preserve variable names in debug output

[art-in]

currently when building with debug info resulting wasm lacks original names for local / global variables and function parameters.

my_lib/src/lib.rs

fn add_two(param1: i32, param2: i32) -> i32 {
    param1 + param2
}

compiling to wasm

wasm-pack build --dev

[INFO]: Checking for the Wasm target...
[INFO]: Compiling to Wasm...
   Compiling my-lib v0.1.0 (D:\projects\wasm\my-lib)
    Finished dev [unoptimized + debuginfo] target(s) in 0.30s
[INFO]: Installing wasm-bindgen...
[INFO]: :-) Done in 0.48s
[INFO]: :-) Your wasm pkg is ready to publish at .\pkg.

converting wasm to wat

wasm2wat pkg/my_lib_bg.wasm
cat pkg/my_lib_bg.wat

...
(func $my_lib::add_two::hf6c60c59956375f2 (type $t6) (param $p0 i32) (param $p1 i32) (result i32)
  (local $l0 i32) (local $l1 i32) (local $l2 i32) (local $l3 i32) (local $l4 i32) ...
...

I can easily find my add_two function, but names of variables / parameters are gone, which is why it's hard to read and debug.

I believe those $p0, $p1, $l0, $l1 names are autogenerated by wasm2wat, since original wasm uses indices instead of custom names.

is it possible to preserve original names?

[danwilhelm]

This requires that (1) wasm-pack exports the parameter names to WASM (in this case as DWARF debug symbols), and (2) wasm2wat supports extracting them from the DWARF info.

Note that wasm-pack can export parameter names, but not by default:

1. Although --dev adds some debug info, it currently does not export parameter names. You can verify this: hexdump -C my_lib_bg.wasm.

2. That said, you can export param names via wasm-pack by adding a few lines to your Cargo.toml (This adds the --keep-debug flag when calling wasm-bindgen.):

[package.metadata.wasm-pack.profile.dev.wasm-bindgen]
dwarf-debug-info = true

I assume this default was intentional, since it is documented in the --dev release notes.

So, this question might be better asked to wasm2wat. (I ran a quick test and also did not see the original param names in the WAT, even when they were included in the WASM file.)

[art-in]

@danwilhelm

thanks, i've tried same investigation myself and see same results. posted issue to wasm2wat (WebAssembly/wabt#1290)

and i'm still curios why would dwarf-debug-info be disabled by default.
maybe it's not stable, and that's why wasm2wat cannot read debug symbols. hmmm....

[ashleygwilliams]

hey folks! it seems like we've found a resolution for this- do we think additional documentation would be helpful to other users here? let me know and i can make this a ticket for that! otherwise we can close :)

[art-in]

here's main unanswered question from WebAssembly/wabt#1290 (comment)

what's more appropriate to do right now? ask wasm-pack to generate Name section, or wasm2wat to read DWARF section?

[ashleygwilliams]

@art-in thanks for clarifying! that's definitely an interesting question. @fitzgen do you have any thoughts?