f597367581
hlfir.declare with a fir.box input always lead to a fir.rebox being created to ensure the lower bounds and attributes are set correctly in the local descriptor for the entity. This is really needed for hlfir.declare using fir.box function argument that do not come with any guarantees with regards to the lower bounds. Sometimes however, this fir.rebox just adds a lot of noise in the SSA chain, especially at the LLVM level and it is known that the input descriptor is already correct. I am making this patch in the context of OpenACC where I want to remap the variables inside the compute region, creating a fir.rebox on the way. This fir.rebox cannot be optimized away by FIR because of the OpenACC ops in the SSA chain. This patch adds a flag to indicate the the fir.box is known to have the correct lower bounds and attributes so that it can have a simpler code generation to FIR.
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.