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

Daniel Berlin dberlin at dberlin.org
Fri May 1 17:38:00 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?

I'm trying to understand why you've chosen the design you have.
Given that what you are designing is something that is likely to have
to be supported for a pretty long time, it makes sense to make sure
that design is the right one for the use case you are trying to

>> 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?

Plenty of them are?

> Are they
> suffering because Clang currently emits DWARF in ways that cause them
> problems, or that bloats the debug info unnecessarily?

Yes, sure.

>  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.

This does not do a lot to explain why your design choice of "debugger
names" over "features" makes sense.

> 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.

What does this have to do anything?

If they were just features, you wouldn't have to define *any of these options*.

> Show me another _real_ case of _real_ differences that matter to the
> community, and we can talk about another debugger tuning option.

Again, i'm asking you to define why "debugger target" is the right
abstraction for what you are trying to achieve, when history has shown
1. Feature tests/description have worked better for compiler
differentiation than target "gcc 2.95"
2. The same is highly likely to be true of debuggers.

Particularly in light of the fact that what i'm suggesting you do does
not in any way prevent or change what you want to achieve, but simply
abstract it in a way that requires no work on the part of clang/llvm
to get any particular subset of features they want for their debugger.
IE people do not have to change source in response to debugger

In response, you have mostly just told me "a lot of users are in pain
right now".

>> 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'.)


It's not a strawman. I am asking you *why you have designed it this
way when plenty of other abstractions, at face value, seem to make
more sense to me"

> (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"?
The lang-feature stuff is exactly how people test for what level of
c++11 support something has.
-std=c++11, in fact, means different things to different compilers,
and thus, has in fact *not* provided a good abstraction for people
trying to say "i want to be able to compile anything that is standard
c++11".  Why do you expect debuggers to be different here?
Honestly. I'm really just trying to understand why you think debuggers
are going to turn out different.
This is despite the fact that C++11 is in fact a standard. If there
was "standard gdb", yes, it may make sense for llvm to *try* have
-debugger-std=gdb.  But there *is no standard gdb*.  So i'm trying to
understand why you are tying you design to the notion that you can
have a "gdb debugger tuning", instead of having a list of features,
and then having something that understands what the user wants (like
your IDE, or even a driver), set the feature flags that make sense for
whatever profiles you care to define.

>> 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.

None of this has been particularly helpful in understanding why you've
made the design tradeoffs you have.  I'm not calling your design good
or bad. I'm trying to understand why you have chosen this set of
things from among the various alternatives.

Is there a document, a thread, or anything that goes through the
various alternatives you've considered before you arrived at this one?

> --paulr

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