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

Scott Smith via llvm-dev llvm-dev at lists.llvm.org
Tue Apr 25 12:42:20 PDT 2017

On Tue, Apr 25, 2017 at 12:36 PM, Vedant Kumar <vsk at apple.com> 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?

I'm confused too.  I'm new here :-)

> > 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.

I don't know, and frankly I don't like using it.  It was more "to get the
conversation started."  I can change all the string routines to take the
arena as a parameter, it'll just make the diff look larger.

But if libcxxapi has already done the heavy lifting then maybe I should
just benchmark their demangler instead.
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