/Coarray feature. stat_failed_image is added
Form team statement	N	Multi-image/Coarray feature
Change team construct	N	Multi-image/Coarray feature
Coarrays allocated in teams	N	Multi-image/Coarray feature
Critical construct	N	Multi-image/Coarray feature
Lock and unlock statements	N	Multi-image/Coarray feature
Events	N	Multi-image/Coarray feature
Sync team construct	N	Multi-image/Coarray feature
Image selectors	N	Multi-image/Coarray feature
Intrinsic functions get_team and team_number	N	Multi-image/Coarray feature
Intrinsic function image_index	N	Multi-image/Coarray feature
Intrinsic function num_images	N	Multi-image/Coarray feature
Intrinsic function this_image	N	Multi-image/Coarray feature
Intrinsic move_alloc extensions	P	Multi-image/Coarray feature
Detecting failed and stopped images	N	Multi-image/Coarray feature
Collective subroutines	N	Multi-image/Coarray feature
New and enhanced atomic subroutines	N	Multi-image/Coarray feature
Failed images and stat= specifiers	N	Multi-image/Coarray feature
Intrinsic function coshape	N	Multi-image/Coarray feature

Fortran 2008

All features except those listed in the following table are supported.

Feature	Status	Comments
Coarrays	N	Lowering and runtime support is not implemented
do concurrent	P	Sequential execution works. Parallel support in progress
Internal procedure as an actual argument or pointer target	Y	Current implementation requires stack to be executable. See Proposal

Fortran 2003

All features except those listed in the following table are supported.

Feature	Status	Comments
Parameterized Derived Types	P	PDT with length type parameters is not supported. See Proposal
Assignment to allocatable	P	Assignment to whole allocatable in FORALL is not implemented
Asynchronous input/output	P	IO will happen synchronously
MIN/MAX extensions for CHARACTER	P	Some variants are not supported

Fortran 95

All features are supported.

Fortran 90

All features are supported.

FORTRAN 77

All features are supported.