This post is part of the GSoC21: LFortran series.

Delving into language standards and back-ends for lfortran


As discussed in a previous post in this series, I have been spending roughly half of each working day with LFortran as part of the 2021 Google Summer of Code under the fortran-lang organization, mentored by Ondrej Certik.


Some of the meeting points are to be expanded on below.

  • Met with Ondrej on Tuesday, as discussed previously
    • Talked about language server implementations
      • Looked into rtags and generating a compilation-database
      • Discussed how the C++ concept of having file based units makes this simpler than the Fortran form, which recognizes no file based program units
  • Talked about the status of the different back-ends
  • Discussed LLVM and MLIR, in the context of Flang (the f18 compiler)
    • Also briefly touched upon legacy-flang and historical issues
  • Discussed standardization of the mod-files
    • Decided this is not a good idea, because a lot of build systems expect Fortran compilers to generate .mod files, even if they are completely incompatible
    • The standard does not specify what should be contained in a mod file
    • Every mod file of every compiler is unique and needs to be handled separately
    • The goal (of this project too) is to handle at the very least conversion of gfortran module files to lfortran module files


For the second week of my project on getting lfortran to compile dftatom and in general form a usable compiler ecosystem to facilitate greater adoption in the community, I opted to take a bit of a step back and delve into the historical evolution of both the Fortran standard itself Lyon (1980) and also the compiler ecosystem. This was also in no small part due to the fact that I ended up moving during this week, which forced me to spend a lot of time cleaning and playing with adult LEGO 1. This naturally left me with plenty of time to both participate 2 in the monthly Fortran-lang call (which was rather explosive 3) and also contemplate the overall ecosystem of the standards committee and compilers. Many more issues were set up and will be completed over the weekend.

Merge Requests

Minor installation bug (985)
Fixed a small issue with the cmake files

Assigned Issues

Handling kind in the ASR (357)
A straightforward correction to make the ASR handle more common, but non-standard use cases
GFortran Module v15 support (355)
Currently only supports v14
Runtime Math and the CPP back-end (354)
Might take a little longer, does not compile at the moment

Additional Tasks

I intend to write more about the Fortran standard and go through more of the compiler code I can get my hands on, since that should give me a better handle on the different way the standards have been implemented (and augmented!). As part of this, I’ll probably handle some of issue 350; regarding the main site.

LFortran and Back-ends


This is the default, and the most performance oriented. It is however, slightly more complicated to extend due to the intricacies of the LLVM syntax.


This is a newer back-end, perfect for rapid prototyping; and can be selected by passing the --backend=cpp flag to lfortran (e.g. with FFLAGS). This is easier to work with in that many features boil down to stdlib calls. However, this back-end requires KOKKOS. It also has uglier error handling as shown in Fig. 1. Since this is more attractive as a candidate for the math intrinsics as a first approximation, it is of considerable interest to me.

It is however, still fairly straightforward to work with.

 1cd $LFROOT # LFortran root
 2export LFORTRAN_KOKKOS_DIR=$LFROOT/ext/kokkos
 3mkdir extsrc && cd extsrc
 4gh repo clone kokkos/kokkos
 5cd kokkos && mkdir build && cd build
 7make -j$(nproc)
 8make install
 9cd $LFORTRAN_KOKKOS_DIR # Need to make a symlink
10ln -sf lib64/ lib
11cd ../../examples/project1
12FC=lfortran FFLAGS="--backend=cpp" cmake .
14./project1 # Profit

Figure 1: Contrasting backends

Figure 1: Contrasting backends

Inspecting intermediates

The module files cannot currently be read or queried from lfortran, however a representation of what gets compressed into these files can be obtained with the --show-asr flag. Additionally, a very python inspired --show-stacktrace is implemented to help narrow down areas which need to be augmented.


 1cd $DFTATM/src
 2gfortran -c types.f90
 3# Read gfortran module
 4zcat types.mod
 5# Output
 6GFORTRAN module version '15' created from types.f90
 7(() () () () () () () () () () () () () () () () () () () () () () () ()
 8() () ())
21IMPLICIT-SAVE 0 0) () (INTEGER 4 0 0 0 INTEGER ()) 0 0 () (CONSTANT (
22INTEGER 4 0 0 0 INTEGER ()) 0 '8' ()) () 0 () () () 0 0)
25('dp' 0 2)


