[LLVMdev] About the "debugger target"

[Robinson, Paul]

While I've already posted reviews for the initial patches for this (see
http://reviews.llvm.org/D8506 and http://reviews.llvm.org/D8599), the
grapevine suggests I should post a lengthier description of my intent
for the "debugger target." The idea was prompted by a suggestion from 
Eric Christopher, and I'm running with it.

Various bits of the DWARF we produce are conditional on the target
platform, or on command-line options with defaults based on the target
platform.  Other bits aren't but probably ought to be.  Some of these 
are little things:
- various Apple-defined attributes (DW_AT_APPLE_*)
- TLS opcode (DW_OP_form_tls_address v. DW_OP_GNU_push_tls_address)

Some are bigger things:
- accelerator tables
- pubnames/pubtypes

Really these aren't so much _platform_ specific things as they are
_debugger_ specific things. For example:
- accelerator tables: default on for Darwin, but really on for LLDB.
- pubnames/pubtypes: off for Darwin and PS4, but really on for GDB.

We're co-opting platform as a proxy for the debugger.  Sometimes they
are closely tied, but in other cases maybe not so much.  I think it is
already the case that LLDB is used in more places than just Darwin?
But the things that look like they ought to be LLDB-specific are actually
implemented as Darwin-specific.  That's sub-optimal for the non-Darwin 
but LLDB-using platforms.  I mean, LLDB will still *work* on those 
platforms, because 99% of what's going on is still standard DWARF, it 
just won't be "tuned" for how LLDB wants to operate.

In most cases on the LLVM side, control of one of these tuning things
is via a bool flag in DwarfDebug, and the DwarfDebug ctor is where the
actual decision-making takes place.  My initial patch to define a 
debugger target starts in the direction of setting those flags based on
debugger, not platform.  More could be done, but as I'm not clear on 
LLDB's scope I didn't want to go too far right away.

For example, it would not make me sad to suppress the DW_AT_APPLE* stuff 
except when the debugger would actually understand it.  It's not clear
to me whether these really ought to be LLDB attributes, or Darwin, or
what, because currently we emit them unconditionally. :-P

And quite frankly, I'm looking ahead to fiddling things for PS4. We have 
no use for linkage names in DWARF, for example, so I'd like to turn that 
off and save some space.  We do want .debug_aranges so I'd like to turn 
that on by default.  And there's a little bit of fine-tuning on exactly
what -flimit-debug-info applies to, that we'd like to do (that part is
obviously on the Clang side rather than the LLVM side).

I hope this helps explain what's going on with the "debugger target"
and what I want to do with it. (And apologies for not posting this
sooner.)
--paulr