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This patch adds a flag to mark hlfir.declare of host variables that are captured in some internal procedure. It enables implementing a simple fir.call handling in fir::AliasAnalysis::getModRef leveraging Fortran language specifications and without a data flow analysis. This will allow implementing an optimization for "array = array_function()" where array storage is passed directly into the hidden result argument to "array_function" when it can be proven that arraY_function does not reference "array". Captured host variables are very tricky because they may be accessed indirectly in any calls if the internal procedure address was captured via some global procedure pointer. Without flagging them, there is no way around doing a complex inter procedural data flow analysis: - checking that the call is not made to an internal procedure is not enough because of the possibility of indirect calls made to internal procedures inside the callee. - checking that the current func.func has no internal procedure is not enough because this would be invalid with inlining when an procedure with internal procedures is inlined inside a procedure without internal procedure.
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.