[Clang][CIR][Doc] Document CIR code duplication plans

clang/docs/ClangIRCodeDuplication.rst

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+================================
+ClangIR Code Duplication Roadmap
+================================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+This document describes the general approach to code duplication in the ClangIR
+code generation implementation. It acknowledges specific problems with the
+current implementation, discusses strategies for mitigating the risk inherent in
+the current approach, and describes a general long-term plan for addressing the
+issue.
+
+Background
+==========
+
+The ClangIR code generation is very closely modeled after Clang's LLVM IR code
+generation, and we intend for the CIR produced to eventually be semantically
+equivalent to the LLVM IR produced when not going through ClangIR. However, we
+acknowledge that as the ClangIR implementation is under development, there will
+be differences in semantics, both because we have not yet implemented all
+features of the classic codegen and because the CIR dialect is still evolving
+and does not yet have a way to represent all of the necessary semantics.
+
+We have chosen to model the ClangIR code generation directly after the classic
+codegen, to the point of following identical code structure, using similar names
+and often duplicating the logic because this seemed to be the most certain path
+to producing equivalent results. Having such nearly identical code allows for
+direct comparison between the CIR codegen and the LLVM IR codegen to find what
+is missing or incorrect in the CIR implementation.
+
+However, we recognize that this is not a sustainable permanent solution. As
+bugs are fixed and new features are added to the classic codegen, the process of
+keeping the analogous CIR code up to date will be a purely manual process.
+
+Long term, we need a more sustainable approach.
+
+Current Strategy
+================
+
+Practical considerations require that we make steady progress towards a working
+implementation of ClangIR. This necessity is directly opposed to the goal of
+minimizing code duplication.
+
+For this reason, we have decided to accept a large amount of code duplication
+in the short term, even with the explicit understanding that this is producing
+a significant amount of technical debt as the project progresses.
+
+As the CIR implementation is developed, we often note small pieces of code that
+could be shared with the classic codegen if they were moved to a different part
+of the source, such as a shared utility class in some directory available to
+both codegen implementations or by moving the function into a related AST class.
+It is left to the discretion of the developer and reviewers to decide whether
+such refactoring should be done during the CIR development, or if it is
+sufficient to leave a comment in the code indicating this as an opportunity for
+future improvement. Because much of the current code is likely to change when
+the long term code sharing strategy is complete, we will lean towards only
+implementing refactorings that make sense independent of the code sharing
+problem.
+
+We have discussed various ways that major classes such as CGCXXABI/CIRGenCXXABI
+could be refactored to allow parts of there implementation to be shared today
+through inheritence and templated base classes. However, this may prove to be
+wasted effort when the permanent solution is developed, so we have decided that
+it is better to accept significant amounts of code duplication now, and defer
+this type of refactoring until it is clear what the permanent solution will be.
+
+Mitigation Through Testing
+==========================
+
+The most important tactic that we are using to mitigate the risk of CIR diverging
+from classic codegen is to incorporate two sets of LLVM IR checks in the CIR
+codegen LIT tests. One set checks the LLVM IR that is produced by first
+generating CIR and then lowering that to LLVM IR. Another set checks the LLVM IR
+that is produced directly by the classic codegen.
+
+At the time that tests are created, we compare the LLVM IR output from these two
+paths to verify (manually) that any meaningful differences between them are the
+result of known missing features in the current CIR implementation. Whenever
+possible, differences are corrected in the same PR that the test is being added,
+updating the CIR implementation as it is being developed.
+
+However, these tests serve a second purpose. They also serve as sentinels to
+alert us to changes in the classic codegen behavior that will need to be
+accounted for in the CIR implementation. While we appreciate any help from
+developers contributing to classic codegen, our current expectation is that it
+will be the responsibility of the ClangIR contributors to update the CIR
+implementation when these tests fail.
+
+As the CIR implementation gets closer to the goal of IR that is semantically
+equivalent to the LLVM IR produced by the classic codegen, we would like to
+enhance the CIR tests to perform some automatic verification of the equivalence
+of the generated LLVM IR, perhaps using a tool such as Alive2.
+
+Eventually, we would like to be able to run all existing classic codegen tests
+using the CIR path as well.
+
+Other Considerations
+====================
+
+The close modeling of CIR after classic codegen has also meant that the CIR
+dialect often represents language details at a much lower level than it ideally
+should.
+
+In the interest of having a complete working implementation of ClangIR as soon
+as is practical, we have chosen to take the approach of following the classic
+codegen implementation closely in the initial implementation and only raising
+the representation in the CIR dialect to a higher level when there is a clear
+and immediate benefit to doing so.
+
+Over time, we expect to progressively raise the CIR representation to a higher
+level and remove low level details, including ABI-specific handling from the
+dialect. However, having a working implementation in place makes it easier to
+verify that the high level representation and subsequent lowering are correct.
