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

Tue Apr 25 13:51:18 PDT 2017

On 4/25/17 2:37 PM, Asiri Rathnayake wrote:
>
>
> On Tue, Apr 25, 2017 at 8:36 PM, Vedant Kumar via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>
>     > On Apr 25, 2017, at 12:24 PM, Scott Smith
>     <scott.smith at purestorage.com <mailto: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 ?).

A copy of the libcxxabi one was added to libSupport relatively recently 
so that lldb could use it.... so they should be almost the same.

lldb has its own because it has different constraints w.r.t. memory 
allocation and speed compared to the __cxa_* one. (I don't know much 
about the details there though). If falls back on the __cxa_* 
implementation for some cases where the "fast" one's implementation is 
incomplete (again, repeating what I remember... I don't know the details).

Jon
>
>
>
>     > 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 <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 
> <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 
> <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
>     <mailto: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 <mailto: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 <mailto:llvm-dev at lists.llvm.org>
>     > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>     <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>     >
>     >
>
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>

-- 
Jon Roelofs
jonathan at codesourcery.com
CodeSourcery / Mentor Embedded / Siemens

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