erik.pilkington added inline comments.
================
Comment at: source/Core/Mangled.cpp:310
+#elif defined(LLDB_USE_LLVM_DEMANGLER)
+ llvm::ItaniumPartialDemangler IPD;
+ bool demangle_err = IPD.partialDemangle(mangled_name);
----------------
sgraenitz wrote:
> erik.pilkington wrote:
> > sgraenitz wrote:
> > > sgraenitz wrote:
> > > > sgraenitz wrote:
> > > > > erik.pilkington wrote:
> > > > > > I think this is going to really tank performance:
> > > > > > ItaniumPartialDemangler dynamically allocates a pretty big buffer
> > > > > > on construction that it uses to store the AST (and reuse for
> > > > > > subsequent calls to partialDemangle). Is there somewhere that you
> > > > > > can put IPD it so that it stays alive between demangles?
> > > > > >
> > > > > > An alternative, if its more convenient, would be to just put the
> > > > > > buffer inline into ItaniumPartialDemangler, and `= delete` the move
> > > > > > operations.
> > > > > Thanks for the remark, I didn't dig deep enough to see what's going
> > > > > on in the `BumpPointerAllocator` class. I guess there is a reason for
> > > > > having `ASTAllocator` in the `Db` struct as an instance and thus
> > > > > allocating upfront, instead of having a pointer there and postponing
> > > > > the instantiation to `Db::reset()`?
> > > > >
> > > > > I am not familiar enough with the code yet to know how it will look
> > > > > exactly, but having a heavy demangler in every `Mangled` per se
> > > > > sounds unreasonable. There's just too many of them that don't require
> > > > > demangling at all. For each successfully initiated
> > > > > `partialDemangle()` I will need to keep one of course.
> > > > >
> > > > > I will have a closer look on Monday. So far thanks for mentioning
> > > > > that!
> > > > Well, right the pointer to `BumpPointerAllocator` won't solve anything.
> > > > Ok will have a look.
> > > > ItaniumPartialDemangler dynamically allocates a pretty big buffer on
> > > > construction
> > >
> > > I think in the end each `Mangled` instance will have a pointer to IPD
> > > field for lazy access to rich demangling info. This piece of code may
> > > become something like:
> > > ```
> > > m_IPD = new ItaniumPartialDemangler();
> > > if (bool err = m_IPD->partialDemangle(mangled_name)) {
> > > delete m_IPD; m_IPD = nullptr;
> > > }
> > > ```
> > >
> > > In order to avoid unnecessary instantiations, we can consider to filter
> > > symbols upfront that we know can't be demangled. E.g. we could duplicate
> > > the simple checks from `Db::parse()` before creating a
> > > `ItaniumPartialDemangler` instance.
> > >
> > > Even the simple switch with the above code in place shows performance
> > > improvements. So for now I would like to leave it this way and review the
> > > issue after having the bigger patch, which will actually make use of the
> > > rich demangle info.
> > >
> > > What do you think?
> > Sure, if this is a performance win then I can't think of any reason not to
> > do it.
> >
> > In the future though, I don't think that having copies of IPD in each
> > Mangled is a good idea, even if they are lazily initialized. The partially
> > demangled state held in IPD is significantly larger than the demangled
> > string, so I think it would lead to a lot more memory usage. Do you think
> > it is possible to have just one instance of IPD that you could use to
> > demangle all the symbols to their chopped-up state, and just store that
> > instead?
> Yes if that will be a bit more work, but also a possibility. I did a small
> experiment making the IPD in line 288 `static`, to reuse a single instance.
> That didn't affect runtimes much. I tried it several times and got the same
> results again, but have no explanation.
>
> You would expect a speedup from this right? Is there maybe cleanup work that
> eats up time when reusing an instance? Maybe I have to revisit that.
Weird, I would have expected a speedup for making it static. My only guess is
that `malloc` is just quickly handing back the buffer it used for the last IPD
or something. I think the cleanup work IPD does should be about the same as the
cost of construction.
https://reviews.llvm.org/D49612
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