[llvm-dev] RFC: Cleaning up the Itanium demangler

[Erik Pilkington via llvm-dev]

On 6/21/17 5:42 PM, Rui Ueyama wrote:
> I'm very interested in your work because I've just started writing a 
> demangler for the Microsoft mangling scheme. What I found in the 
> current Itanium demangler is the same as you -- it looks like it 
> allocates too much memory during parsing and concatenates std::strings 
> too often. I could see there's a (probably big) room to improve. 
> Demangler's performance is sometimes important for LLD, which is my 
> main project, as linkers often have to print out a lot of symbols if a 
> verbose output is requested. For example, if you link Chrome with the 
> -map option, the linker has to demangle 300 MiB strings in total, 
> which currently takes more than 20 seconds on my machine if 
> single-threaded.
>
> The way I'm trying to implement a MS demangler is the same as you, 
> too. I'm trying to create an AST to describe type and then convert it 
> to string. I guess that we can use the same AST type between Itanium 
> and MS so that we can use the same code for converting ASTs to strings.
Using the same AST is an interesting idea. The AST that I wrote isn't 
that complicated, and is pretty closely tied to the libcxxabi demangler, 
so I bet it would be easier to have separate representations, especially 
if your intending on mimicking the output of MS's demangler. I'm also 
not at all familiar with how MS mangles their C++, which might imply a 
slightly different representation.
> It's unfortunate that my work is overlapping with yours. Looks like 
> you are ahead of me, so I'll take a look at your code to see if 
> there's something I can do for you.
>
> On Wed, Jun 21, 2017 at 4:42 PM, Erik Pilkington via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>     Hello all,
>     The itanium demangler in libcxxabi (and also, llvm/lib/Demangle)
>     is really slow. This is largely because the textual representation
>     of the symbol that is being demangled is held in a std::string,
>     and manipulations done during parsing are done on that string. The
>     demangler is always concatenating strings and inserting into the
>     middle of strings, which is terrible. The fact that the parsing
>     logic and the string manipulation/formatting logic is interleaved
>     also makes the demangler pretty ugly. Another problem was that the
>     demangler used a lot stack space, and has a bunch of stack
>     overflows filed against it.
>
>     I've been working on fixing this by parsing first into an AST
>     structure, and then traversing that AST to produce a demangled
>     string. This provides a significant performance improvement and
>     also make the demangler somewhat more clean. Attached you should
>     find a patch to this effect. This patch is still very much a work
>     in progress, but currently passes the libcxxabi test suite and
>     demangles all the symbols in LLVM identically to the current
>     demangler. It also provides a significant performance improvement:
>     it demangles the symbols in LLVM about 3.7 times faster than the
>     current demangler. Also, separating the formatting code from the
>     parser reduces stack usage (the activation frame for parse_type
>     reduced from 416 to 144 bytes on my machine). The stack usage is
>     still pretty bad, but this helps with some of it.
>
>     Does anyone have any early feedback on the patch? Does this seem
>     like a good direction for the demangler?
>
>     As far as future plans for this file, I have a few more
>     refactorings and performance improvements that I'd like to get
>     through. After that, it might be interesting to try to replace the
>     FastDemangle.cpp demangler in LLDB with this, to restore the one
>     true demangler in the source tree. The FastDemangler.cpp is only
>     partially completed, and calls out to ItaniumDemangle.cpp in llvm
>     (which is a copy of cxa_demangle.cpp) if it fails to parse the symbol.
>
>     Any thoughts here would be appreciated!
>     Thanks,
>     Erik
>
>     _______________________________________________
>     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>
>
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170621/37742309/attachment.html>