[Lldb-commits] [PATCH] D33426: Introduce new command: thread backtrace unique

Zachary Turner via lldb-commits lldb-commits at lists.llvm.org
Tue May 30 09:34:35 PDT 2017

DenseSet<> provides a method named find_as which lets you use an arbitrary
type as the lookup key.  The only requirement is that your DenseMapInfo<>
that you've specialized provide two methods for each key type you want to
use.  So using this approach, even if you have a DenseSet<StackInfoPtr> S,
you can still write S.lookup_as(AddrList);

On Tue, May 30, 2017 at 9:29 AM Greg Clayton via Phabricator via
lldb-commits <lldb-commits at lists.llvm.org> wrote:

> clayborg added a comment.
> Zach made some comments via e-mail:
> > Couple of things:
> >
> > 1. Unless you really really want iteration over this map to be in some
> deterministic order, better to use unordered_map.  And if you do want it to
> be in some deterministic order, you should provide a comparison function,
> as the default one is probably not what you want.'
> True. The previous solution had the same ordering as this map I proposed.
> > 2. std::map<std::vector<lldb::addr_t>, std::vector<uint32_t>> is pretty
> horrible to read, and offers no insight into what the keys and values are.
> At the very least, add some typedefs like: typedef
> std::vector<lldb::addr_t> StackID; typedef std::vector<tid_t> ThreadIDList;
> std::unordered_map<StackID, ThreadIDList> TheMap;
> Agreed.
> > 3. If performance is a concern here (700+ threads seems like it could
> exhibit slowdown when uniquing stacks), the best performance would be to
> use DenseSet<std::unique_ptr<StackInfo>>, where StackInfo contains both the
> address list and the thread is list as well as a pre-computed hash value,
> and then use a custom implementation of DenseMapInfo that just returns the
> hash value directly.  See llvm/lib/DebugInfo/CodeView/TypeSerializer.cpp
> for an example.
> Not sure I would go that far. When you include both things in the type
> (StackInfo here) then you must make one up in order to search for it right?
> We are mainly trying to create a collection of stacks that we can use to
> point the the thread IDs. I agreed it doesn't need to be sorted so
> std::unordered_map would work just fine.
> The more I think about it, the previous approach was probably fine with
> the std::set and with the UniqueStack class. Lets just stick with that for
> now, but we still need to fix the display to not show variable values and
> claim that many threads have the same variable values when showing
> ${function.name-with-args}.
> https://reviews.llvm.org/D33426
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