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[Flang][OpenMP] Add OpenMP standards support doc (#132707)
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‎flang/docs/OpenMPSupport.md

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<!--===- docs/OpenMPSupport.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 OpenMP 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 outlines the OpenMP API features supported by Flang. It is intended as a general reference.
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For the most accurate information on unimplemented features, rely on the compiler’s TODO or “Not Yet Implemented”
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messages, which are considered authoritative. With the exception of a few corner cases, Flang
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offers full support for OpenMP 3.1 ([See details here](#openmp-31-openmp-25-openmp-11)).
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Partial support for OpenMP 4.0 is also available and currently under active development.
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The table below outlines the current status of OpenMP 4.0 feature support.
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Work is ongoing to add support for OpenMP 4.5 and newer versions; a support statement for these will be shared in the future.
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The table entries are derived from the information provided in the Version Differences subsection of the Features History section in the OpenMP standard.
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The feature support information is provided as a table with three columns that are self explanatory. The Status column uses
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the letters **P**, **Y**, **N** for the implementation status:
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- **P** : Partial. When the implementation is incomplete for a few cases
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- **Y** : Yes. When the implementation is complete
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- **N** : No. When the implementation is absent
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Note : No distinction is made between the support in Parser/Semantics, MLIR, Lowering or the OpenMPIRBuilder.
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## OpenMP 4.0
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| Feature | Status | Comments |
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|------------------------------------------------------------|--------|---------------------------------------------------------|
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| proc_bind clause | Y | |
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| simd construct | P | linear clause is not supported |
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| declare simd construct | N | |
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| do simd construct | P | linear clause is not supported |
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| target data construct | P | device clause not supported |
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| target construct | P | device clause not supported |
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| target update construct | P | device clause not supported |
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| declare target directive | P | |
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| teams construct | P | reduction clause not supported |
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| distribute construct | P | dist_schedule clause not supported |
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| distribute simd construct | P | dist_schedule and linear clauses are not supported |
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| distribute parallel loop construct | P | dist_schedule clause not supported |
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| distribute parallel loop simd construct | P | dist_schedule and linear clauses are not supported |
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| depend clause | P | depend clause with array sections are not supported |
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| declare reduction construct | N | |
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| atomic construct extensions | Y | |
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| cancel construct | N | |
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| cancellation point construct | N | |
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| parallel do simd construct | P | linear clause is not supported |
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| target teams construct | P | device and reduction clauses are not supported |
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| teams distribute construct | P | reduction and dist_schedule clauses not supported |
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| teams distribute simd construct | P | reduction, dist_schedule and linear clauses are not supported |
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| target teams distribute construct | P | device, reduction and dist_schedule clauses are not supported |
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| teams distribute parallel loop construct | P | reduction and dist_schedule clauses are not supported |
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| target teams distribute parallel loop construct | P | device, reduction and dist_schedule clauses are not supported |
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| teams distribute parallel loop simd construct | P | reduction, dist_schedule, and linear clauses are not supported |
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| target teams distribute parallel loop simd construct | P | device, reduction, dist_schedule and linear clauses are not supported |
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## OpenMP 3.1, OpenMP 2.5, OpenMP 1.1
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All features except a few corner cases in atomic (complex type, different but compatible types in lhs and rhs), threadprivate (character type) constructs/clauses are supported.

‎flang/docs/index.md

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GettingInvolved
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GettingStarted
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ImplementingASemanticCheck
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OpenMPSupport
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PullRequestChecklist
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```
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