[cfe-dev] rfc: Adding a mode to -gline-tables-only to include parameter names, namespaces

Wed Apr 29 16:13:55 PDT 2015

On Wed, Apr 29, 2015 at 4:08 PM, Robinson, Paul <
Paul_Robinson at playstation.sony.com> wrote:

>  Ø  4) pointer size - it's specified by the CU header, but not in the
> line table header
>
>
>
> (beats head on table) gotta have that to interpret DW_LNE_set_address
> correctly, afaict that's the *only* reason .debug_line would need it.  Feh.
> But basically you always need that, just to get started.
>
>
>
> Ø  5) inlining info
>
>
>
> For conjuring up fake frames in the traceback, presumably?
>

Yep

>   That sort of thing always felt like the debugger was lying to me,
> personally.
>

The alternative is a frame that looks like this:

outer_function() : inner_function_location.cpp:42

which tends to confuse people (& is ambiguous - which call to
inner_function from outer_function are we in?).

If I'm looking for an asan report, or walking through code in a debugger,
etc - it's often/usually not important that code has been inlined (just as
it's not important that a variable has been put in a register or a stack
slot, etc) - I just want to think about the code as though it wasn't
optimized, insofar as I can.

>  --paulr
>
>
>
> *From:* David Blaikie [mailto:dblaikie at gmail.com]
> *Sent:* Wednesday, April 29, 2015 3:33 PM
> *To:* Robinson, Paul
> *Cc:* Greg Clayton; Eric Christopher; Eric Christopher; David Blaikie;
> Alexey Samsonov; cfe-dev at cs.uiuc.edu Developers
>
> *Subject:* Re: [cfe-dev] rfc: Adding a mode to -gline-tables-only to
> include parameter names, namespaces
>
>
>
>
>
>
>
> On Wed, Apr 29, 2015 at 3:23 PM, Robinson, Paul <
> Paul_Robinson at playstation.sony.com> wrote:
>
> > -----Original Message-----
> > From: cfe-dev-bounces at cs.uiuc.edu [mailto:cfe-dev-bounces at cs.uiuc.edu]
> On
> > Behalf Of Greg Clayton
> > Sent: Wednesday, April 29, 2015 1:55 PM
> > To: Eric Christopher
> > Cc: Eric Christopher; Alexey Samsonov; David Blaikie;
> cfe-dev at cs.uiuc.edu
> > Developers
> > Subject: Re: [cfe-dev] rfc: Adding a mode to -gline-tables-only to
> include
> > parameter names, namespaces
> >
> > Actually parameters and their types and locations would greatly help out.
> > This would increase debug info size, but it would also keep me from
> having
> > to detect borked DWARF and having to avoid parsing any of it in LLDB.
> > Right now the -gline-tables-only will quickly make LLDB assert and die if
> > we try to use this debug info because we try to convert a function into a
> > real clang::ASTContext type and clang will quickly assert and kill the
> > debug session when it becomes unhappy with us trying to create functions
> > with no params or return types (think "operator<" with no params... Boom.
> > Crash).
> >
> > It is too bad that DWARF requires compile units just so we can have line
> > tables. If it wasn't for this, we could just emit a .debug_line section
> > with no .debug_info.
>
> Hmmm why was that, again?  I can think of three possibilities, none of
> which should be insurmountable:
> 1) CU has the base directory and filename of the compilation; .debug_line
> can include these directly, if we know it's -gline-tables-only.
> 2) CU has the use_utf8 flag; just assume UTF-8.
> 3) CU points to the chunk of .debug_line for this CU; we can probably
> work out how to reliably subdivide .debug_line without that assist.
>
>
> I don't think (3) is an issue - the line tables probably have their length
> in them.
>
> 4) pointer size - it's specified by the CU header, but not in the line
> table header
> 5) inlining info (this is pretty much the major point/issue with -gmlt) is
> not included in the line table. Cary's two-level line tables address this,
> though it's not implemented in LLVM. It's the minimal debug_info describing
> function inlining that's what confuses LLDB.
>
> - David
>
>
>
> Thanks,
> --paulr
>
>
> >
> > More comments inline below.
> >
> > > On Apr 29, 2015, at 12:47 PM, Eric Christopher <echristo at gmail.com>
> > wrote:
> > >
> > >
> > >
> > > On Wed, Apr 29, 2015 at 12:41 PM Nico Weber <thakis at chromium.org>
