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Optimize ItaniumDemangle by using an arena allocator
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Authored by scott.smith on Apr 25 2017, 11:49 AM.

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Summary

Use an arena allocator to reduce the number of allocations and frees. Individual allocations become extremely cheap, and individual frees are free. Change from std::string to a custom string implementation that is a reference to immutable data, so that substr and copies are free. Remove the null terminator so that strings can be appended without copying the base.

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rL LLVM

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scott.smith created this revision.Apr 25 2017, 11:49 AM
davide added a subscriber: davide.Apr 25 2017, 1:07 PM

I have some concerns about the approach. Some of the code is non-portable, I guess, but that can be fixed.
You might want to try reducing the amount of memory allocated rather than growing an homemade allocator to amortize malloc()/free() cost. I'm not positive this can be reached with the current design, but there have been several discussions in the past about a full rewrite. (Please note that the demangler has deeper issues, including looking at its own output which has been cause many bugs).

lib/Demangle/ItaniumDemangle.cpp
49–50

I'm not sure this will work with MSVC.

scott.smith added inline comments.Apr 25 2017, 1:22 PM
lib/Demangle/ItaniumDemangle.cpp
49–50

The admittedly non-portable extension works on MSVC, gcc, and clang. So it's mostly portable ;-) But it's a hack, I admit.

Though I would prefer to remove plus the arena through to all the callsites. It would make the diff less readable, so I thought I'd start with this version. Sorry I should have made that clear from the beginning.

scott.smith abandoned this revision.May 3 2017, 11:49 AM

With added parallelism to lldb, this change no longer has an impact, and I sense an uphill battle trying to get this into libcxx, and then llvm.