[Lldb-commits] [PATCH] D61779: FuncUnwinders: General clean up and optimization

Pavel Labath via Phabricator via lldb-commits lldb-commits at lists.llvm.org
Fri May 10 07:59:50 PDT 2019

labath marked an inline comment as done.
labath added inline comments.

Comment at: include/lldb/Symbol/FuncUnwinders.h:117
+  class LazyPlan {
+    lldb::UnwindPlanSP m_plan_sp;
clayborg wrote:
> clayborg wrote:
> > labath wrote:
> > > clayborg wrote:
> > > > maybe use:
> > > > 
> > > > ```
> > > > llvm::Optional<lldb::UnwindPlanSP> m_plan_sp;
> > > > ```
> > > > 
> > > > Then just check if it has no value, and if so compute and set it either to a valid shared pointer or an empty one?
> > > That would work, but it would increase the size of the FuncUnwinders struct by about 80 bytes (8 bytes for each LazyPlan object). I can do that, but given that "uncomputed value" trick is internal to the class and does not leak out or affect the implementation, it seemed like a worthwhile optimization to me.
> > Actually isn't there a pointer union class that can steal bool bits from the aligned values and not increase the size?
> I believe the clang type system steals 3 bits from a Type * to encode "const" and a few other type qualifiers
Yes, there is a `llvm::PointerIntPair`, but AFAICT that only works with a raw pointer, not a shared_ptr. I believe implementing a similar trick for shared_ptr would be possible, but it would definitely be tricky. Way more trickier than this..



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