Flang 12.0.0 (In-Progress) Release Notes


These are in-progress notes for the upcoming LLVM 12.0.0 release. Release notes for previous releases can be found on the Download Page.


This document contains the release notes for the Flang Fortran frontend, part of the LLVM Compiler Infrastructure, release 12.0.0. Here we describe the status of Flang in some detail, including major improvements from the previous release and new feature work. For the general LLVM release notes, see the LLVM documentation. All LLVM releases may be downloaded from the LLVM releases web site.

Note that if you are reading this file from a Git checkout, this document applies to the next release, not the current one. To see the release notes for a specific release, please see the releases page.

Known Issues

These are issues that couldn't be fixed before the release. See the bug reports for the latest status.

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Introducing Flang

Flang is LLVM's Fortran front end and is new for the LLVM 11 release.

Flang is still a work in progress for this release and is included for experimentation and feedback.

Flang is able to parse a comprehensive subset of the Fortran language and check it for correctness. Flang is not yet able to generate LLVM IR for the source code and thus is unable to compile a running binary.

Flang is able to unparse the input source code into a canonical form and emit it to allow testing. Flang can also invoke an external Fortran compiler on this canonical input.

Flang's parser has comprehensive support for:

  • Fortran 2018
  • OpenMP 4.5
  • OpenACC 3.0

Interested users are invited to try to compile their Fortran codes with flang in and report any issues in parsing or semantic checking in bugzilla.

Major missing features

  • Flang is not supported on Windows platforms.

Using Flang

Usage: flang hello.f90 -o hello.bin

By default, Flang will parse the Fortran file hello.f90 then unparse it to a canonical Fortran source file. Flang will then invoke an external Fortran compiler to compile this source file and link it, placing the resulting executable in hello.bin.

To specify the external Fortran compiler, set the F18_FC environment variable to the name of the compiler binary and ensure that it is on your PATH. The default value for F18_FC is gfortran.

When invoked with no source input, Flang will wait for input on stdin. When invoked in this way, Flang performs the same actions as if called with -fdebug-measure-parse-tree -funparse and does not invoke F18_FC.

For a full list of options that Flang supports, run flang --help.

Additional Information

Flang's documentation is located in the flang/docs/ directory in the LLVM monorepo.

If you have any questions or comments about Flang, please feel free to contact us via the mailing list.