[cfe-dev] Make standalone-debug default based on glldb tuning

[David Blaikie via cfe-dev]

On Fri, Apr 12, 2019 at 8:45 AM <paul.robinson at sony.com> wrote:
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>
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> > -----Original Message-----
> > From: aprantl at apple.com [mailto:aprantl at apple.com]
> > Sent: Friday, April 12, 2019 11:21 AM
> > To: David Blaikie
> > Cc: Clang Dev; Robinson, Paul; Douglas Katzman
> > Subject: Re: Make standalone-debug default based on glldb tuning
> >
> >
> >
> > > On Apr 12, 2019, at 8:18 AM, David Blaikie <dblaikie at gmail.com> wrote:
> > >
> > > I realized recently (as folks at Google have started experimenting
> > > with LLDB) that the platform default for debugger tuning and the
> > > default for -fstandalone-debug are both independent. This seems a bit
> > > wrong to me because enabling -glldb doesn't enable -fstandalone-debug.
> > >
> > > I'm wondering if everyone's on board with removing
> > > ToolChain::GetDefaultStandaloneDebug, and using
> > > ToolChain::getDefaultDebuggerTuning() to select the default standalone
> > > debug state?
> > >
> > > Then on the command line, I'd kind of expect one to override the
> > > other, so things like "-glldb -fno-standalone-debug" is respected
> > > (when it comes to hopefully fixing LLDB to cope with standalone debug
> > > input).
> > >
> > > Sound good to everyone? If so I'll make a patch & send it for review.
> > > (well, I'll probably work on the patch now anyway)
> >
> > Since this doesn't change anything for platforms that default to -glldb,
> > I'm fine with this change.
> >
> > -- adrian
>
> I'm somewhat surprised LLDB would want -fstandalone-debug, independent
> of the target.  It would be a source of unnecessary bloat on ELF-using
> targets, no?

LLDB fundamentally (at the moment) doesn't support -fstandalone-debug,
no matter the file format.

As far as I understand it: LLDB constructs ASTs from DWARF on a
per-binary level (so, .so or executable). -fstandalone-debug creates
"impossible" DWARF (eg: if one base class's vtable (& thus DWARF
definition) is in one .so, but a derived class is in another .so -
then the DWARF in the second .so contains a definition that derives
from a declaration (& the DWARF consumer would have to go look in the
other .so to stitch it up to a definition).

(FreeBSD defaults to -fstandalone-debug because it defaults to LLDB as
the debugger - Adrian's reply makes me wonder if that's clear, so
figured I'd mention that)

> --paulr
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