[llvm-dev] [DWARFv5] The new line-table section header (original) (raw)

Robinson, Paul via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 27 13:12:04 PDT 2017

The next feature on my DWARF 5 list is the line-table header. While this is pretty easy generate, it is a real bear to parse, so I thought I should let y'all know what I'm up to and why as I head out to the yak farm. Any thoughts and suggestions would be very much appreciated.

The v5 directory and file tables no longer have a fixed format; instead, we have a list of field descriptors followed by the fields for each entry in the directory or file table. Normally the directory table would have one descriptor: DW_LNCT_path, DW_FORM_string This tells us each entry contains a pathname encoded as an inline string. (Which is essentially how the v4 directory table is encoded.) However, because of the FORM code, we now have whole new worlds of complication regarding where the actual string might be. We might have DW_FORM_strp which puts the actual string in the .debug_string section; eventually we could have DW_FORM_line_str (pointing to .debug_line_str) or even DW_FORM_strx (indirecting through .debug_str_offsets).

Conveniently, we have the DWARFFormValue class which knows how to decode data based on what the form code is.

Inconveniently, DWARFFormValue assumes it is looking at a .debug_info section, and picks up its relocations from a DWARFUnit. But if we're using DWARFFormValue to decode data from .debug_line, then it needs a different relocation map.

It's only the string data that causes a problem; all the other kinds of data in the file table are constants, and retrieving constants with DWARFFormValue is no problem.

I think the right tactic is a "top-down" approach, starting by teaching DWARFDebugLine to parse a v5 line-table header but support only DW_FORM_string for the paths. This should let me use an unmodified DWARFFormValue to parse the directory and file tables.

From there, teaching DWARFFormValue to handle DWFORMstrp from the .debug_line section should be pretty well motivated and it should be straightforward to see what's really needed in terms of the API.

Once we get that far, I would hope that the line_str and strx forms would not require much additional effort. Actually Wolfgang is separately working on the strx forms so with any luck that would Just Work for the .debug_line section.

Oh yeah, after all that I'd actually generate the v5 header from LLVM. The idea is that by then, I can use llvm-dwarfdump to validate it and be very confident that it would all work.

Does all that sound like a plan? The alternative would be to try to teach DWARFFormValue to handle DW_FORM_strp from .debug_line up front, but I think we might rather go at this in smaller pieces.

Thanks, --paulr

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