'unsupported line_ref tag: 0' warning when decoding line refs

[mat-hek]

Hey there,

sometimes I get the unsupported line_ref tag: 0 warning when loading a module and it seems it's correlated with missing entries in a stack trace when an error is raised in that module. I'm going to debug further, but it seems that the warning comes from here:

AtomVM/src/libAtomVM/module.c

Lines 498 to 499 in fc824ad

	// TODO handle integer compact encodings > 2048
	fprintf(stderr, "Unsupported line_ref tag: %u\n", tag);

and it seems a part of 'compact integers decoding' implementation is missing. Is there any place (in OTP?) where I can find some details / a reference implementation of it so that I can add it to AtomVM?

[pguyot]

Thank you for rising this, it is indeed annoying 😀

I'm not aware of any documentation and to handle similar cases in the past I ended up looking at BEAM source code, the VM or the compiler.

[mat-hek]

Gotcha, thanks for quick answer. Do you have any idea about what the 'compact integer' is? Searching for 'compact integer' in the OTP source didn't give anything :(

[pguyot]

Well, I'd search the code in BEAM that builds the line table.
https://github.com/erlang/otp/blob/50c08864b6decbe3561143986bda06663a9da0b7/lib/compiler/src/beam_asm.erl#L295

And more precisely in encode_line_items :
https://github.com/erlang/otp/blob/50c08864b6decbe3561143986bda06663a9da0b7/lib/compiler/src/beam_asm.erl#L324

Reading the code, I don't understand our decoding and I wonder if it's correct as we do the same thing for TAG_COMPACT_INT and TAG_COMPACT_ATOM but we don't do the same thing for TAG_EXTENDED_INT and TAG_EXTENDED_ATOM.

There seems to be four way to encode the value depending, and we only do two. See:
https://github.com/erlang/otp/blob/50c08864b6decbe3561143986bda06663a9da0b7/lib/compiler/src/beam_asm.erl#L579