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[Flang] Add a Fortran Standards Support doc (#132195)
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‎flang/docs/FortranStandardsSupport.md

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<!--===- docs/FortranStandardsSupport.md
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Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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See https://llvm.org/LICENSE.txt for license information.
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SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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-->
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# Flang Fortran Standards Support
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```{contents}
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---
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local:
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---
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```
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This document summarizes Fortran standards support in Flang. The information is only provided as a guideline. The
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TODOs/Not Yet Implemented messages emitted by the compiler for unimplemented features should be treated as authoritative.
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The standards support information is provided as a table with three columns that are self explanatory.
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The Status column uses the letters **Y**, **P**, **N** for the implementation status:
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- **Y** : Yes. When the implementation is complete
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- **P** : Partial. When the implementation is incomplete for a few cases
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- **N** : No. When the implementation is absent
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There is no distinction made between support in the Parser/Semantics and the MLIR or Lowering stages.
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Note: The two major missing features in Flang at present are coarrays and parameterized derived types (PDTs) with length type parameters.
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## Fortran 2023
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See [document](F202X.md) for a brief discussion about the new features in Fortran 2023. The following table summarizes the
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status of all important Fortran 2023 features. The table entries are based on the document [The new features in Fortran 2023](https://wg5-fortran.org/N2201-N2250/N2212.pdf).
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| Feature | Status | Comments |
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|------------------------------------------------------------|--------|---------------------------------------------------------|
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| Allow longer statement lines and overall statement length | Y | |
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| Automatic allocation of lengths of character variables | N | |
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| The specifiers typeof and classof | N | |
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| Conditional expressions and arguments | N | |
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| More use of boz constants | P | All usages other than enum are supported |
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| Intrinsics for extracting tokens from a string | N | |
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| Intrinsics for Trig functions that work in degrees | N | |
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| Intrinsics for Trig functions that work in half revolutions| N | |
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| Changes to system_clock | N | |
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| Changes for conformance with the new IEEE standard | Y | |
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| Additional named constants to specify kinds | Y | |
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| Extensions for c_f_pointer intrinsic | N | |
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| Procedures for converting between fortran and c strings | N | |
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| The at edit descriptor | N | |
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| Control over leading zeros in output of real values | N | |
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| Extensions for Namelist | N | |
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| Allow an object of a type with a coarray ultimate component to be an array or allocatable | N | |
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| Put with Notify | N | |
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| Error conditions in collectives | N | |
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| Simple procedures | N | |
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| Using integer arrays to specify subscripts | N | |
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| Using integer arrays to specify rank and bound of an array | N | |
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| Using an integer constant to specify rank | N | |
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| Reduction specifier for do concurrent | P | Syntax is accepted |
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| Enumerations | N | |
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## Fortran 2018
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All features except those listed in the following table are supported. Almost all of the unsupported features are related to
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the multi-image execution. The table entries are based on the document [The new features in Fortran 2018](https://wg5-fortran.org/N2151-N2200/ISO-IECJTC1-SC22-WG5_N2161_The_New_Features_of_Fortran_2018.pdf).
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| Feature | Status | Comments |
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|------------------------------------------------------------|--------|---------------------------------------------------------|
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| Asynchronous communication | P | Syntax is accepted |
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| Teams | N | Multi-image/Coarray feature |
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| Image failure | P | Multi-image/Coarray feature. stat_failed_image is added |
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| Form team statement | N | Multi-image/Coarray feature |
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| Change team construct | N | Multi-image/Coarray feature |
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| Coarrays allocated in teams | N | Multi-image/Coarray feature |
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| Critical construct | N | Multi-image/Coarray feature |
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| Lock and unlock statements | N | Multi-image/Coarray feature |
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| Events | N | Multi-image/Coarray feature |
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| Sync team construct | N | Multi-image/Coarray feature |
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| Image selectors | N | Multi-image/Coarray feature |
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| Intrinsic functions get_team and team_number | N | Multi-image/Coarray feature |
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| Intrinsic function image_index | N | Multi-image/Coarray feature |
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| Intrinsic function num_images | N | Multi-image/Coarray feature |
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| Intrinsic function this_image | N | Multi-image/Coarray feature |
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| Intrinsic move_alloc extensions | P | Multi-image/Coarray feature |
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| Detecting failed and stopped images | N | Multi-image/Coarray feature |
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| Collective subroutines | N | Multi-image/Coarray feature |
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| New and enhanced atomic subroutines | N | Multi-image/Coarray feature |
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| Failed images and stat= specifiers | N | Multi-image/Coarray feature |
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| Intrinsic function coshape | N | Multi-image/Coarray feature |
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## Fortran 2008
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All features except those listed in the following table are supported.
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| Feature | Status | Comments |
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|------------------------------------------------------------|--------|---------------------------------------------------------|
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| Coarrays | N | Lowering and runtime support is not implemented |
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| do concurrent | P | Sequential execution works. Parallel support in progress|
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| Internal procedure as an actual argument or pointer target | Y | Current implementation requires stack to be executable. See [Proposal](InternalProcedureTrampolines.md) |
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## Fortran 2003
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All features except those listed in the following table are supported.
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| Feature | Status | Comments |
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|------------------------------------------------------------|--------|---------------------------------------------------------|
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| Parameterized Derived Types | P | PDT with length type parameters is not supported. See [Proposal](ParameterizedDerivedTypes.md) |
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| Assignment to allocatable | P | Assignment to whole allocatable in FORALL is not implemented |
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| The VOLATILE attribute | P | VOLATILE in procedure interfaces is not implemented |
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| Asynchronous input/output | P | IO will happen synchronously |
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| MIN/MAX extensions for CHARACTER | P | Some variants are not supported |
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## Fortran 95
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All features are supported.
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## Fortran 90
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All features are supported.
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## FORTRAN 77
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All features are supported.

‎flang/docs/index.md

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C++style
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DesignGuideline
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FortranForCProgrammers
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FortranStandardsSupport
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GettingInvolved
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GettingStarted
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ImplementingASemanticCheck

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