[lldb-dev] [cfe-dev] What does "debugger tuning" mean?

[Robinson, Paul]

Another example would be .debug_pubnames and .debug_pubtypes sections.
Currently these default to omitted for Darwin and PS4, but included
everywhere else. My initial patch for "tuning" changes the PS4 platform
criterion to the SCE debugger predicate; quite likely the "not Darwin"
criterion ought to be "not LLDB" or in other words "on for GDB only."

"not LLDB" wouldn't be "on for GDB only" (it'd be "on for GDB and SCE" given the current debuggers)

Poorly phrased on my part.  The "not Darwin" part would become "not LLDB" and _when combined with the existing "not SCE" part_ could be refactored into "on for GDB only."

Re extra artificial entries, the specific example being variables for local anonymous unions:

I was inclined to just say it's a debugger bug and only enable the workaround when targeting that debugger specifically, and Eric wasn't. We came to the conclusion/agreement that maybe having it on by defaut but off if targeting any /specific/ non-GDB debugger.

Well, that's consistent with the default tuning being for GDB.  Emitting it under GDB means you do get them by default but not if you're knowingly targeting something else.

If you're envisioning a "no specific tuning" mode, I flip-flopped on that one a couple times while I was working up the idea, and came down on the side of not doing it.  Not opposed to having it, but we haven't really thought about what it would look like.  (Emit every kind of standard thing we know about, and no non-standard things at all?  So we'd get pubnames and pubtypes and aranges but no Apple attributes or accelerator tables and not the GNU TLS opcode.  Is that a useful mode to have?)

Re where to put the guidelines:

ought to be committed to the repo somewhere, although whether it's as
a lump of text in a docs webpage or a lump of commentary in some source
file is not clear; opinions welcome.)

In the source, probably - somewhere near the enum or major entry point to querying it.

By the definitions of the predicate functions, WFM.

We might want to talk a bit more about when to err on the side of caution & put something in for every debugger by default, an opt out of it when tuning for a debugger that doesn't need it.

That part of the visioning would imagine a new debugger (there are more of them out there in the world) and how to phrase the condition to work in the way, how to say this, least likely to be a problem.  The TLS opcode should default to standard, except GDB.  Soon enough I'll be using the tuning feature to turn off linkage names for SCE, but it'll be on by default.  Like that.
--paulr

From: David Blaikie [mailto:dblaikie at gmail.com]
Sent: Friday, May 01, 2015 1:58 PM
To: Robinson, Paul
Cc: cfe-dev at cs.uiuc.edu Developers (cfe-dev at cs.uiuc.edu); LLVM Developers Mailing List (llvmdev at cs.uiuc.edu); lldb-dev at cs.uiuc.edu
Subject: Re: [cfe-dev] What does "debugger tuning" mean?



On Fri, May 1, 2015 at 1:06 PM, Robinson, Paul <Paul_Robinson at playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>> wrote:
This is basically a reboot of the previous thread titled
  About the "debugger target"
except that "target" was really too strong a term for what I had intended
to use this feature for.  "Debugger tuning" is more like it.  You don't
need to have read the previous thread, I'll recap here.

Fundamentally, Clang/LLVM uses DWARF as the specification for the _format_
of information provided by the compiler to a variety of "consumers," which
primarily means debuggers (but not exclusively).  [For a long time it was
the only format supported by LLVM. Lately, Microsoft debug info has started
appearing, but being a less widely used format, the issues that DWARF runs
into aren't a concern for that format.  So "debugger tuning" is unlikely
to be an issue for Microsoft debug info.]

DWARF is a permissive standard, meaning that it does not rigidly require
that source-language construct X must be described using the DWARF
construct Y.  Instead, DWARF says something more like, "If you have a
source construct that means something like X, here's a mechanism Y that
you could use to describe it."  While this gives compilers a lot of nice
flexibility, it does mean that there's a lot of wiggle room for how a
compiler describes something and in how a debugger interprets that
description.  Compilers and debuggers therefore need to do a bit of
negotiation in determining how the debug-info "contract" will work, when
it comes to nitty-gritty details.  DWARF itself (the standard, as well
as the committee that owns the standard) refuses to get involved in this
negotiation, referring to all that as "quality of implementation issues."

It is readily apparent that different debuggers have different ideas
about certain DWARF features, for example whether they are useful or
irrelevant, or whether a certain source construct should be described
this way or that way.  As these generally fall into the QOI realm, the
DWARF spec itself is no help, and it comes down to a matter of opinion
about whether "the debugger should just know this" or "the compiler
really ought to just emit it that way."

Clang/LLVM is in the position of being a compiler that wants to support
several different debuggers, all of which have slightly different ideas
about what they want from the DWARF info for a program.  Our first line
of defense of course is the DWARF standard itself, but as we've seen,
that is not a universally definitive reference.

