[lldb-dev] Adding D language demangling support

Wed Sep 21 15:50:30 PDT 2016

On Wed, Sep 21, 2016 at 3:35 PM, Greg Clayton via lldb-dev <
lldb-dev at lists.llvm.org> wrote:

> Sounds like you could then make a setting that is a dictionary where you
> say what the prefix is (like maybe "_D") and the value is the path to the
> tool to use? This would be easy to implement. Demangling does tend to be
> one of the most expensive parts of symbol file and debug info parsing, so
> if you do this, you will want to make sure the shell tool can be spawned
> and kept running maybe?
>
> Greg
>

where in the lldb code would be such entry point?

instead of a binary it can just be a library dynamically loaded via dlopen
(as i wrote, though I should've called it LLDB_DEMANGLER_LIB instead
of LLDB_DEMANGLER_EXE),
and the dynamically loaded symbol be cached to make sure it's dlopen'd at
most once per process.

Then it's easy enough for us to write a demangleCustom that is fast on the
D side of things. It can also work with a binary instead of a dllib but
would be a bit slower (could have a client server model, but that's more
complex than the simple dllib solution i was proposing).

yes, we could use a prefix for that as well.

>
> > On Sep 21, 2016, at 3:30 PM, Timothee Cour <timothee.cour2 at gmail.com>
> wrote:
> >
> >
> >
> > On Wed, Sep 21, 2016 at 3:10 PM, Greg Clayton <gclayton at apple.com>
> wrote:
> > There is no external demangling plug-in infrastructure at the moment,
> but you could add functionality that would allow it. No one is going to
> have D installed by default. Where do you expect your demangler dylib to
> live?
> > Would you just add code that tries to locate the dylib in N places on
> the current system and try to dlopen it? Avoiding duplication and just not
> having the functionality at all unless something else is might not make it
> that useful. Is D stable? Is the mangling changing at all? Will you require
> a demangler to be vended with each new version of the tool? Are all
> previous demanglings still valid in newer versions? Can you figure out the
> version of the D from a compiled executable so that you might be able to
> locate one of 5 different installs of D and select the right one? Let me
> know what you use case is.
> >
> > Greg
> >
> >
> > one simple flexible backward compatible option would be to have a
> generic environment variable:
> >
> > ```
> > export LLDB_DEMANGLER_EXE="/usr/bin/ddemangle"
> > lldb myprog
> > ```
> >
> > inside lldb (D-like pseudo code):
> >
> > ```
> > bool demangle(string symbol, string* output){
> >   auto path=env["LLDB_DEMANGLER_EXE"];
> >   if(!path.empty) {
> >      auto demangleCustom=cast(proper_type) dlopen(path);
> >      if(demangleCustom(symbol, output)) return true;
> >      // fallsback to default code if custom code didn't handle symbol
> >   }
> >   return run_default_lldb_demangle(symbol, output);
> > }
> > ```
> >
> > user defined demangler (eg D's demangler)
> > ```
> > // return true if can demangle symbol (ie it's a D symbol in our case)
> > bool demangleCustom(string symbol, string* output);
> >
> > ```
> >
> > >> Is the mangling changing at all?
> >
> > yes, there's some ongoing work on making the mangling scheme produce
> much shorter symbols. The logic is complex, and it'd be a lot of work to
> reproduce this.
> >
> > Bottomline: this scheme is very flexible, and it'd be no less useful
> than current situation, where lldb just returns the symbol unchanged if it
> can't demangle.
> >
> >
> >
> >
> > > On Sep 21, 2016, at 3:00 PM, Timothee Cour <thelastmammoth at gmail.com>
> wrote:
> > >
> > > Is there a way to provide a hook (eg, via an extern(C) function, or
> using a dynamically loaded shared library) to do this, so as to simply
> reuse D's https://dlang.org/phobos/std_demangle.html and make sure it's
> always in sync with D's demangling instead of duplicating code
> > >
> > > On Wed, Sep 21, 2016 at 10:24 AM, Greg Clayton via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> > > It might be nice to add demangling support to llvm and then use it by
> modifying "Mangled::GetDemangledName()" in Mangled.cpp. This is where all
> demangling happens. Hopefully you have a great prefix that won't conflict
> with other languages "_Z" for C++, "_T" for swift. But the code in
> Mangled::GetDemangledName() will look at the prefix and attempt to demangle
> the name based on what prefix it starts with.
> > >
> > >
> > > > On Sep 21, 2016, at 5:52 AM, Johan Engelen via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> > > >
> > > > Hi all,
> > > >   I recently looked into adding demangling support for D in LLDB,
> but got lost in the code.
> > > > (right now, basic D support is there with: https://reviews.llvm.org/
> D24794)
> > > >
> > > > I'd like some pointers to where demangling is done for the other
> languages, and to where I should add D support for it.
> > > >
> > > > Thanks a lot,
> > > >   Johan
> > > >
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> >
> >
>
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