1cd $DFTATM/src
2lfortran --show-asr -c types.f90
3# Output
4(TranslationUnit (SymbolTable 1 {lfortran_intrinsic_kind: (Module (SymbolTable 4 {dkind: (Function (SymbolTable 5 {r: (Variable 5 r ReturnVar () Default (Integer 4 []) Source Public), x: (Variable 5 x In () Default (Real 8 []) Source Public)}) dkind [(Var 5 x)] [(= (Var 5 r) (ConstantInteger 8 (Integer 4 [])))] (Var 5 r) Intrinsic Public Implementation), kind: (Function (SymbolTable 6 {r: (Variable 6 r ReturnVar () Default (Integer 4 []) Source Public), x: (Variable 6 x In () Default (Logical 4 []) Source Public)}) kind [(Var 6 x)] [(= (Var 6 r) (ConstantInteger 4 (Integer 4 [])))] (Var 6 r) Intrinsic Public Implementation), lkind: (Function (SymbolTable 7 {r: (Variable 7 r ReturnVar () Default (Integer 4 []) Source Public), x: (Variable 7 x In () Default (Logical 4 []) Source Public)}) lkind [(Var 7 x)] [(= (Var 7 r) (ConstantInteger 4 (Integer 4 [])))] (Var 7 r) Intrinsic Public Implementation), selected_int_kind: (Function (SymbolTable 8 {R: (Variable 8 R In () Default (Integer 4 []) Source Public), res: (Variable 8 res ReturnVar () Default (Integer 4 []) Source Public)}) selected_int_kind [(Var 8 R)] [(If (Compare (Var 8 R) Lt (ConstantInteger 10 (Integer 4 [])) (Logical 4 [])) [(= (Var 8 res) (ConstantInteger 4 (Integer 4 [])))] [(= (Var 8 res) (ConstantInteger 8 (Integer 4 [])))])] (Var 8 res) Intrinsic Public Implementation), selected_real_kind: (Function (SymbolTable 9 {R: (Variable 9 R In () Default (Integer 4 []) Source Public), res: (Variable 9 res ReturnVar () Default (Integer 4 []) Source Public)}) selected_real_kind [(Var 9 R)] [(If (Compare (Var 9 R) Lt (ConstantInteger 7 (Integer 4 [])) (Logical 4 [])) [(= (Var 9 res) (ConstantInteger 4 (Integer 4 [])))] [(= (Var 9 res) (ConstantInteger 8 (Integer 4 [])))])] (Var 9 res) Intrinsic Public Implementation), skind: (Function (SymbolTable 10 {r: (Variable 10 r ReturnVar () Default (Integer 4 []) Source Public), x: (Variable 10 x In () Default (Real 4 []) Source Public)}) skind [(Var 10 x)] [(= (Var 10 r) (ConstantInteger 4 (Integer 4 [])))] (Var 10 r) Intrinsic Public Implementation)}) lfortran_intrinsic_kind [] .true.), types: (Module (SymbolTable 2 {dp: (Variable 2 dp Local (FunctionCall 2 kind () [(ConstantReal "0.d0" (Real 4 []))] [] (Integer 4 [])) Parameter (Integer 4 []) Source Public), kind: (ExternalSymbol 2 kind 4 kind lfortran_intrinsic_kind kind Private)}) types [lfortran_intrinsic_kind] .false.)}) [])

Note that, lfortran is expected to eventually interoperate with existing mod files from compilers like gfortran.

 1lfortran mod --show-asr types.mod
 2# Output
 3Traceback (most recent call last):
 4  File "/build/glibc-2.32/csu/../sysdeps/x86_64/start.S", line 120, in _start()
 5  Binary file "/nix/store/hp8wcylqr14hrrpqap4wdrwzq092wfln-glibc-2.32-37/lib/", in __libc_start_main()
 6  File "$HOME/Git/Github/Fortran/mylf/src/bin/lfortran.cpp", line 1076, in ??
 7    asr = LFortran::mod_to_asr(al, arg_mod_file);
 8  File "$HOME/Git/Github/Fortran/mylf/src/lfortran/mod_to_asr.cpp", line 361, in LFortran::mod_to_asr(Allocator&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)
 9    return parse_gfortran_mod_file(al, s);
10LFortranException: Only GFortran module version 14 is implemented so far

Which is the source of an issue assigned to me.


As I cross my fingers and hope my tenuously tethered wifi is equal to the task of uploading this post; I can only hope that the installation of my WiFi on Monday goes smoothly enough to be restored to full working capacity for the next week. Until then, I shall trawl the code-bases and standards for an inkling of what makes compiler design the art form as described in Cooper and Torczon (2011).


Cooper, Keith, and Linda Torczon. 2011. Engineering a Compiler. Elsevier.

Lyon, G. E. 1980. Using Ans Fortran. National Bureau of Standards.

  1. I am convinced the concept behind IKEA and its Icelandic counterpart Rumfatalagerinn is that some people enjoy assembling their furniture the same way kids enjoy LEGO ↩︎

  2. Owing to the lack of a stable internet I was reduced to text-only participation ↩︎

  3. There were some interesting allegations of the standards committee being far too conservative, though many of the concerns reflect on the dearth of community efforts compared to other languages; Fortran is not after-all coupled to a compiler unlike modern languages like Rust and Julia ↩︎

Series info

GSoC21: LFortran series

  1. GSoC21 W1: LFortran Kickoff
  2. GSoC21 W2: LFortran Unraveling <-- You are here!
  3. GSoC21 W3: Kind, Characters, and Standards
  4. GSoC21 W4: LFortran, Backends and Bugs
  5. GSoC21 W5: LFortran Design Details and minidftatom
  6. GSoC21 W6: LFortran ASR and Values
  7. GSoC21 W7: LFortran Workflow Basics
  8. GSoC21 W8: LFortran Refactors, and Compile Time Evaluations
  9. GSoC21 W9: LFortran Bug Hunting Bonanza
  10. GSoC21 W10: LFortran Runtime Library Design
  11. GSoC21 LFortran and Computational Chemistry