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[Flang] Add a Fortran Standards Support doc #132195

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[Flang] Add a Fortran Standards Support doc
kiranchandramohan Mar 20, 2025
8499992
Address review comments
kiranchandramohan Mar 21, 2025
6c4b380
Add support tables for Fortran 2023 and Fortran 2018 standards
kiranchandramohan Mar 23, 2025
84165a7
Move MIN MAX extension to Fortran 2003 table
kiranchandramohan Mar 23, 2025
8f9db84
Remove line about extending support for Fortran 2018 and Fortran 2023
kiranchandramohan Mar 24, 2025
8eb16e4
Address review comments
kiranchandramohan Mar 24, 2025
6781534
Address review comment about multi-image execution
kiranchandramohan Mar 24, 2025
66199ad
Partially address comments from Mark
kiranchandramohan Apr 19, 2025
04fb498
Address Steve's comments
kiranchandramohan Apr 20, 2025
27260b6
Remove duplicate sentence
kiranchandramohan Apr 21, 2025
c36aebc
Address comment from h.vetinari
kiranchandramohan Apr 22, 2025
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116 changes: 116 additions & 0 deletions flang/docs/FortranStandardsSupport.md
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<!--===- docs/FortranStandardsSupport.md

Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
See https://llvm.org/LICENSE.txt for license information.
SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

-->

# Flang Fortran Standards Support

```{contents}
---
local:
---
```

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.
Standards support is provided upto Fortran 2008 for now. It will be extended later for Fortran 2018 and Fortran 2023.

The standards support information is provided as a table with three columns that are self explanatory. The Status column uses
the letters **P**, **Y**, **N** for the implementation status:
- **P** : When the implementation is incomplete for a few cases
- **Y** : When the implementation is complete
- **N** : When the implementation is absent
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@h-vetinari h-vetinari Apr 20, 2025 •
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I still think we should make this easier for readers (explain the letters, order them by degree of completion)

Suggested change
the letters **P**, **Y**, **N** for the implementation status:
- **P** : When the implementation is incomplete for a few cases
- **Y** : When the implementation is complete
- **N** : When the implementation is absent
the letters **Y**, **P**, **N** for the implementation status:
- **Y** (Yes): When the implementation is complete
- **P** (Partial): When the implementation is incomplete
- **N** (No): When the implementation is absent

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@kiranchandramohan kiranchandramohan Apr 22, 2025

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Done. The following format was used.

- **Y** : Yes. When the implementation is complete


Note : No distinction is made between the support in the Parser/Semantics and MLIR or Lowering support.

## Fortran 2023
See [document](F202X.md) for a brief discussion about the new features in Fortran 2023. The following table summarizes the
status of all important Fortran 2023 features.

| 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 | N | |
| Intrinsics for Trig functions that work in half revolutions| N | |
| 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 | N | |
| 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 | Y | |
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@rouson rouson Mar 24, 2025

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I think this is still in progress. @ergawy can confirm. Should the status be N or P?

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@ergawy ergawy Mar 24, 2025 •
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This is supported for the pure Fortran case, for the do concurrent -> OpenMP case (or do concurrent parallelization in general), this is not supported yet. So I think @kiranchandramohan is indeed correct here.

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@kiranchandramohan kiranchandramohan Mar 24, 2025

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As @ergawy mentions, this is supported for the sequential Fortran case.
But I understand @rouson 's point that this might cause a user to think that reduction works in a parallel sense. So I am changing it to P (partial).

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@rouson rouson Mar 24, 2025

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@kiranchandramohan actually I hadn't thought about the difference between serial support and parallel support. My point was that I didn't even realize that serial support for reduce existed yet on the main branch of llvm-project. I'm building flang now and will try it out. If serial support works, then I think either "Y" or "P" is fine and I leave it up to you to decide.

| Enumerations | N | |

## Fortran 2018
All features except those listed in the following table are supported. Almost all of the unsupported features are related to
coarrays.
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@rouson rouson Mar 24, 2025

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Suggested change
coarrays.
multi-image execution.

Many multi-image parallel features have do not necessarily involve coarrays (e.g., collective subroutines and teams).

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@rouson rouson Mar 24, 2025

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Similarly to how the parallel support for do concurrent is described below as in progress, please state the support for multi-image features is in progress.

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@kiranchandramohan kiranchandramohan Mar 24, 2025

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I now have the following sentence:

 Almost all of the unsupported features are related to the additional parallel features (coarrays/teams/collectives).

The support status of collectives and teams are individually called out in the status table for Fortran 2018.

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@rouson rouson Mar 24, 2025

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@kiranchandramohan thanks for making the edits. "Parallel features" could be interpreted to include parallelization of do concurrent so I'd still suggest "multi-image" to replace "parallel."

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@kiranchandramohan kiranchandramohan Mar 24, 2025

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Done. Made the suggested change.


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

## 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](InternalProcedureTrampolines.md) |

## 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](ParameterizedDerivedTypes.md) |
| Assignment to allocatable | P | Assignment to whole allocatable in FORALL is not implemented |
| The VOLATILE attribute | P | VOLATILE in procedure interfaces 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.
1 change: 1 addition & 0 deletions flang/docs/index.md
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Expand Up @@ -26,6 +26,7 @@ on how to get in touch with us and to learn more about the current status.
C++style
DesignGuideline
FortranForCProgrammers
FortranStandardsSupport
GettingInvolved
GettingStarted
ImplementingASemanticCheck
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