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So far flang generates runtime derived type info global definitions (as opposed to declarations) for all the types used in the current compilation unit even when the derived types are defined in other compilation units. It is using linkonce_odr to achieve derived type descriptor address "uniqueness" aspect needed to match two derived type inside the runtime. This comes at a big compile time cost because of all the extra globals and their definitions in apps with many and complex derived types. This patch adds and experimental option to only generate the rtti definition for the types defined in the current compilation unit and to only generate external declaration for the derived type descriptor object of types defined elsewhere. Note that objects compiled with this option are not compatible with object files compiled without because files compiled without it may drop the rtti for type they defined if it is not used in the compilation unit because of the linkonce_odr aspect. I am adding the option so that we can better measure the extra cost of the current approach on apps and allow speeding up some compilation where devirtualization does not matter (and the build config links to all module file object anyway).
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.