[llvm-dev] RFC: Improving the performance of ItaniumDemangle

Wed May 3 02:46:32 PDT 2017

One of the things we need in lldb is to find a "basename" of a
function. This is so that you can set a breakpoint on a function named
"foo", even if "foo" is a method of a template class declared inside
another function, which is inside 5 namespaces, a couple of them being
anonymous... :)

Currently we do that by parsing the demangled name, which is neither
fast nor easy, especially when function pointers come into the game
(D31451). We could probably save some runtime if the demangler could
provide us with a bit more structured information instead of just the
final demangled string. I expect the demangler to be in a much better
position to do that then us trying to reverse engineer the string.

I have no idea how this fits in with the rest of the goals of llvm's
demangler, but I guess it doesn't hurt throwing the idea out there.

cheers,
pl

On 30 April 2017 at 21:54, Saleem Abdulrasool via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
>
> On Tue, Apr 25, 2017 at 1:37 PM, Asiri Rathnayake
> <asiri.rathnayake at gmail.com> wrote:
>>
>>
>>
>> On Tue, Apr 25, 2017 at 8:36 PM, Vedant Kumar via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>>>
>>>
>>> > On Apr 25, 2017, at 12:24 PM, Scott Smith <scott.smith at purestorage.com>
>>> > wrote:
>>> >
>>> > well, top-of-branch lldb uses this code, that's how I found it.  Do you
>>> > mean libc++'s demangler?
>>>
>>> Thanks for explaining, this is the first time I'm looking at the
>>> demangler situation. It looks like libcxxabi has an arena-based demangler,
>>> and that the one in llvm is different.
>>>
>>> I'm confused by this because the comment in llvm says that libcxxabi is
>>> supposed to reuse the llvm demangler. This doesn't seem to be happening,
>>> right?
>>
>>
>> This seems correct. libcxxabi demangler [1] is different from the one used
>> by llvm [2]. I'm hoping Saleem, Eric or Jon (copied) knows a bit of history
>> as to why this is so (perhaps because the two projects evolved independently
>> ?).
>
>
> They didnt really evolve independently, the version in LLVM was imported
> from libc++.  However, we simplified it to make it more portable.  The
> simpifications naturally led to the ability to remove the arena allocation
> routines.  The copy in libc++ needs to retain a certain amount of
> flexibility due to the exporting of the interface into the user's address
> space (via the __cxa_demangle interface).  However, making adjustments that
> improve performance in the LLVM version should be acceptable.
>
>>>
>>>
>>>
>>> > FYI when I said 14+% (and now it's 17%), I mean the overall performance
>>> > of starting lldb, not just the demangler itself.  It's probably several
>>> > times faster now with this change (https://reviews.llvm.org/D32500)
>>>
>>> Do you know what the llvm policy is on using TLS in library code? I can't
>>> find any mention of this in the programmer's manual, and my officemates
>>> don't know either.
>>
>>
>> Both libcxx and libcxxabi use __libcpp_tls_*() functions of the threading
>> API [2] (which call pthread functions on most platforms) for thread-local
>> storage needs. IIRC thread_local is not implemented across all the platforms
>> that llvm support.
>>
>> If the idea is to improve libcxxabi's demangler, then it should be
>> straightforward to use these functions instead of thread_local.
>>
>> [1]
>> https://github.com/llvm-mirror/libcxxabi/blob/master/src/cxa_demangle.cpp
>> [2]
>> https://github.com/llvm-mirror/llvm/blob/master/lib/Demangle/ItaniumDemangle.cpp
>> [3]
>> https://github.com/llvm-mirror/libcxx/blob/master/include/__threading_support
>>
>> PS: Here's a particularly amusing bug of the current libcxxabi demangler:
>> https://bugs.llvm.org//show_bug.cgi?id=31031
>>
>> Cheers,
>>
>> / Asiri
>>
>>>
>>>
>>> vedant
>>>
>>>
>>> > On Tue, Apr 25, 2017 at 12:19 PM, Vedant Kumar <vsk at apple.com> wrote:
>>> > I thought the plan of record was (r280732):
>>> >
>>> > '''
>>> > Once the fast demangler in lldb can handle any names this
>>> > implementation can be replaced with it and we will have the one true
>>> > demangler.
>>> > '''
>>> >
>>> > What is the status of lldb's fast demangler? Is it available on Ubuntu
>>> > 16.04?
>>> >
>>> > vedant
>>> >
>>> >
>>> > > On Apr 24, 2017, at 6:02 PM, Scott Smith via llvm-dev
>>> > > <llvm-dev at lists.llvm.org> wrote:
>>> > >
>>> > > (Again), while trying to improve the performance of lldb, I ran into
>>> > > a bottleneck with the demangler.  This may be specific to my platform -
>>> > > Ubuntu 16.04, probably using libstdc++, not libc++.  It makes extensive use
>>> > > of std::string and std::vector, and I see memory allocation at the top.  I
>>> > > prototyped a version that uses an arena-style memory allocator (you can
>>> > > allocate, but you can't ever free).  It is approximately 14+% faster.  I
>>> > > think I can further optimize it by making repeated appends zero-copy (for
>>> > > the string being appended too).
>>> > >
>>> > > The code right now is a little ugly, because it uses a thread local
>>> > > variable to pass around the arena pointer, rather than change every + and +=
>>> > > to be function calls that take db.arena as a parameter.  I'm not sure what
>>> > > you guys would prefer for that either (thread local variable vs api change).
>>> > >
>>> > > _______________________________________________
>>> > > LLVM Developers mailing list
>>> > > llvm-dev at lists.llvm.org
>>> > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>> >
>>> >
>>>
>>> _______________________________________________
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>>> llvm-dev at lists.llvm.org
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>>
>>
>
>
>
> --
> Saleem Abdulrasool
> compnerd (at) compnerd (dot) org
>
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