[llvm-dev] [RFC] Upstream ObjCMetadata Reader Library

[David Chisnall via llvm-dev]

Hi Steven,

This sounds like a very useful tool.  In Clang, we've been very careful 
to maintain an abstraction layer over different Objective-C runtime 
implementations.  Your email talked only about the Apple runtime - do 
you already have an abstraction layer for the runtime-specific details, 
or is this something that will need to be added prior to upstreaming?

David

On 22/03/2019 18:34, Steven Wu via llvm-dev wrote:
> Overview
> =======
> We are planning to open source our internal implementation of 
> Objective-C metadata reader library to LLVM. Unlike the C style 
> objective-c metadata reader in llvm-objdump, it is designed to be a 
> reusable C++ library to parse objective-c metadata from binaries with 
> LLVM style error handling. The current implementation has an interface 
> that resembles libObject in and it supports parsing objective-c metadata 
> from MachO object file or LLVM bitcode that is generated by clang/swift 
> compiler. It has the flexibility to be extended to support other 
> objective-c metadata format if needed.
> You can find an old open-source version of implementation here[1]. The 
> version we planned to open source has the same design but it will come 
> with some bug fixes and improvements, and it will be rebased to the trunk.
> 
> Background
> =========
> We think it is very beneficial to have an objective-c metadata library 
> in LLVM. We use the library in many tools, including tapi which we are 
> currently in the process of open sourcing. Some have been asking for 
> this libraries on the list or off the list to use it in the contexts 
> like JIT[2] or macports.
> 
> Originally, the open source of this library is delayed because we 
> thought it would be good if we can achieve following two goals before 
> upstreaming:
> 
>   * An improved libObject interface: we are expecting libObject
>     interfaces to be changed which ObjCMetadata library should be
>     updated to match.
>   * Replace the parser inside llvm-objdump.
> 
> The first one has been talked for years but never put into action. The 
> second one requires a strict output match because llvm-objdump is a 
> replacement for otool-classic on darwin platform, and doing so will 
> require LLVM infrastructure improvements on how to handle errors. With 
> some discussion, we believe both tasks can be done in trunk when it is 
> time, rather than waiting for all the requirements are met.
> 
> Design
> =====
> ObjCMetadata library comes with headers (in 
> llvm/include/llvm/ObjCMetadata)[3], implementations (in 
> llvm/lib/ObjCMetadata)[4] and we will also implement a tool similar to a 
> stripped down version of [5] for testing.
> Here is an example how to use the library by extracting and printing the 
> objc class names:
> ```
>    MachOMetadata ObjCInfo(InputObject);
> *if* (*auto* ObjCClasses = ObjCInfo.classes()) {
> *for* (*auto* c : *ObjCClasses) {
> *auto* ObjCClass = *c;
> *if* (!ObjCClass) {
>          handleError(ObjCClass.takeError());
> *continue*;
>        }
> *auto* name = ObjCClass->getName();
> *if* (!name) {
>          handleError(name.takeError());
> *continue*;
>        }
>        outs() << *name << "\n";
>    }
> ```
> 
> I am preparing a patch and will send out when it is ready. Let me know 
> if you have any questions or feedbacks.
> 
> Thanks
> 
> Steven
> 
> [1] https://opensource.apple.com/source/clang/clang-800.0.38
> [2] http://lists.llvm.org/pipermail/llvm-dev/2016-November/106995.html
> [3] 
> https://opensource.apple.com/source/clang/clang-800.0.38/src/lib/ObjCMetadata/
> [4] 
> https://opensource.apple.com/source/clang/clang-800.0.38/src/include/llvm/ObjCMetadata/
> [5] 
> https://opensource.apple.com/source/clang/clang-800.0.38/src/tools/api-analyzer/
> 
> 
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