2e353a635b
OpenACC declare statements are restricted from having having clauses
that reference assumed size arrays. It should be the case that we can
implement `deviceptr` and `present` clauses for assumed-size arrays.
This is a first step towards relaxing this restriction.
Note running flang on the following example results in an error in
lowering.
```
$ cat t.f90
subroutine vadd (a, b, c, n)
real(8) :: a(*), b(*), c(*)
!$acc declare deviceptr(a, b, c)
!$acc parallel loop
do i = 1,n
c(i) = a(i) + b(i)
enddo
end subroutine
$ flang -fopenacc -c t.f90
error: loc("/home/akuhlenschmi/work/p4/ta/tests/openacc/src/t.f90":3:7): expect declare attribute on variable in declare operation
error: Lowering to LLVM IR failed
error: loc("/home/akuhlenschmi/work/p4/ta/tests/openacc/src/t.f90":4:7): unsupported OpenACC operation: acc.private.recipe
error: loc("/home/akuhlenschmi/work/p4/ta/tests/openacc/src/t.f90":4:7): LLVM Translation failed for operation: acc.private.recipe
error: failed to create the LLVM module
```
I would like to to share this code, because others are currently working
on the implementation of `deviceptr`, but it is obviously not running
end-to-end. I think the cleanest approach to this would be to put this
exception to the rule behind some feature flag, but I am not certain
what the precedence for that is.
Flang
Flang is a ground-up implementation of a Fortran front end written in modern C++. It started off as the f18 project (https://github.com/flang-compiler/f18) with an aim to replace the previous flang project (https://github.com/flang-compiler/flang) and address its various deficiencies. F18 was subsequently accepted into the LLVM project and rechristened as Flang.
Please note that flang is not ready yet for production usage.
Getting Started
Read more about flang in the docs directory. Start with the compiler overview.
To better understand Fortran as a language and the specific grammar accepted by flang, read Fortran For C Programmers and flang's specifications of the Fortran grammar and the OpenMP grammar.
Treatment of language extensions is covered in this document.
To understand the compilers handling of intrinsics, see the discussion of intrinsics.
To understand how a flang program communicates with libraries at runtime, see the discussion of runtime descriptors.
If you're interested in contributing to the compiler, read the style guide and also review how flang uses modern C++ features.
If you are interested in writing new documentation, follow LLVM's Markdown style guide.
Consult the Getting Started with Flang for information on building and running flang.