DebugLocationPropagation: pass debuglocation from parent node to chil…

[XMadrid]

This PR creates a pass to propagate debug location from parent node to child nodes which has no debug location with pre-order traversal. This is useful for compilers that use Binaryen API to generate WebAssembly modules.

It behaves like wasm-opt read text format file: children are tagged with the debug info of the parent, if they have no annotation of their own.

For compilers that use Binaryen API to generate WebAssembly modules, it is a bit redundant to add debugInfo for each expression, Especially when the compiler wrap expressions.

With this pass, compilers just need to add debugInfo for the parent node, which is more convenient.

For example:

(drop
  (call $voidFunc)
)

Without this pass, if the compiler only adds debugInfo for the wrapped expression drop, the call expression has no corresponding source code mapping in DevTools debugging, which is obviously not user-friendly.

[kripken]

This makes sense to me to add, I agree it looks useful for compilers.

Before I look at the code in detail, though, @tlively didn't you have plans to add something like this, to copy debug info from parents to (unmarked) children? Maybe for the new parser somehow? If so perhaps code could be shared here.

[tlively]

Yep https://github.com/WebAssembly/binaryen/blob/main/src/parser/wat-parser.cpp#L94-L144

[kripken]

Thanks @tlively !

Then @XMadrid I think the best thing would be to refactor that existing code out into a pass, and use the pass in the current place.

[XMadrid]

OK, Thank you! I will move existing code out to a pass. And then we can use this pass in wasm-opt options and wat-parser.

[XMadrid]

Hi, I refactored the existing code into a pass. Use a stack to keep track of traversed expressions, which has a bit lower space complexity. The functionality should be the same. Could you please kindly review this @kripken @tlively ?

[tlively]

Thanks, I have a better undstanding of how this works now.

[XMadrid]

Thanks, I think this PR can be merged.

[tlively]

Oops, I had forgotten to hit the approve button. Thanks for working on this!