> wrote:
> > > On Wed, Apr 29, 2015 at 12:35 PM, Reid Kleckner <rnk at google.com>
> wrote:
> > > On Wed, Apr 29, 2015 at 12:25 PM, Nico Weber <thakis at chromium.org>
> > wrote:
> > > Hi,
> > >
> > > the motivation for -gline-tables-only was to make debug info much
> > smaller, but still include enough to get usable stack frames [1]. We
> > recently tried using it in Chromium and discovered that the stack frames
> > aren't all that usable: Function parameters disappear, as do function
> > namespaces.
> > >
> > > Are there any concerns about adding a mode to -gline-tables-only (or a
> > second flag -gline-tables-full, or similar) that includes function
> > parameter info and namespace info but still omits most debug info?
> >
> > I would like this due to reasons mentioned above.
> > >
> > > (Background: We rely on dsym files to let our crash server symbolize
> > crash dumps we get from the wild. dsymutil uses debug info, but it
> > apparently crashes if debug info is > 4GB. We hit that recently. So we
> > started using -gline-tables-only, but now all our stacks are close to
> > unusable, even though the motivation for gline-tables-only was to still
> > have usable stacks. We can't easily use symbol names as they get stripped
> > very early in our pipeline, and changing that is apparently involved.)
> >
> > You can still recover function bounds using the LC_FUNCTION_STARTS load
> > command even if you have a stripped executable. You won't have the
> > function names, but you can still backtrace and you can get accurate
> > function bound info for backtracing.
> >
> > >
> > > Is there some way to change the pipeline to use the mangled name stored
> > in .symtab instead of the DW_AT_name in the DWARF? My understanding is
> > that the sanitizers already do this.
> >
> > They probably just get the info from the symbol table.
> >
> > >
> > > See "We can't easily use symbol names as they get stripped very early
> in
> > our pipeline." above – from what I understand, one machine does the
> build,
> > strips, runs dsymutil to get a .dSYM bundle, and then that is shipped to
> > the crash servers.
> >
> > It better strip _after_ you run dsymutil to make the dSYM or you will get
> > empty dSYM files if the debug map is stripped. It really depends on what
> > kind of strip you are doing.
> >
> > >
> > >
> > > (adding Greg for dsymutil issues, but I don't think there's much in the
> > way of solving it)
> > > Parameter names is something we could probably add. I'd like to see the
> > impact of it from a size perspective. I mean, next people will want
> > locations of parameters. Then... and we're right back to just full debug
> > info.
> >
> > My main issue with -gline-tables-only (which LLDB doesn't support
> > currently due to the partial DWARF that is emitted that causes clang to
> > assert and kill the debugger) is that we now have DWARF that we can't
> > trust to use when creating types because all functions are potentially
> > invalid to create in a clang::ASTContext since everything has been
> > removed. Clang has the notion that certain types of functions must have
> > certain number of arguments, like C++ operators, and it gets unhappy if
> we
> > try to convert this DWARF into clang::AST types. We have a radar for
> clang
> > to somehow mark the debug info has "-gline-tables-only" so we can know
> > that any DW_TAG_subprogram tags we run into need to be converted to clang
> > AST types that return "UnknownAnyTy" and taking varargs as the parameters
> > so that they will be callable from the expression parser. We still might
> > run into names that can't be used as function names, especially if the
> > namespaces and class decl contexts are removed, so LLDB might have to
> just
> > say "I am not parsing any DWARF from anything that had -gline-tables-only
> > enabled..".
> >
> > Greg Clayton
> >
> >
> > _______________________________________________
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> > cfe-dev at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>
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