Flang (also known as "Classic Flang") is an out-of-tree Fortran compiler targeting LLVM. It is an open-sourced version of pgfortran, a commercial Fortran compiler from PGI/NVIDIA. It is different from the new Flang (formerly known as "F18"; see https://flang.llvm.org/), which has been part of the LLVM project since 2020, although both are developed by the same community. It is also unrelated to other projects of the same name, such as https://github.com/llvm-flang/flang and https://github.com/isanbard/flang.
Classic Flang is used in several downstream commercial projects like the AMD, Arm and Huawei compilers, and continues to be maintained, but the plan is to replace Classic Flang with the new Flang in the future.
Visit the Flang wiki for more information:
https://github.com/flang-compiler/flang/wiki
To sign up for the developer mailing lists for announcements and discussions, visit:
https://lists.llvm.org/cgi-bin/mailman/listinfo/flang-dev
We have a flang-compiler channel on Slack. Slack is invitation-only but anyone can join with the invitation link below:
https://join.slack.com/t/flang-compiler/shared_invite/MjExOTEyMzQ3MjIxLTE0OTk4NzQyNzUtODQzZWEyMjkwYw
Instructions for building Flang can be found on the Flang wiki:
https://github.com/flang-compiler/flang/wiki/Building-Flang
For a list of compiler options, enter:
% flang -help
Flang accepts all Clang compiler options and supports many, as well as the following Fortran-specific compiler options:
-noFlangLibs Do not link against Flang libraries
-mp Enable OpenMP and link with with OpenMP library libomp
-nomp Do not link with OpenMP library libomp
-Mbackslash Treat backslash in quoted strings like any other character
-Mnobackslash Treat backslash in quoted strings like a C-style escape character (Default)
-Mbyteswapio Swap byte-order for unformatted input/output
-Mfixed Assume fixed-format source
-Mextend Allow source lines up to 132 characters
-Mfreeform Assume free-format source
-Mpreprocess Run preprocessor for Fortran files
-Mrecursive Generate code to allow recursive subprograms
-Mstandard Check standard conformance
-Msave Assume all variables have SAVE attribute
-module path to module file (-I also works)
-Mallocatable=95 Select Fortran 95 semantics for assignments to allocatable objects
-Mallocatable=03 Select Fortran 03 semantics for assignments to allocatable objects (Default)
-static-flang-libs Link using static Flang libraries
-M[no]daz Treat denormalized numbers as zero
-M[no]flushz Set SSE to flush-to-zero mode
-Mcache_align Align large objects on cache-line boundaries
-M[no]fprelaxed This option is ignored
-fdefault-integer-8 Treat INTEGER and LOGICAL as INTEGER*8 and LOGICAL*8
-fdefault-real-8 Treat REAL as REAL*8
-i8 Treat INTEGER and LOGICAL as INTEGER*8 and LOGICAL*8
-r8 Treat REAL as REAL*8
-fno-fortran-main Don't link in Fortran main
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