Re: dwarflint: DW_AT_location being empty vs. missing check

[Roland McGrath <roland at redhat dot com>]

> a future FEAT, possibly to file it in some BZ?

We are not using BZ for dwarf branch development tasks, just this mailing
list and the wiki.  This sort of check on producer high-level semantics is
something we vaguely plan for the future, but is not on our critical path
for development.

It would be great to collect these ideas on the DwarfLint wiki page or
another new wiki page for planning this particular sort of check.

> dwarflint could warn on existing zero-length DW_FORM_block* DW_AT_location for
> DW_AT_variable with neither DW_AT_declaration nor DW_TAG_const_value.  This
> should mean the same as missing DW_AT_location - which has smaller DWARF code.

I had actually contemplated the converse check.  That is, call questionable
a variable/formal_parameter DIE with no location attrs (nor alternatives
like const_value).  I thought of this because the buggy/poor GCC generation
for e.g. inlines' parameters' locations produced no attrs at all and looks
like the compiler just forgot they could have them.  In contrast, an attr
with an empty expression is a clear and explicit sign that the compiler is
saying, "I know about it, but it's optimized out."

It's true that by the spec missing and empty mean the same.  The space cost
of the attr with empty expr is about a byte (modulo the abbrev) and so
seemed negligible to me.  OTOH, they could add up and I recognize that.
Your idea may well be a better idea.  For these kinds of checks I think the
deciding criterion is what is in practice most helpful in diagnosing the
cases where the compiler is not doing what it should.


Thanks,
Roland