Verify that generated DWARF debugging information works in Chrome DevTools

[swiftwasm]

Verify that generated DWARF debugging information works in Chrome DevTools

[MaxDesiatov]

I think adding breakpoints and stepping through source code is as good as it can get right now. The rest depends on browser implementors and how quickly the debugging format is standardized, as seen here https://github.com/WebAssembly/debugging. I'm happy to investigate if something can be fixed on SwiftWasm side that browser do support already in a documented way, but I'm not sure if watching variable content is one of such features.

[kateinoigakukun]

As far as I know, DWARF support on Chrome is still in progress, and it only works with binaries built on the local machine because dwarf information has absolute path to source file.
At least, my debugger wasminspect works with Swift WebAssembly binary well.
So I think Swift compiler emits enough debug information through LLVM.

[kateinoigakukun]

Thank you! I wrote a document, but we need to improve that more.

As you say, wasminspect can't debug binaries using browser-specific features. In the long run, DWARF support in Chrome DevTool is an essential feature to popularize Swift on Web.

(I also planned to contribute to Chrome DevTools, but the project was too big, so I gave up at that time.)

[fjtrujy]

Hello,
I have tried as well to use Chrome DevTools for debugging Swift WebAssembly code.

I have been able to read clearly Swift files and classes, I can put breakpoints and I can go during debugging, step by step, however, I'm not able to watch variables content and so on.

Do we have any updates about how to use it?

Thanks.

[MaxDesiatov]

wasminspect is fantastic, and I much appreciate your work on that, we definitely should link to it as we improve user-visible documentation. I'm not sure though if it would work well if a user needs to debug issues with DOM, as far as I understand it doesn't provide a simulated browser DOM environment?

[MaxDesiatov]

They've posted an update, and looks like it's fully working with Emscripten 🤔

https://developers.google.com/web/updates/2020/12/webassembly

[fjtrujy]

I think adding breakpoints and stepping through source code is as good as it can get right now. The rest depends on browser implementors and how quickly the debugging format is standardized, as seen here https://github.com/WebAssembly/debugging. I'm happy to investigate if something can be fixed on SwiftWasm side that browser do support already in a documented way, but I'm not sure if watching variable content is one of such features.

Thanks for the quick reply, please keep us updated with your investigation.

[kateinoigakukun]

I did some investigation but unfortunately, it's difficult to support frame-variable inspection for now.

Generally, to support variable printing, a debugger needs to know the memory layout of the type of a variable, and DWARF supports primitive types (e.g. integer with endianness) and structural memory layout expression.

But Swift doesn’t use that mechanism and uses .swiftmodule's AST info in debugger to have more enriched formatting. Even for Int or String, we need AST info for printing because DWARF in a binary doesn’t have their layouts.

Therefore, in-browser debugging, we can see only C-oriented variables for now.

If Chrome DevTools would have Swift support, it would be delighted, but it’s an unrealistic story…

Perhaps, we may be able to interact with LLDB, which already has Swift supports, to get variable layout but I'm not sure it's feasible.