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

Sean Silva chisophugis at gmail.com
Fri May 1 18:20:34 PDT 2015

On Fri, May 1, 2015 at 4:57 PM, Robinson, Paul <
Paul_Robinson at playstation.sony.com> wrote:

> > -----Original Message-----
> > From: Daniel Berlin [mailto:dberlin at dberlin.org]
> > Sent: Friday, May 01, 2015 3:15 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: [LLVMdev] What does "debugger tuning" mean?
> >
> > On Fri, May 1, 2015 at 1:06 PM, Robinson, Paul
> > <Paul_Robinson at playstation.sony.com> wrote:
> >
> > >
> > > 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.
> >
> > How is this not actually "we expect the debugger to want his described
> > as X, Y, and Z" instead of "we expect the debugger to by X or Y or Z".
> Different ways of describing things let us reason about them in different
> ways.  Are Cartesian coordinates a reason to reject polar coordinates?
> > Debuggers change over time.
> Well, they're software too.  They're allowed.
> > GDB 4's level of support != GDB 5 != GDB6 != GDB7.
> >
> > Heck, the same version of GDB on different platforms can be very
> > different (HP's GDB was very different, as was Apple's).
> >
> > Do you plan on having "debugger tuning" flags for each of these?
> Are all of these current supported Clang/LLVM environments?  Are they
> suffering because Clang currently emits DWARF in ways that cause them
> problems, or that bloats the debug info unnecessarily?  Are vendors
> carrying a pile of private patches to get LLVM to emit DWARF that looks
> like what their debugger wants?  If the answer to these questions is
> Yes then my answer to your question is Sure, we could, if we find
> those differences to be important.
> I promise you that these things are true at Sony, and I would be
> astounded if it wasn't true for Apple. Guess what? LLDB and SCE are
> two of the debugger-tuning options for a reason.
> Show me another _real_ case of _real_ differences that matter to the
> community, and we can talk about another debugger tuning option.
> > I am having a lot of trouble understanding how this is about what
> > debuggers expect and not "ways of representing things".
> Sometimes there's not much difference in the result, although there
> is a difference in how we think about it.
> > Otherwise, i see you getting stuck introducing tons and tons of
> > debugger tunings, instead of people who want it to work a certain way
> > building an option profile consisting of
> > "--debugging-feature=use-tls-operator
> > --debugging-feature=explicit-anonymous-namespace", and getting exactly
> > what they want.
> (That sort of alternate proposal is way more useful than the straw-man
> stuff you started out with.  Just sayin'.)
> (Also incidentally, I see gcc 4.8.2 is emitting an explicit import of
> anonymous namespaces, so that's becoming _less_ different over time.)
> In fact this is more or less how DwarfDebug operates now; there is a
> pile (small pile, but still a pile) of individual feature flags that
> get set various ways depending on target or command-line options.
> The problem I'm looking at is that the defaults for a lot of these
> things are based on target, which AFAICT is based on an out-of-date
> assumption about how target correlates to debugger.
> I'm proposing to package up known sets of stuff that we're having to
> contend with _today_ in a way that's easier to talk about and do
> something about than the target-oriented mish-mash we have now.
> It's a whole lot simpler to say something like "FreeBSD implies LLDB"
> instead of "FreeBSD implies accelerator tables and standard TLS opcode
> and no pubnames or pubtypes."
> Or would you rather say "--lang-feature=auto --lang-feature=rvalue-ref
> --lang-feature=move_ctor ..." than "-std=c++11"?

Is there a reason that we couldn't implement "--debugger-target=lldb" as
syntax sugar for "--debugger-feature=acceleratortables
--debugger-feature=standardtlsopcode --debugger-feature=no-pubnames

I'm not strongly convinced that we would necessarily need to expose the
--debugger-feature flag to users (or at all on the command line), if that's
what is bothering you. But at some point inside the code there needs to be
an explicit factorization of the different "debugger targets" into what
would be individual --debugger-feature options, so that it is easy to
describe new debugger targets.

-- Sean Silva

> > IE a year from now, somebody comes along with a version of GDB that
> > doesn't match what your current "tuning profile" is, and asks for it
> > to change.
> If it's meaningfully different, i.e. makes enough of a functional and/or
> size difference, and it's an environment we collectively want to support,
> and it's unreasonable to get the necessary effect some other way, then
> sure, maybe we would want to invent a new tuning profile.
> But, if it doesn't meet that bar, I don't see why we'd want to.
> --paulr
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