[llvm-dev] Making library calls for obj2yaml functionalities

[James Henderson via llvm-dev]

Hi Rahman,

Traditionally, the ability to read sections is a feature added to
llvm-readobj/llvm-readelf. For some sections, it delegates to methods in
places like the Object library and BinaryFormat, but for the more
specialised sections, it typically has code local to itself doing the work.
The same is true for other dumping tools like obj2yaml and llvm-objdump,
which means in some cases, we have multiple varieties of parsers for the
same thing. I'm not sure there's necessarily a strong motivation for doing
so, however, so I'd be happy to support functionality being added elsewhere
in one of those libraries, which tools like obj2yaml and llvm-readobj can
hook into. I'm also happy to support refactoring that improves code reuse
within the tools, though I don't have any further ideas on this.

Can I ask what your motivation for using obj2yaml is in this context? If
it's just for testing purposes, adding support to llvm-readobj/llvm-readelf
would be the more normal way, as it allows you to dump just that section.

Only tangentially relatedly, I've only just seen your previous patch/email
thread, and I do have one thing I'd like to ask if it can be changed. At
the moment, the section type is SHT_PROGBITS, but I think it would be
better, if possible, to define a new SHT_* type for the new section? In
general, it is bad design to rely on section names to distinguish between
different kinds of sections - this requires the linker and other tools to
have to do unnecessary string comparisons, which are slower and messier
than switching on the sh_type field.

James

On Wed, 16 Sep 2020 at 22:00, Rahman Lavaee <rahmanl at google.com> wrote:

> Hi All,
>
> Following up on
> https://lists.llvm.org/pipermail/llvm-dev/2020-July/143512.html, and
> https://reviews.llvm.org/D85408, we would like to consider a design which
> allows external tools to read the structured contents of the .bb_addr_map
> section with library calls into an LLVM library. At the same time, we need
> to have tools/obj2yaml tests in place for bb_addr_map. So it sounds like
> the perfect place to do it. However, the current structure does not expose
> the obj2yaml functionalities under lib/ObjectYAML.
>
> In general, there seems to be an inconsistency regarding how obj2yaml and
> yaml2obj tools are structured. The latter has a nice wrapper which calls
> environment-dependent functions under lib/ObjectYAML, but the former has
> environment-dependent source files under tools/obj2yaml (elf2yaml.cc,
> coff2yaml.cc, etc).
>
> I wanted to reach out to ask if there is any alternative or get an idea
> about the amount of refactoring work that is required to make the structure
> friendlier.
>
> bests,
>
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