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The semantic of pointer assignments inside FORALL requires evaluating the targets (RHS) and pointer variables (LHS) of all iterations before evaluating the assignments. In practice, if the compiler can prove that the RHS and LHS evaluations are not impacted by the assignments, the evaluation of the FORALL assignment statement can be done in a single loop. However, if the compiler cannot prove this, it needs to "save" the addresses of the targets and/or the pointer descriptors of each iterations before doing the assignments. This patch implements the most common cases where there is no lower bound spec, no bounds remapping, the LHS is not polymorphic, and the RHS is not NULL. The HLFIR operation used to represent assignments inside FORALL can be used for pointer assignments to (the only difference being that the LHS is a descriptor address). The analysis for intrinsic assignment can be reused, with the distinction that the RHS data is not read during the assignment. The logic that is used to save LHS in intrinsic assignments inside FORALL is extracted to be used for the RHS of pointer assignments when needed (saving a descriptor value). Pointer assignment LHS are just descriptor addresses and are saved as int_ptr values.
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.