+
+Mixing With Other Dialects
+==========================
+
+Mixing of dialects is a central design feature of MLIR. The CIR dialect is
+currently more self-contained than most dialects, but even now we generate
+the ACC (OpenACCC) dialect in combination with CIR, and when support for OpenMP
+and CUDA are added, similar mixing will occur.
+
+We also expect CIR to be at least partially lowered to other dialects during
+the optimization phase to enable features such as data dependence analysis, even
+if we will eventually be lowering it to LLVM IR.
+
+Therefore, any plan for generating LLVM IR from CIR must be integrated with the
+general MLIR lowering design, which typically involves lowering to the LLVM
+dialect, which is then transformed to LLVM IR.
+
+Other Consumers of CIR and MLIR
+===============================
+
+We must also consider that we will not always be lowering CIR to LLVM IR. CIR,
+usually mixed with other dialects, will also be directed to offload targets
+and other code generators through interfaces that are opaque to Clang. We must
+still produce semantically correct CIR for these consumers.
+
+Long Term Vision
+================
+
+As the CIR implementation matures, we will eliminate target-specific handling
+from the high-level CIR generated by Clang. The high-level CIR will then be
+progressively lowered to a form that is closer to LLVM IR, including a pass
+that inserts ABI-specific handling, potentially representing the target-specific
+details in another dialect.
+
+As we raise CIR to this higher level implementation, there will naturally be
+less code duplication, and less need to have the same logic repeated in the
+CIR generation.
+
+We will continue to use that same basic design and structure for CIR code
+generation, with classes like CIRGenModule and CIRGenFunction that serve the
+same purpose as their counterparts in classic codegen, but the handling there
+will be more closely tied to core semantics and therefore less likely to require
+frequent changes to stay in sync with classic codegen.
+
+As the handling of low-level details is moved to later lowering phases, we will
+need to move away from the current tight coupling of the CIR and classic codegen
+implementations. As this happens, we will look for ways that this handling can
+be moved to new classes that are specifically designed to be shared among
+clients that are targeting different IR substrates. That is, rather than trying
+to overlay reuse onto the existing implementations, we will replace relevant
+parts of the existing implementation, piece by piece, as appropriate, with new
+implementations that perform the same function but with a more general design.
+
+Example: C Calling Convention Handling
+======================================
+
+C calling convention handling is an example of a general purpose redesign that
+is already underway. This was started independently of CIR, but it will be
+directly useful for lowering from high-level call representation in CIR to a
+representation that includes the target- and calling convention-specific details
+of function signatures, parameter type coercion, and so on.
+
+The current CIR implementation duplicates most of the classic codegen handling
+for function call handling, but it omits several pieces that handle type
+coercion. This leads to an implementation that has all of the complexity of the
+class codegen without actually achieving the goals of that complexity. It will
+be a significant improvement to the CIR implementation to simplify the function
+call handling in such a way that it generates a high-level representation of the
+call, while preserving all information that will be needed to lower the call to
+an ABI-compliant representation in a later phase of compilation.
+
+This provides a clear example where trying to refactor the classic codegen in
+some way to be reused by CIR would have been counterproductive. The classic
+codegen implementation was tightly coupled with Clang's LLVM IR generation. The
+implementation is being completely redesigned to allow general reuse, not just by
+CIR, but also by other front ends.
+
+The CIR calling convention lowering will make use of the general purpose C
+calling convention library that is being created, but it should create an MLIR
+transform pass on top of that library that is general enough to be used by other
+dialects, such as FIR, that also need the same calling convention handling.
+
+Significant Areas For Improvement
+=================================
+
+The following list enumerates some of the areas where significant restructuring
+of the code is needed to enable better code sharing between CIR and classic
+codegen. Each of these areas is relatively self-contained in the codegen
+implementation, making the path to a shared implementation relatively clear.
+
+- Constant expression evaluation
+- Complex multiplication and division expansion
+- Builtin function handling
+- Exception Handling and C++ Cleanups
+- Inline assembly handling
+- C++ ABI Handling
+
+  - VTable generation
+  - Virtual function calls
+  - Constructor and destructor arguments
+  - Dynamic casts
+  - Base class address calculation
+  - Type descriptors
+  - Array new and delete
+
+Pervasive Low-Level Issues
+==========================
+
+This section lists some of the features where a non-trivial amount of code
+is shared between CIR and classic codegen, but the handling of the feature
+is distributed across the codegen implementation, making it more difficult
+to design an abstraction that can easily be shared.
+
+- Global variable and function linkage
+- Alignment management
+- Debug information
+- TBAA handling
+- Sanitizer integration
+- Lifetime markers