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This PR applies the changes discussed in [[RFC] Rationale for Flang AliasAnalysis pointer component logic](https://discourse.llvm.org/t/rfc-rationale-for-flang-aliasanalysis-pointer-component-logic/79252). In summary, this PR replaces the existing pointer component logic in Flang's AliasAnalysis implementation. That logic focuses on aliasing between pointers and non-pointer, non-target composites that have pointer components. However, it is more conservative than necessary, and some existing tests expect its current results when less conservative results seem reasonable. This PR splits the logic into two cases: 1. Source values are the same: Return MayAlias when one value is the address of a composite, and the other value is statically the address of a pointer component of that composite. 2. Source values are different: Return MayAlias when one value is the address of a composite (actual argument), and the other value is the address of a pointer (dummy arg) that might dynamically be a component of that composite. In both cases, the actual implementation is still more conservative than described above, but it can be improved further later. Details appear in the comments. Additionally, this PR revises the logic that reports MayAlias for a pointer/target vs. another pointer/target. It constrains the existing logic to handle only isData cases, and it adds less conservative handling of !isData cases elsewhere. First, it extends case 2 listed above to cover the case where the actual argument is the address of a pointer rather than a composite. Second, it adds a third case: where target attributes enable aliasing with a dummy argument.
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.