GSoC21 W4: LFortran, Backends and Bugs

This post is part of the GSoC21: LFortran series.

Towards the mid-summer evaluation and redirecting efforts

Background

Serialized update for the 2021 Google Summer of Code under the fortran-lang organization, mentored by Ondrej Certik.

Overview

This week was a bit of a sticky wicket. My unfamiliarity with the LLVM backend and its internals caught up with me just around the same time I ran into several fixture and rent related idiosyncrasies which led to yet another shift in my weekly meeting. Thanks to a timely reminder from my mentor Ondrej, I managed, nevertheless, to make some concrete progress working on the compilation of dftatom modules.

New Merge Requests

ASR Bug with Arrays (997): Fixes a bug in the existing handling of arrays and allocate calls

Freshly Assigned Issues

Semantics: Implement TINY (388): A classic intrinsic for error handling
Semantics: Array Initializer Expressions (389): A bugfix related to the merge request
ODE1 Error (390): Placeholder issue to be narrowed and chiseled into something actionable
ASR mismatch between `gfortran` modules and source (395): Issue relating to the completeness and equivalence of mod formats

Additional Tasks

I’d like to make some time to continue exploring the LLVM, and continue familiarizing myself with formal language and automata theory.

Logistics

I met with Ondrej on Friday this week
- He was kind enough to make a standing meeting twice a week, Tuesdays and Fridays
- This was done to ensure concrete progress is made beyond intangibles
Discussed the possibility of extending the lfortran mod interface
- Currently this has a copy of the ASR
- In principle then, later modules are larger since they have relevant code
- While gfortran does not bother with having a concrete representation in the mod file
Discussed forming a functional subset of the dftatom code for now
- This is related to the slight change in direction which is being considered
Rather than stick to one feature / function from front end to back end, it was decided that a more breadth first approach would be better
- Though I am still encouraged to delve into the backend, it is good to set up a fully working rational ASR pass first
Eventually, backends aside, work will need to be undertaken for the runtime library as well

Misc Log

Some other things which came my way this week.

Linking Troubles

Sometime down the line of modules and the roadmap to dftatom described in the master issue here, I ran into an odd linker error. These seem to show up with both miniconda and nix, however, it is unclear to me at the moment if it warrants an issue until I check on a native ArchLinux machine. Another natural fix might be to actually bring in and compile llvm; then use the lld linker instead of the bfd.

1lfortran mod types.mod --show-asr
2Traceback (most recent call last):
3  File "/build/glibc-2.32/csu/../sysdeps/x86_64/start.S", line 120, in _start()
4  Binary file "/nix/store/hp8wcylqr14hrrpqap4wdrwzq092wfln-glibc-2.32-37/lib/libc.so.6", in __libc_start_main()
5  File "/users/home/rog32/Git/Github/Fortran/mylf/src/bin/lfortran.cpp", line 1086, in ??
6    asr = LFortran::mod_to_asr(al, arg_mod_file);
7LFortranException: Unknown module file format

In any case, for now, switching to micromamba and using direnv just works.

Missing Symbols

This actually seems related to the more pertinent issue namely, that my lapack module didn’t seem to contain relevant symbols at all:

1lfortran -c types.f90 -o types.o
2lfortran -c lapack.f90 -o lapack.o
3lfortran -c constants.f90 -o constants.o
4lfortran -c interpolation.f90 -o interpolation.o
5Semantic error: The symbol 'dgesv' not found in the module 'lapack'

This is true actually:

1cat lapack.mod | grep dgesv

Truncated Stacktraces

Also related, probably, was an issue I noticed while co-working with Ondrej, namely, that my stacktraces in nix were and are a good deal less informative than his, probably due to the default nix lack of debug symbols. This can be quickly reproduced with:

 1lfortran character_array.f90
 2BFD: DWARF error: could not find variable specification at offset 2722f
 3BFD: DWARF error: could not find variable specification at offset 2723e
 4BFD: DWARF error: could not find variable specification at offset 2736f
 5BFD: DWARF error: could not find variable specification at offset 2722f
 6BFD: DWARF error: could not find variable specification at offset 2723e
 7BFD: DWARF error: could not find variable specification at offset 2736f
 8BFD: DWARF error: could not find variable specification at offset 2722f
 9BFD: DWARF error: could not find variable specification at offset 2723e
10BFD: DWARF error: could not find variable specification at offset 2736f
11BFD: DWARF error: could not find variable specification at offset 2722f
12BFD: DWARF error: could not find variable specification at offset 2723e
13BFD: DWARF error: could not find variable specification at offset 2736f
14BFD: DWARF error: could not find variable specification at offset 2722f
15BFD: DWARF error: could not find variable specification at offset 2723e
16BFD: DWARF error: could not find variable specification at offset 2736f
17BFD: DWARF error: could not find variable specification at offset 2722f
18BFD: DWARF error: could not find variable specification at offset 2723e
19BFD: DWARF error: could not find variable specification at offset 2736f
20BFD: DWARF error: could not find variable specification at offset 615b9
21Traceback (most recent call last):
22  File "/build/glibc-2.32/csu/../sysdeps/x86_64/start.S", line 120, in _start()
23  Binary file "/nix/store/hp8wcylqr14hrrpqap4wdrwzq092wfln-glibc-2.32-37/lib/libc.so.6", in __libc_start_main()
24  File "/users/home/rog32/Git/Github/Fortran/mylf/src/bin/lfortran.cpp", line 1250, in ??
25    err = compile_to_object_file(arg_file, tmp_o, false,
26  File "/users/home/rog32/Git/Github/Fortran/mylf/src/bin/lfortran.cpp", line 601, in (anonymous namespace)::compile_to_object_file(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool, bool) [clone .isra.0]
27    result = fe.get_asr2(input);
28  File "/users/home/rog32/Git/Github/Fortran/mylf/src/lfortran/codegen/evaluator.cpp", line 469, in LFortran::FortranEvaluator::get_asr2(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)
29    asr = ast_to_asr(al, *ast, symbol_table);
30  File "/users/home/rog32/Git/Github/Fortran/mylf/src/lfortran/semantics/ast_to_asr.cpp", line 3091, in LFortran::ast_to_asr(Allocator&, LFortran::AST::TranslationUnit_t&, LFortran::SymbolTable*)
31    v.visit_TranslationUnit(ast);
32  File "/users/home/rog32/Git/Github/Fortran/mylf/src/lfortran/ast.h", line 3235, in LFortran::SymbolTableVisitor::visit_TranslationUnit(LFortran::AST::TranslationUnit_t const&)
33    case modType::Program: { v.visit_Program((const Program_t &)x); return; }
34  File "/users/home/rog32/Git/Github/Fortran/mylf/src/lfortran/ast.h", line 3555, in LFortran::AST::BaseVisitor<LFortran::SymbolTableVisitor>::visit_program_unit(LFortran::AST::program_unit_t const&)
35    void visit_program_unit(const program_unit_t &b) { visit_program_unit_t(b, self()); }
36  File "/users/home/rog32/Git/Github/Fortran/mylf/src/lfortran/ast.h", line 3561, in LFortran::AST::BaseVisitor<LFortran::SymbolTableVisitor>::visit_unit_decl2(LFortran::AST::unit_decl2_t const&)
37    void visit_unit_decl2(const unit_decl2_t &b) { visit_unit_decl2_t(b, self()); }
38  File "/users/home/rog32/Git/Github/Fortran/mylf/src/lfortran/ast.h", line 3624, in LFortran::AST::BaseVisitor<LFortran::SymbolTableVisitor>::visit_expr(LFortran::AST::expr_t const&)
39    void visit_expr(const expr_t &b) { visit_expr_t(b, self()); }
40  File "/users/home/rog32/Git/Github/Fortran/mylf/src/lfortran/ast.h", line 3369, in void LFortran::AST::visit_expr_t<LFortran::SymbolTableVisitor>(LFortran::AST::expr_t const&, LFortran::SymbolTableVisitor&)
41    static void visit_expr_t(const expr_t &x, Visitor &v) {
42  Binary file "/nix/store/hp8wcylqr14hrrpqap4wdrwzq092wfln-glibc-2.32-37/lib/libc.so.6", in killpg()
43Segfault: Signal SIGSEGV (segmentation fault) received

With a rather simple test problem I was using for my allocatable length character feature.