LLVM already emits DWARF slightly differently for different *targets*;
primarily Darwin, in a few cases PS4.  But in at least some cases, the
target is just a (somewhat unreliable) proxy for which *debugger* the
compiler expects to be consuming the DWARF.  The most instructive case
is the exact DWARF expression used to describe the location of a thread-
local variable.  DWARF v3 defined an operator to find the base address
of the thread-local storage area; however, GDB has never learned to
recognize it.  Therefore, for targets where we "know" GDB isn't used,
we can emit the standard operator; for targets where GDB *might* be
used, we need to emit the equivalent (non-standard) GNU operator.

It would be semantically more meaningful to base decisions like this on
whether we expected the debugger to be X or Y or Z.  Therefore I've
proposed (http://reviews.llvm.org/D8506) a "debugger tuning" option that
will make the reasoning behind these choices more obvious, and ultimately
give users a way to control the tuning themselves, when the platform's
default isn't what they want. (I'll have a follow-up patch exposing the
tuning option to the Clang driver.)

So, what kinds of things should be based on the debugger tuning option?
Are there still things that should be based on the target platform?
Simplest to consider these questions together, because it is often clear
which criterion is important if you consider (a) the same debugger run
on different targets, versus (b) different debuggers running on the same
target.  Basically, if the same debugger on different targets wants to
have something a certain way, that's probably a debugger-tuning thing.
And if different debuggers on the same target doesn't mean you should
change how the DWARF looks, that's likely a platform-specific thing.

The most obvious example of a debugger-tuning consideration is the TLS
operator mentioned above. That's something that GDB insists on having.
(It turns out that the standard operator was defined in DWARF 3, so we
also have to emit the GNU operator if we're producing DWARF 2.  Tuning
considerations don't trump what the standard says.)

Another example would be .debug_pubnames and .debug_pubtypes sections.
Currently these default to omitted for Darwin and PS4, but included
everywhere else. My initial patch for "tuning" changes the PS4 platform
criterion to the SCE debugger predicate; quite likely the "not Darwin"
criterion ought to be "not LLDB" or in other words "on for GDB only."

"not LLDB" wouldn't be "on for GDB only" (it'd be "on for GDB and SCE" given the current debuggers)

Eric, Adrian, and I hit another case of positive/negative checking recently for some DWARF feature... local anonymous unions. GDB likes to have explicit (possibly artificial) local variables for the unions members, LLDB can manage without them.

Eric & I discussed that there's a bit of a sliding scale of compatibility we should bother with - how much LLVM bends over backwards to cope with debugger bugs/limitations. I was inclined to just say it's a debugger bug and only enable the workaround when targeting that debugger specifically, and Eric wasn't. We came to the conclusion/agreement that maybe having it on by defaut but off if targeting any /specific/ non-GDB debugger.

And having the code actually reflect the correct semantic purpose seems
like an overall goodness.

An example of a target-dependent feature might be the .debug_aranges
section. As it happens, we don't emit this section by default, because
apparently no debugger finds it useful, although there's a command-line
option (-gdwarf-aranges) for it.  But, for PS4 we do want to emit it,
because we have non-debugger tools that find it useful.  We haven't yet
done the work to make that change on llvm.org<http://llvm.org>, but it's on the list.
I would conditionalize this on the target, not the debugger, because
the debugger is not why we want to generate the section.

Okay, so I've been pretty long-winded about all this, can I possibly
codify it all into a reasonably succinct set of guidelines?  (which
ought to be committed to the repo somewhere, although whether it's as
a lump of text in a docs webpage or a lump of commentary in some source
file is not clear; opinions welcome.)

In the source, probably - somewhere near the enum or major entry point to querying it.

We might want to talk a bit more about when to err on the side of caution & put something in for every debugger by default, an opt out of it when tuning for a debugger that doesn't need it.


o Emit standard DWARF if possible.
o Omitting standard DWARF features that nobody uses is fine.
  (example: DW_AT_sibling)
o Extensions are okay, but think about the circumstances where they
  would be useful (versus just wasting space).  These are probably a
  debugger tuning decision, but might be a target-based decision.
  (example: DW_AT_APPLE_* attributes)
o If some debugger can't tolerate some piece of standard DWARF, that's
  a missing feature or a bug in the debugger.  Accommodating that in
  the compiler is a debugger tuning decision.
  (example: DW_OP_form_tls_address not understood by GDB)
o If some debugger has no use for some piece of standard DWARF, and
  it saves space to omit it, that's a debugger tuning decision.
  (example: .debug_pubnames/.debug_pubtypes sections)
o If a debugger wants things a certain way regardless of the target,
  that's probably a debugger tuning decision.
o If "system" software on a target (other than the debugger) wants
  things a certain way regardless of which debugger you're using,
  that's NOT a debugger tuning decision, but a target-based decision.
  (example: .debug_aranges section)

Let me know if this all seems reasonable, and especially if you have
a good idea where to keep the guidelines.
Thanks,
--paulr


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