 1! character_array.f90
 2program character_array
 3  implicit none
 4
 5  character(15) :: something
 6  something = "sUpErWeiRdCaSe"
 7
 8  call upcase(something)
 9
10  something = upcase_func(something)
11
12  contains
13
14subroutine upcase(s)
15! Returns string 's' in uppercase
16character(*), intent(in) :: s
17character(len(s)) :: t
18integer :: i, diff
19t = s; diff = ichar('A')-ichar('a')
20do i = 1, len(t)
21    if (ichar(t(i:i)) >= ichar('a') .and. ichar(t(i:i)) <= ichar('z')) then
22        ! if lowercase, make uppercase
23        t(i:i) = char(ichar(t(i:i)) + diff)
24    end if
25end do
26print*, "Subroutine"
27print*, "Length of arg is ", len(s)
28print*, "Converted " // s // " to " // t
29
30end subroutine
31
32function upcase_func(s) result(t)
33! Returns string 's' in uppercase
34character(*), intent(in) :: s
35character(len(s)) :: t
36integer :: i, diff
37t = s; diff = ichar('A')-ichar('a')
38do i = 1, len(t)
39    if (ichar(t(i:i)) >= ichar('a') .and. ichar(t(i:i)) <= ichar('z')) then
40        ! if lowercase, make uppercase
41        t(i:i) = char(ichar(t(i:i)) + diff)
42    end if
43end do
44! print*,  new_line('c')
45print*, "Function"
46print*, "Length of arg is ", len(s)
47print*, "Converted " // s // " to " // t
48end function
49
50end program character_array

Gfortran Modules

One of the early goals was inter-operability with the gfortran module format. This is a lisp like syntax which changes occasionally.

 1! simple module
 2module b
 3implicit none
 4private
 5public g
 6contains
 7integer function g()
 8g = 5
 9end function
10end module

The corresponding asr is given by:

1(TranslationUnit (SymbolTable 1 {b: (Module (SymbolTable 2 {g: (Function (SymbolTable 3 {g: (Variable 3 g ReturnVar () Default (Integer 4 []) Source Public Required)}) g [] [(= (Var 3 g) (ConstantInteger 5 (Integer 4 [])))] (Var 3 g) Source Public Implementation)}) b [] .false.)}) [])

For the currently supported gfortran module (v14) we get:

1lfortran mod b14.mod --show-asr
2(TranslationUnit (SymbolTable 1 {g: (Subroutine (SymbolTable 2 {}) g [] [] GFortranModule Public Interface)}) [])

Which is contains a lower amount of information. The differences between the module versions are not major:

More importantly however, the module versions correspond to being compressed and this needs to be eventually handled as well.

Conclusions

For the next week, I shall be finalizing work on the allocatable character lengths and moving on to some more semantics and intrinsics. Naturally as these are completed there will be more progress down the dftatom module listing. The main takeaway from this week for me was the retargeting of efforts. I do wish to continue struggling with the backend simply because I feel it is exciting and fun, but I shall endeavor to be more productive by applying a breadth first approach by contributing more concrete code every week. I have high hopes for being able to power through the intended project goals by the end of the program, and can always clean up / document afterwards.

GSoC21 W4: LFortran, Backends and Bugs

Background

Overview

New Merge Requests

Freshly Assigned Issues

Additional Tasks

Logistics

Misc Log

Linking Troubles

Missing Symbols

Truncated Stacktraces

Gfortran Modules

Conclusions

Series info

GSoC21: LFortran series